OzVMX Forum
Clubroom => Tech Talk => Topic started by: John Orchard on December 12, 2014, 08:03:31 am
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So I buy a Honda CR500RE, I read-up on them, "oh they are the ping-king", ok so I get the head machined before I even ride it, first time out on it, it pings like crazy after 2 laps of the track. Hmmm I thinks, sounds lean at part throttle BUT it starts cold without the choke, Hmmm tells me it's rich on the pilot. I pull the plug .... blaaack.
So I figure it's lean on the first part of the needle, so I buy another needle with a smaller diameter first half, still has the same lean ping! Hmmmm (deep thinking again), so I sand-down the needle to make it real rich, it still pings, then blubbers and fouls a plug!
ok I think maybe slide cutaway? I gets the big file out and go to town on the bottom of the slide to reduce the amount of cutaway slide (making more rich) :-) So I've whipped about a 1mm off the slide, still pings like a bitch! There is no wear at all on the slide, looks new, and is still the standard 3.5.
There are no air-leaks anywhere, not the manifold, not the base gasket, the compression ratio is no higher than standard even with the squish closed up to 1.2mm.
So I think ignition timing, I read that the extra turbulence from a closed-up squish can cause the flame front to travel faster, maybe at that certain rpm/throttle position the CDI advance curve has more advance at that point, combined with the faster burning combustion chamber, causing it to ping? So I slot the stator backing-plate (so I can rotate it in the direction that the rotor travels), first 1mm, then 2mm, if anything it seems to run/ping worse, so I go back to standard position.
Now I've partially fixed the problem and now know what is causing it, I'll just see if anyone else thinks they know what it is before I tell? ;-)
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Mine pinged like a barstard on the standard needle. I changed to the magic needle and that made a world of difference, but I still need to "clear its throat" before it runs properly.
Looking forward to seeing what the answer is John :)
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Which part of the head did you get machined? Was it to reduce compression or change the squish angle?
What size main jet are you running in what carby?
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Probably shouldn't be related what main I'm running as the ping was at part throttle? It's 160 main. Standard Keihin PE38.
The squish was made narrower to 14mm and the inner dome blended to the squishband with a good radius. Then the mating surface machined .7mm to bring the comp back up and close the squish to 1.2mm.
Of the 100's of Google leads I have followed, only once is this matter touched on and no one has offered the fix that worked for me.
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Yeah 160 main is on the money. I have a few mates with 500 RE's and we work on them and they're not that far off the mark std.
I think you've prob got too much comp now, try and extra head gasket just to see if that cures the pinging.
Some of my mates are running the round slide Keihin and some are using a pwk flatslide, but they all run pretty good and weren't that difficult to jet
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I wouldn't put in another head gasket, the squish & comp are right.
I've found that some Harley's & scooters can suffer with this problem. They touch on the subject here http://vintagebikemagazine.com/technical-articles/mikuni-and-general-carburetor-tuning/
Just repeating, I have found the problem, just interested if you guys can guess it :-)
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Have you tried running BP 100 fuel?
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Yep, made no difference :-)
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Vibration? The big bores vibrate as we know which can aerate the fuel in the bowl causing a lean mix.
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Vibration? The big bores vibrate as we know which can aerate the fuel in the bowl causing a lean mix.
Well done I must say, took me ages to work it out, spent the whole day yesterday riding around as per Steve McQueen style (topless cap on backwards), trying everything I could on a hot sunny arvo at The Farm.
My fix was to get a piece of fuel line shove'd in the pilot jet nozzle, so that it was collecting fuel from the bottom of the fuel bowl ..... fixed it. But with the extra vibration from revving in neutral it still pinged on race starts. I'm just about to rubber mount the airbox to reduce the vibes more.
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Pilot jet is higher in the bowl than the main, so at higher rpm (when vibrating more) and at part throttle (still on the pilot) the frothing/airated fuel was incredibly leaning out the mixture.
Yes I must be a woose to be on the pilot jet, anything smaller than a 500 and I'd be on the needle (plus it wouldn't be vibrating as much) and not noticing it.
I guess riding on the dry & slippery surface at Heathcote had me hunting for traction on the pilot.
I've noticed a similar problem with the KX400, it was only slight, going larger with the pilot in that case wrongly improved things, I will be going down on the pilot in that and extending the pilot jet pick-up point to the bottom of the bowl.
I can't believe that there isn't a lot written about this on the net?!!!
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Some of my mates are running the round slide Keihin and some are using a pwk flatslide, but they all run pretty good and weren't that difficult to jet
Can't run PWK's in Pre 85 can you?
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All interesting.
I haven't had enough time on mine to make any pronouncements on the goodness or otherwise of the squish modification and consequential uncorrected decompression compared to standard. My PWK is running a 155MJ which seems to be right with the stock and the Pro Circuit pipes.
I have a spare head as a control and am tempted to close the squish up on the already modified one and see how it goes.
Comparing the PE with the PWK is revealing. The float bowls will interchange but the PWK's pilot well extends down 31 mm from the gasket face or as far down as the bottom of the MJ baffle. The PD's sits 21 mm down; so a full 10 mm higher ;).
PWK has gotta be legal. The earlier RM500 has a flat slide TM; the PWK is not a round slide and has a partially flat profile.
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Comparing the PE with the PWK is revealing. The float bowls will interchange but the PWK's pilot well extends down 31 mm from the gasket face or as far down as the bottom of the MJ baffle. The PD's sits 21 mm down; so a full 10 mm higher ;).
Yep I just pulled a bowl off a PJ carb, found a plastic shroud around the pilot jet running down to the bottom of the bowl.
Jeez takes 30 years to discover the problem, poor suckers that raced them back in the day! I wonder what carb was on Jeff Leisk's CR500RE back in the day when he won Mr Motocross?!!
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Well done John. I discovered something similar with my then '74 CR250M . There are 2 different length pilot jets for the early Keihins. One about 10mm longer than the other. When I first got my CR, I couldn't get the thing to pull cleanly off the bottom for the love of jets and jet drills. The thing was like a light switch and pinged with the DG head. Then I found longer PJ's at a dealer in Taree NSW. Grabbed a couple of different sizes and then set about trying them. Even with a too lean PJ the bike stopped pinging! I used a #60 PJ and then drilled that bigger again. I ended up with an Elsinore that actually had bottom end response ;D
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Well done John. I discovered something similar with my then '74 CR250M . There are 2 different length pilot jets for the early Keihins. One about 10mm longer than the other. When I first got my CR, I couldn't get the thing to pull cleanly off the bottom for the love of jets and jet drills. The thing was like a light switch and pinged with the DG head. Then I found longer PJ's at a dealer in Taree NSW. Grabbed a couple of different sizes and then set about trying them. Even with a too lean PJ the bike stopped pinging! I used a #60 PJ and then drilled that bigger again. I ended up with an Elsinore that actually had bottom end response ;D
That's awesome, I don't know why I haven't seen more info about this?
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I just ripped the bowls off my PE & PJ carbs, the PJ is on the left, you can just see the shroud around the pilot jet.
(http://i1093.photobucket.com/albums/i423/JohnnyRacer89/PJandPEcarbs_zpsfc41c330.jpg) (http://s1093.photobucket.com/user/JohnnyRacer89/media/PJandPEcarbs_zpsfc41c330.jpg.html)
This is my make-shift extension that fixed the problem out at the track, just a bit of fuel hose pushed in with a wedge cut out of the end so it doesn't seal on the bowl bottom..
(http://i1093.photobucket.com/albums/i423/JohnnyRacer89/Keihinpilotextension_zpsc170fa99.jpg) (http://s1093.photobucket.com/user/JohnnyRacer89/media/Keihinpilotextension_zpsc170fa99.jpg.html)
I'm looking at stuffing some foam in there somewhere, plus I'll raise the float level another 1mm from 16 to 15.
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Good stuff! I'll look at the PE on the 480 tomorrow although it hasnt really pinged until it gets really hot. Seems like a good little upgrade. Planning to ride it on Sunday. I guess there is more likely hood of the jet blocking though.
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Well done John. I discovered something similar with my then '74 CR250M . There are 2 different length pilot jets for the early Keihins. One about 10mm longer than the other. When I first got my CR, I couldn't get the thing to pull cleanly off the bottom for the love of jets and jet drills. The thing was like a light switch and pinged with the DG head. Then I found longer PJ's at a dealer in Taree NSW. Grabbed a couple of different sizes and then set about trying them. Even with a too lean PJ the bike stopped pinging! I used a #60 PJ and then drilled that bigger again. I ended up with an Elsinore that actually had bottom end response ;D
That's awesome, I don't know why I haven't seen more info about this?
I didn't see any info about it either. I just stumbled onto it when comparing jets at the bike shop and thought there had to be a reason for the longer jets. The '75 and '76 CR's came out with the longer jet which might help to explain why they run so much smoother than the '74....
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"I wonder what carb was on Jeff Leisk's CR500RE back in the day when he won Mr Motocross?!!"
Jeff Leisk doesn't use a pilot or needle jet.......the slide is out of the carb and the throttle just switches the ign on or off..... ::)
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I wonder what carb was on Jeff Leisk's CR500RE back in the day when he won Mr Motocross?!!
[/quote]
Jeff rode a pre proddy or semi factory air cooled 500 in 83 to some moto wins in MR MX, But won the title on a watercooled job the next year;)
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Was it in '83 I saw pics of the rear disc air-cooled CR500 at Broadford?
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I was wrong JO, It must have been 85 when he rode the watercooler, This sticker on Ebay shows an aircooler & it says 1984 MR MX :-[ :-[ :-[ :-[
http://www.ebay.it/itm/Jeff-Leisk-1984-Mr-Motocross-Honda-Australia-sticker-CR500-Moto-X-motorcycle-/121463482050
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Interesting stuff! I knew there was another reason, besides being fast, that I have you as my factory rider. :D
The CR500RE has a different crank balancing than the CR480, maybe that is why it might vibrate more? This makes me wonder about the infamous YZ490 and it's pinging problems. I think vibration is almost always mentioned in the YZ490 tests.
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I would be interested to what difference a probably balanced crank made to the pinging?
Frank did my 84 495 crank.
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I guess with regards to balancing a single cyl, it can only be balanced for smoother operation at a certain rpm, and not balanced either side of the rev range? Need some input from Lozza, he would have an idea.
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The hose on the pilot jet thing was a common "fix" for the big flat spot bog that Honda XR's had when landing from jumps or going over a long stretch of rough ground. Never seen in on a CR till now though.
K
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I wonder also if the tin baffle plates in the fuel bowl help contribute to the frothing fuel from vibration?
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The CR500RE has a different crank balancing than the CR480, maybe that is why it might vibrate more?
Jay the 500 has a longer stroke also, which would also help contribute to the vibration.
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there is a bloke in qland who cured his husky from vibrating by balancing and matching his crank halves. it was a 250 and he milled a scary amount of metal off the crank cheeks before he panicked and stopped but the result was superb.
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The hose on the pilot jet thing was a common "fix" for the big flat spot bog that Honda XR's had when landing from jumps or going over a long stretch of rough ground. Never seen in on a CR till now though.
K
Yeh Mick I remember making all sorts of baffle plates to cure a lean 'bog' after jumps or big hits back in the day, I'd never considered airation from vibration though.
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Single cylinder engine balancing is a subject all on its own. Ideally you shift vibes to a rpm range that's never used. Default factor is 60% reciprocating over rotating masses. High revving kart engines can be as low as 55% and low revving mxers can be as high as 67%. Main thing is to sdd mass to the light side of the crank rather than try to take it from the heavy side. With the CR I would expect the balance would need to be raised rubber mounting at the head stay helps a lot to. Has anyone actualy checked the balance factor with the 500?
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on that vibration bit,bitd engine vibes on Bathurst's conrod straight bounced the fuel off the tank bottom,causing stavation,clever guys lengthened the tap internal pickup spigots, :P
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Crank balancing according to rev range is one thing, the crank halves being trued is another. Never a bad idea to get the crank checked for how true it is if you pull the motor apart.
I found 1/4" hydraulic tubing has a good interference fit in the pilot jet holder, soput in the extension in my PE38 today
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Interesting stuff! I knew there was another reason, besides being fast, that I have you as my factory rider. :D
The CR500RE has a different crank balancing than the CR480, maybe that is why it might vibrate more? This makes me wonder about the infamous YZ490 and it's pinging problems. I think vibration is almost always mentioned in the YZ490 tests.
there is a yz 490 in number 46 of the vmx mag
The yz 490 featured has had some head work and a few other big mods to stop the pinging , maybe a good read.
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Ok I'm going testing out The Farm this week, I've rubber mounted the airbox, raised the float level from the 16mm float height to 15mm, I've also totally removed the tin baffle plates, they were sure to vibrate and make the fuel airate with any rpm on.
Still keeping the rubber fuel hose pilot jet extension, with 160 main jet, 50 pilot jet, DGG needle with B8ES plug.
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Ok went testing ....
I didn't find any further improvement over my previous pilot jet extension, maybe a touch worse? With the airbox rubber mounted, it appeared to cause the carb to vibrate around a bit more? I tried pulling the carb body hard against the tension of the rubber joints with a big cable-tie to the frame, it seemed to reduce the vibration going through the carb but didn't reduce the high rpm/low throttle lean ping. I ended up putting the tin fuel bowl baffle plates back in and kept the higher float-level.
In an effort to establish other area's that could be causing the leanness; I did notice plenty of smoke coming from the pipe join to the barrel, while this is not an odd thing as many bikes leak at the header joint, I have to wonder if a big bore is more likely to take-in some fresh air with a negative pulse travelling past the leak? I'll put an 'o' ring in for the next test.
My next area of concern is with the carburetor 'pilot air bleed', from my understanding & experience on how the air-bleeds work in a carb, the effects of bleeding air into a fuel circuit (pilot or main circuit), the higher the rpm, the more air is bled into the circuit.
So, with the pilot jet down from the stock 68 to a 50, the bike still started cold without need for the choke, so the pilot jet is still too big, or there could be some other reason for the super-low rpm (kickstart speed) richness?
As the rpm's increase, while still with a slight throttle opening (still on the pilot circuit) the affects of the pilot air-bleed (air-screw) is leaning-out the mixture too much? The mixture screw was set at the stock 1.5 turns. So I figure for the next test I'll try going down to a 40 or 45 pilot jet with the air-screw turned right in, maybe around 1/4 turn? I wouldn't be surprised if I have to go even smaller with the pilot jet, maybe a 30 or 35 and seat the air-screw all the way home? But we'll see.
I have found in the past that the 'pilot air bleed' can have an affect mixtures during larger throttle openings, found by accident one time with an RM125B, that with the air-screw seated closed I got a high rpm/full throttle rich miss-fire, so with that in mind, if I end up seating the CR's air-screw home I may have to compensate further up the rpm/throttle opening settings?
I am surprised that an engine that is close to stock would require such a drastic change from the stock fuel curve? I did find many years ago when racing a Honda VTR1000 Firestorm, that once I opened up the airbox, I had to go from the stock 175 main jets to 300 (1.75mm to 3.00mm), this gave perfect power at 10,000 rpm but a very bad rich stumble at 6,000 rpm (at WOT), so I went back to the 175 mains but completely blocked-off the main air-bleeds ... perfect all the way through the rev range.
Stay tuned, will test again next week when the new jets arrive.
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lots of good work happening there John :-X keep at it
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hi john
I recently did the squish on a 77 yz400 ,as it ran ok before this, the amount removed from the dome of head was the same as removed to get a 1mm squish. but it pinged real bad ,so I read up about it and checked comp ratio .it ended up being 22 to 1
,Shane ended up fixing for us he had to remove 18cc extra from what I had done, now its a rocket.
cheers arno
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I would still be looking at the cylinder head
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But why ping when only on the pilot circuit? Surely the combustion chamber is under more load to make it ping with more throttle on.
I'll down-size the pilot and screw in the air-screw and see :-)
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But why ping when only on the pilot circuit? Surely the combustion chamber is under more load to make it ping with more throttle on.
I'll down-size the pilot and screw in the air-screw and see :-)
Personally, I think the bike isn't getting enough fuel at low rpm John. I'd try a fatter pilot jet and perhaps remove some of the slide cut away if it ends up with a bit of a bog down low.... Just a thought......
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But why ping when only on the pilot circuit? Surely the combustion chamber is under more load to make it ping with more throttle on.
I'll down-size the pilot and screw in the air-screw and see :-)
In my experience (spent 10 years driving a dyno) the idle - pilot circuit responds a differently to air correction compared to main. The air screw is called the idle air screw and has it's greatest effect at idle. I would go back to the stock pilot or slightly bigger.
Would you descibe the ping as only roll on-off throttle at high or on pipe RPM? If it is try a lower or richer slide cutaway to get it on the main curcuit with a little less throttle opening.
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Thanks for the input guy's, spent another day at The Farm today; I think I've nearly worn-out the screws holding that carb in & together lol.
Just about perfect now, waiting on a needle coming from Keihin in Japan to finish it off. It has all been carburation related and is what I thought it was ... running lean at part throttle.
Extending the pilot pick-up got me 50% there but a change in needle was also required, I've posted before that "where you shift power to, you must shift fuel to", the reed & combustion changes boosted low-down so I needed more fuel at part throttle.
What had me confused was how early the straight section of the needle came-in, in this case it comes into play at idle but with more dominance once the throttle slide starts to lift; I guess a 500 has such a strong intake pulse that it draws from the needle jet immediately?
So I've gone down on the pilot, from 68 to 50, stayed with the same air-screw setting (1.5 turns) and gone to a needle with a smaller 2.645 straight section (DEE) on the leanest clip, going to the leanest clip cleaned up the blubbering half-throttle, stock needle is an N28 (DGJ / R1468N) with a 2.685 straight section, my DEE replica is a sanded-down needle, when I get the factory DEE will do a final confirmation test.
:-D
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i haven't read it all but had strange things happen on an old ktm125 think about "86 & did a crank for a guy, off down the paddock & went through a blubbery stage, not idle or just above but about half revs maybe about where it realy wants to get on the boil, & although it was carburation symptoms the cause was 'reeds', someone sugested that & i poo poo'd it but learn't something that day. By memory were fibre reeds & must've lost their tension because new ones damn well fixed it.
With your CR500, if it has a round slide carb [suppose it's keihin but i'm only familiar with mikuni] i think they have a progression hole at an angle coming off the pilot, just in front of the pilot outlet & fed by pilot & think you can increase the richness [bit like less cutaway on the slide] by increasing the jet size then i suppose back the pilot air screw out a bit to keep idle ok [if it was ok before]. Does seem strange if you have to go too far from std though!!, don't suppose it's been ported [over ported] . Had to lengthen the header by heaps on an IT400 motor i bought for vmx & someone had got in there & given it 192 degrease on the exhaust port & "went" like a roadracer but no good in the dirt--not knowing this, i was trying to carburate the flat spot out [almost blubbery], was a pain till i went looking for other causes.
Anyway--don't talk carburetors to me----had to fix a honda quad starter motor the other day for someone not far up the road & could see an "85 XJ600 Yamy in the shed--do you want it for nothing, it's been under a hedge since 2003. raced home & got the trailer, had to knock the calipers off so as to be able to wheel it. I "will" get it going, carbs [4 of the buggers] are the worst mess i've ever seen [& i've said that before] but realy this time. Touched a battery [screwdriver trick] onto the starter & instantly spat a pint of dirty water all over the shed. Anyway, sorry, gettin off track again.--where was i.
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** EDITED **
Well couldn't sleep, spent the night with my mind being 'a fly on the venturi wall' trying to understand my carb.
So, even with a closed twist-grip (there is still a tiny air-gap under the slide), the contributing circuits to fueling the engine are; 'pilot circuit' (pilot jet & air screw) and needle straight section in the needle jet. Yes I know all the charts say that it is only the pilot circuit but in this case I feel it is both circuits.
At Idle speeds with a closed throttle grip, the major contributing circuit is the pilot circuit, due to minimal 'concentrated' airflow over the needle jet to pull the same amounts as the pilot circuit can supply. At a certain point the vacuum pulse over the pilot outlet becomes less and air-speed picks-up over the needle-jet then making the needle the major contributing fuel supplier.
With the two circuits supplying fuel at idle, this is like having a choke passage open, now realizing why I don't need choke to start the bike cold, even with dropping from a 68 to a 50 pilot jet. Having dropped that far in pilot jet makes me think that the little less fuel supply at further throttle openings might require stepping-up the main just a touch, might jump up from the 160 to a 165 (168 is stock).
With a closed twist-grip, the needle is on the straight section at the needle-jet, and stays on the straight section for the first 10mm of throttle slide movement (to about 1/4 throttle in a 38mm venturi). There is no denying that with a fresh 500cc cylinder, the intake vacuum at a closed throttle IS still enough to pull fuel from the needle-jet. Whether this is just a design fault with the Keihin PE38 I don't know, but I will say that I swear the same thing is happening with the VM38 Mikuni on the KX400 (carb is off a KX250A5).
For a bit I thought that because I have the bike set so it would idle, that the slightly lifted slide might bring the needle-jet into play at a closed twist-grip (and I guess it does) but then I realized that with any amount of slide lift, the depression over the needle jet will draw fuel fuel past the needle, so with the same given intake vacuum from the 500cc piston, and, the fact that the straight section of the needle will still 'allow' fuel to flow out, fuel HAS to flow out if the slide lets any air past.
I guess the next step is to try (even though I don't want to disturb something's that now working ;-) ) letting the slide drop to completely to the bottom to see if I can stop fuel supply from the needle-jet with a closed twist-grip?
To keep the status-quo in the fuel curve, it makes sense now that running a smaller (richer) straight section needle required a drop in pilot jet size, it just took me a while to juggle the correct ratio to balance closed throttle & 1/8th throttle, as both circuits are contributing, varied amounts at various openings.
I now wonder how many other open-class two-strokes have this issue, with the them having such strong intake vacuum at closed throttle?
** EDIT ..... I just realized since I made this post that I have been confusing 'closed throttle' (slide all the way to the bottom of the carb) with 'closed twist-grip', because the high idle rpm setting meaning the slide is open (yes on the needle!).
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In my experience (spent 10 years driving a dyno) the idle - pilot circuit responds a differently to air correction compared to main. The air screw is called the idle air screw and has it's greatest effect at idle.
Thanks, I now tend to agree with you.
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just another probably [probably not relevant] thought, have seen lots of needle jets simply worn out & causing loading up on the line [more than normal], constant throat clearing plus ritchening on a closed throttle heading into a corner [rich when opening up after that]. Worst one of mine was a 125k back when it was only a few years old. Could [i have] block the needle jet off & start on the pilot with closed throttle just to check the pilot lean/rich etc but any throttle above a fraction will snuff it of course. Or is the needle designed to bleed a bit anyway on closed throttle to help avoid seizing after a long full song then shutting off for a long slow down. Someone hasn't drilled the air bleed to the needle jet &/or pilot jet [if they have brass air bleeds in the holes]. Seen that done but of course the customer doesn't tell you he's done it--just i've tried everything & of course you fit a new needle & emulsion tube & then go chasing the next problem. Anyway--off track again as i see the heading is 'pinging', run her on av gas!!
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But why ping when only on the pilot circuit? Surely the combustion chamber is under more load to make it ping with more throttle on.
I'll down-size the pilot and screw in the air-screw and see :-)
With the throttle not open the reeds are not opening fully which leads to irregular combustion. The needle won't be dominant until around a 1/4 throttle due to the annulus being smaller than the pilot jet with the throttle closed, or when the annulus between the needle and emulsion tube becomes bigger than the pilot jet area. You should be able to pick the transition(pilot to needle) that will be where the air screw no longer has any effect on rpm. I don't believe the needle jet will bleed fuel at a closed throttle if it does it would not be as much as the the pilot circuit. The needle shroud is still inside the slide. I've never had idling change by raising or lowering the needle clip or changing the needle. To check shine a timing light down the carby bore when running
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** EDITED **
Well couldn't sleep, spent the night with my mind being 'a fly on the venturi wall' trying to understand my carb.
So, even with a closed twist-grip (there is still a tiny air-gap under the slide), the contributing circuits to fueling the engine are; 'pilot circuit' (pilot jet & air screw) and needle straight section in the needle jet. Yes I know all the charts say that it is only the pilot circuit but in this case I feel it is both circuits.
At Idle speeds with a closed throttle grip, the major contributing circuit is the pilot circuit, due to minimal 'concentrated' airflow over the needle jet to pull the same amounts as the pilot circuit can supply. At a certain point the vacuum pulse over the pilot outlet becomes less and air-speed picks-up over the needle-jet then making the needle the major contributing fuel supplier.
With the two circuits supplying fuel at idle, this is like having a choke passage open, now realizing why I don't need choke to start the bike cold, even with dropping from a 68 to a 50 pilot jet. Having dropped that far in pilot jet makes me think that the little less fuel supply at further throttle openings might require stepping-up the main just a touch, might jump up from the 160 to a 165 (168 is stock).
With a closed twist-grip, the needle is on the straight section at the needle-jet, and stays on the straight section for the first 10mm of throttle slide movement (to about 1/4 throttle in a 38mm venturi). There is no denying that with a fresh 500cc cylinder, the intake vacuum at a closed throttle IS still enough to pull fuel from the needle-jet. Whether this is just a design fault with the Keihin PE38 I don't know, but I will say that I swear the same thing is happening with the VM38 Mikuni on the KX400 (carb is off a KX250A5).
For a bit I thought that because I have the bike set so it would idle, that the slightly lifted slide might bring the needle-jet into play at a closed twist-grip (and I guess it does) but then I realized that with any amount of slide lift, the depression over the needle jet will draw fuel fuel past the needle, so with the same given intake vacuum from the 500cc piston, and, the fact that the straight section of the needle will still 'allow' fuel to flow out, fuel HAS to flow out if the slide lets any air past.
I guess the next step is to try (even though I don't want to disturb something's that now working ;-) ) letting the slide drop to completely to the bottom to see if I can stop fuel supply from the needle-jet with a closed twist-grip?
To keep the status-quo in the fuel curve, it makes sense now that running a smaller (richer) straight section needle required a drop in pilot jet size, it just took me a while to juggle the correct ratio to balance closed throttle & 1/8th throttle, as both circuits are contributing, varied amounts at various openings.
I now wonder how many other open-class two-strokes have this issue, with the them having such strong intake vacuum at closed throttle?
** EDIT ..... I just realized since I made this post that I have been confusing 'closed throttle' (slide all the way to the bottom of the carb) with 'closed twist-grip', because the high idle rpm setting meaning the slide is open (yes on the needle!).
Hi John,
The 38mmVM Mikuni on the CR480 you raced ran the UFO slide mod and had the pilot jet completely blocked off.
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Yes I remember Jay, at kickstart speeds we had to have the twist-grip at 1/4 throttle for it get fuel from the needle jet before it would start. Proving an open-class machine has the suction to pull it out.
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OKAY FOUND ANOTHER ISSUE.
I swapped the crank seals around so that crankcase vacuum pulls the seal lips into the crank rather than away (seal recess away from the crank flywheels).
Gearbox oil was causing inconsistent readings as it partially sealed the crank seal.
The resulting vacuum in the gearbox was bringing in dust, making the gear oil dirty quicker. I didn't follow my suggestion of a fuel filter on the breather ( I was getting around to it).
I feel crankcase vacuum is much stronger than compression, especially at high rpm/low throttle, I'd much rather have the chance of losing some crankcase compression, rather than sucking in air and causing a lean mixture.
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If the throttle isn't completely open nor are the reeds ;)
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If the throttle isn't completely open nor are the reeds ;)
A bike set to idle will have a partially open throttle valve that will let 'some' air past, with a piston rising in the bore and creating a vacuum, the reeds will only open enough to let available air through it. So as you say "completely closed throttle valve means completely closed reeds".
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Great thread John. A logical progression of analytical thinking, informed decisions and seat of the pants testing - well written and well executed. You're braver than me though, diving in the deep end and changing half a dozen things at the same time, but that makes it all the more impressive.
This thread is what this forum is all about,
Cheers,
Pete.
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What would happen with a one way valve only letting pressure out of the crankcase solve this? Tried different seal brands or some teflon seals?
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What would happen with a one way valve only letting pressure out of the crankcase solve this? Tried different seal brands or some teflon seals?
Why would we want to let 'pressure' out of the crankcase, it is not getting as extreme as the vacuum (lower pressure) in the crankcase? The negative pulse from the expansion chamber and the opening transfers into the combustion chamber would be stopping any excessive 'positive pressure' build-up?
I think keeping a very low pressure in the crankcase would give an instant throttle response once the throttle is opened, yeh? Now I've rotated the seals it seems to be holding the 'low pressure' (vacuum).
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I meant the gearbox part of the crank case :) As explained before the highest pressure in the crankcase is lower than residual cylinder pressure. Throttle response more carburation than anything else
Pressure in the crank case(red),transfers(purple green blue) inlet (black) and exhaust(brown)
(http://i155.photobucket.com/albums/s297/Lozza85_2007/pindex_zpsecd496e2.png) (http://s155.photobucket.com/user/Lozza85_2007/media/pindex_zpsecd496e2.png.html) Pressure in cylinder(red) exhaust(black) crankcase(green)
(http://i155.photobucket.com/albums/s297/Lozza85_2007/Untitled_zpscbe9734e.png) (http://s155.photobucket.com/user/Lozza85_2007/media/Untitled_zpscbe9734e.png.html)
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I see crankcase pressure exceeding exhaust port pressure between TPO & IPO.
Cylinder pressure = exhaust port pressure, after blow-down.
Second graph shows nothing useful, because the scales of the measurements are so different.
Resolution of a combustion pressure transducer is not suitable for very low & negative pressures.
(Explained by the manufacturer, previously).
Cheers, Daryl
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Thanks Lozza, very thought provoking, I'm sure I'll come back with questions & idea's after studying them :-)
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I meant the gearbox part of the crank case :) As explained before the highest pressure in the crankcase is lower than residual cylinder pressure. Throttle response more carburation than anything else
Pressure in the crank case(red),transfers(purple green blue) inlet (black) and exhaust(brown)
(http://i155.photobucket.com/albums/s297/Lozza85_2007/pindex_zpsecd496e2.png) (http://s155.photobucket.com/user/Lozza85_2007/media/pindex_zpsecd496e2.png.html) Pressure in cylinder(red) exhaust(black) crankcase(green)
(http://i155.photobucket.com/albums/s297/Lozza85_2007/Untitled_zpscbe9734e.png) (http://s155.photobucket.com/user/Lozza85_2007/media/Untitled_zpscbe9734e.png.html)
I've got to disagree again. From the graph it is clear that the crankcase pressure stays higher than both the transfer and the exhaust port pressure for the entire tranfer open period which allows the fresh mixture to flow up into the cylinder. It is also nice to see the chamber doing it job to scavange the cylinder/ exhaust port pressure down to almost equal that of the crankcase just before the transfer open piont and then to see the exhaust port reverse pulse peak just before the exhaust port closes. If the cylinder pressure stayed high as you say the moment the transfers opened their pressure would spike high which they don't.
The graph also clearly shows that the peak pressure in the crankcase of around 1.6 bar which is .6 bar over atmospheric is greater than the amount it drops below of about .7 which is only .3 below.
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I see crankcase pressure exceeding exhaust port pressure between TPO & IPO.
Cylinder pressure = exhaust port pressure, after blow-down.
Second graph shows nothing useful, because the scales of the measurements are so different.
Resolution of a combustion pressure transducer is not suitable for very low & negative pressures.
(Explained by the manufacturer, previously).
Cheers, Daryl
No not at all. The cylinder pressures and the other traces can't exist on the same graph as the cylinder pressure dominates that turns the other traces into flat lines at the bottom of the graph
I've got to disagree again. From the graph it is clear that the crankcase pressure stays higher than both the transfer and the exhaust port pressure for the entire tranfer open period which allows the fresh mixture to flow up into the cylinder. It is also nice to see the chamber doing it job to scavange the cylinder/ exhaust port pressure down to almost equal that of the crankcase just before the transfer open piont and then to see the exhaust port reverse pulse peak just before the exhaust port closes. If the cylinder pressure stayed high as you say the moment the transfers opened their pressure would spike high which they don't.
The graph also clearly shows that the peak pressure in the crankcase of around 1.6 bar which is .6 bar over atmospheric is greater than the amount it drops below of about .7 which is only .3 below.
A trip down to Spec Savers must be on the cards ;) The blue trace clearly shows a spike after transfer port open, crankcase pressure peaks AFTER the transfers open how can the crankcase pressurise with the transfers open???? Look at the black trace just as the transfer opens the resonance in the inlet tract moves into the crankcase and again just as the case pressure drops from the ascending piston. red trace shows a corresponding rise in pressure.
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Not even worth the time to argue, your so far from understanding what is going on inside a 2 stroke. You can't even see what is happening on the graph
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I guess these graphs will give a broad idea on what happens in a two-stroke engine, things would change quite radically from engine to engine.
Lozza do you know any details of the engine (model, mods, cc etc) that the graphs are from?
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Lozza, you wrote the "transfers are purple, green & blue", would the green & blue be each side transfer and the purple a rear boost/C port?
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As explained before the highest pressure in the crankcase is lower than residual cylinder pressure.
Stated repeatedly, but neither 'explained' or 'proven'.
Please explain how the pressure in the cylinder (after blow-down) is higher than the pressure at the exhaust port.
Cheers, Daryl.
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A trip down to Spec Savers must be on the cards ;) The blue trace clearly shows a spike after transfer port open,
Let's try to keep our discussions to Real Facts,
Colour Blindness cannot be corrected with glasses from Spec Savers.
(https://s3.amazonaws.com/WEBPOSTS/OZVMX/Green+spike.JPG)
As we have been given no details on whether these are Real measurements, what the sample rate is, or if this is just a simulation, a small bump at this point is hardly proof of the proposition that: "Residual cylinder pressure ALWAYS exceeds crankcase pressure".
l
The graph suggests that the 'effective' TPO point is actually several degrees after the point indicated on the X axis.
Cheers, Daryl.
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... crankcase pressure peaks AFTER the transfers open how can the crankcase pressurise with the transfers open????
This is probably not of any value to Lozza, but others might be interested in this stuff.
When the transfer ports open, as the piston descends, the initial opening is Very Small.
The transfers are still a restriction at this point, they are not fully open until BDC.
The fresh charge in the transfer ports is stationary. It has Mass that needs to be accelerated.
It takes time to get it moving across the cylinder toward the exhaust port.
Meanwhile....the piston is still descending, reducing the volume in the crankcase.
Volume decrease = pressure increase.
Other stuff is happening, the Intake port has closed.
Air & fuel previously drawn into the Crankcase will warm up, from say 20C. to maybe 100C or even more.
This is through contact with cases, crank, rod, lower part of cylinder and the piston, especially the underside of the crown.
As the pressure increases the heat transfer increases even further ( like pushing the steak onto the BBQ plate).
Increase of Temperature X5 = effective pressure increase X5.
That would be significant enough if we were inducting just air, but unless we are running on LPG,
we have a significant proportion of raw fuel particles in the charge. The change in volume of petrol,
as it vapourises in the hot case, is X160. More opportunity for pressure increases in the crank-case.
As the transfers open wider (increasing the volumetric flow), temperature increase stabilises and the rate of volume change falls, as the piston approaches BDC, then the pressures in the transfers and the crankcase equalise. (visible in the graph).
There are a lot of thing going on in a 2-Stroke engine Temperatures, Pressures, Mass flows, and we haven't even looked at fixed amplitude sound waves...yet.
Cheers, Daryl.
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Lozza, you wrote the "transfers are purple, green & blue", would the green & blue be each side transfer and the purple a rear boost/C port?
Excellent question, John.
The purple (pink) one is responding directly to the pressure drop at the exhaust port.
The green & blue appear to be responding to the pressure in the crank-case.
Cheers, Daryl.
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Lozza, you wrote the "transfers are purple, green & blue", would the green & blue be each side transfer and the purple a rear boost/C port?
Excellent question, John.
The purple (pink) one is responding directly to the pressure drop at the exhaust port.
The green & blue appear to be responding to the pressure in the crank-case.
Cheers, Daryl.
Daryl you have a lot more patience than me.
I would think that the green trace would be the rear boost port. It normaly opens a little earlier than the others which could account for the slight pressure spick. As the piston is going down at that piont and the cylinder pressure is dropping fast and it would be quite conseivable that the first opened port could see a small back flow increasing it's pressure for a split second.
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Look at the black trace just as the transfer opens the resonance in the inlet tract moves into the crankcase and again just as the case pressure drops from the ascending piston. red trace shows a corresponding rise in pressure.
I see pressure in the inlet tract rising as soon as IPC . (Piston ported engine, port closed)
Pressure rise from stopping the flow.
Pressure wave bouncing back and forward along the inlet.
No effect on case pressure at second pressure peak. (Piston ported engine, port closed)
Pressure rise in crank-case at 3rd peak corresponding to IPO. (Piston ported engine, port open)
Perhaps some-one can explain what is driving the intake pressure so Negative from 60degrees ATDC.??
The gradual pressure rise to TDC and beyond doesn't look like it's got the energy to do it.
Cheers, Daryl
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Hi Sleepy,
I love this stuff.
Do you have any ideas on what's driving the dramatic negative pressure in the intake AFTER TDC?
It'd suck the air cleaner through the carby.
Perhaps the graph is just a simulation, with some strange parameters.
Cheers DJ
PS. Get some Sleep, I only have to do work around home tomorrow.
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... crankcase pressure peaks AFTER the transfers open how can the crankcase pressurise with the transfers open????
This is probably not of any value to Lozza, but others might be interested in this stuff.
When the transfer ports open, as the piston descends, the initial opening is Very Small.
The transfers are still a restriction at this point, they are not fully open until BDC.
The fresh charge in the transfer ports is stationary. It has Mass that needs to be accelerated.
It takes time to get it moving across the cylinder toward the exhaust port.
Meanwhile....the piston is still descending, reducing the volume in the crankcase.
Volume decrease = pressure increase.
Other stuff is happening, the Intake port has closed.
Air & fuel previously drawn into the Crankcase will warm up, from say 20C. to maybe 100C or even more.
This is through contact with cases, crank, rod, lower part of cylinder and the piston, especially the underside of the crown.
As the pressure increases the heat transfer increases even further ( like pushing the steak onto the BBQ plate).
Increase of Temperature X5 = effective pressure increase X5.
That would be significant enough if we were inducting just air, but unless we are running on LPG,
we have a significant proportion of raw fuel particles in the charge. The change in volume of petrol,
as it vapourises in the hot case, is X160. More opportunity for pressure increases in the crank-case.
As the transfers open wider (increasing the volumetric flow), temperature increase stabilises and the rate of volume change falls, as the piston approaches BDC, then the pressures in the transfers and the crankcase equalise. (visible in the graph).
There are a lot of thing going on in a 2-Stroke engine Temperatures, Pressures, Mass flows, and we haven't even looked at fixed amplitude sound waves...yet.
Cheers, Daryl.
Thank you Daryl Jones , for a clear & rational explanation, interesting reading.
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Yep very interesting stuff, bizarre stuff happens in a two-stroke :-) I am wondering about another point; the faster air moves, the more the drop in pressure, could that be changing pressure readings?
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... crankcase pressure peaks AFTER the transfers open how can the crankcase pressurise with the transfers open????
This is probably not of any value to Lozza, but others might be interested in this stuff.
When the transfer ports open, as the piston descends, the initial opening is Very Small.
The transfers are still a restriction at this point, they are not fully open until BDC.
The fresh charge in the transfer ports is stationary. It has Mass that needs to be accelerated.
It takes time to get it moving across the cylinder toward the exhaust port.
Meanwhile....the piston is still descending, reducing the volume in the crankcase.
Volume decrease = pressure increase.
Other stuff is happening, the Intake port has closed.
Air & fuel previously drawn into the Crankcase will warm up, from say 20C. to maybe 100C or even more.
This is through contact with cases, crank, rod, lower part of cylinder and the piston, especially the underside of the crown.
As the pressure increases the heat transfer increases even further ( like pushing the steak onto the BBQ plate).
Increase of Temperature X5 = effective pressure increase X5.
That would be significant enough if we were inducting just air, but unless we are running on LPG,
we have a significant proportion of raw fuel particles in the charge. The change in volume of petrol,
as it vapourises in the hot case, is X160. More opportunity for pressure increases in the crank-case.
As the transfers open wider (increasing the volumetric flow), temperature increase stabilises and the rate of volume change falls, as the piston approaches BDC, then the pressures in the transfers and the crankcase equalise. (visible in the graph).
There are a lot of thing going on in a 2-Stroke engine Temperatures, Pressures, Mass flows, and we haven't even looked at fixed amplitude sound waves...yet.
Cheers, Daryl.
Thank you Daryl Jones , for a clear & rational explanation, interesting reading.
And that would pretty much sum it up for me too. Thanks
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Not even worth the time to argue, your so far from understanding what is going on inside a 2 stroke. You can't even see what is happening on the graph
Well that's your opinion backed by....................old wives tales? Even Bells book talks about this.(http://i155.photobucket.com/albums/s297/Lozza85_2007/IMG_0002.jpg) (http://s155.photobucket.com/user/Lozza85_2007/media/IMG_0002.jpg.html) As explained before the highest pressure in the crankcase is lower than residual cylinder pressure.
Stated repeatedly, but neither 'explained' or 'proven'.
Please explain how the pressure in the cylinder (after blow-down) is higher than the pressure at the exhaust port.
Cheers, Daryl.
They are the same never said that port pressure was higher. I said higher than crankcase pressureA trip down to Spec Savers must be on the cards ;) The blue trace clearly shows a spike after transfer port open,
Let's try to keep our discussions to Real Facts,
Colour Blindness cannot be corrected with glasses from Spec Savers.
(https://s3.amazonaws.com/WEBPOSTS/OZVMX/Green+spike.JPG)
As we have been given no details on whether these are Real measurements, what the sample rate is, or if this is just a simulation, a small bump at this point is hardly proof of the proposition that: "Residual cylinder pressure ALWAYS exceeds crankcase pressure".
l
The graph suggests that the 'effective' TPO point is actually several degrees after the point indicated on the X axis.
Cheers, Daryl.
The blue and green overlay each other just shows the green ::). The traces come from EngMod2T, that is taken from Blairs calculations validated by a South African Professor (who worked in Yamaha's GP program from the mid 80's to mid 90's) at the uni with about 30 Kistler pressure transducers connected to the YZ250 engine. Same 4T software is used by EVERY V8 Supercar team. The results are very much validated by simulated outputs perfectly matching "at the wheel" figures. Feel free to contact http://www.vannik.co.za/index.htm
I see so trying to convince me that with the transfers OPEN no matter how small that opening the decending piston will cause case pressure rise and "force" charge through the transfers?
Fuel is roughly 80deg C in the cases, measured pressure has never increased by 5 times regardless of expansion and evaporation.
Look at the black trace just as the transfer opens the resonance in the inlet tract moves into the crankcase and again just as the case pressure drops from the ascending piston. red trace shows a corresponding rise in pressure.
I see pressure in the inlet tract rising as soon as IPC . (Piston ported engine, port closed)
Pressure rise from stopping the flow.
Pressure wave bouncing back and forward along the inlet.
No effect on case pressure at second pressure peak. (Piston ported engine, port closed)
Pressure rise in crank-case at 3rd peak corresponding to IPO. (Piston ported engine, port open)
Perhaps some-one can explain what is driving the intake pressure so Negative from 60degrees ATDC.??
The gradual pressure rise to TDC and beyond doesn't look like it's got the energy to do it.
Cheers, Daryl
Assumptions are mistakes awaiting discovery :) Would be the first piston port engine ever with asymetric inlet timing :D The black trace is the pressure trace at the end of the inlet tract. The inlet resonance peaks and troughs shift with rpm that's is just 1 rpm point 12,000 in a Rotax 128 engine. The resonance is not as important on the disc valve as on a reed valve, which (with a reed engine)you can with varying degrees of success shift the peaks to where you want them.
Yes effective opening of the transfers is a tiny bit later than when the window opens, as the port that opens first flows last.Lozza, you wrote the "transfers are purple, green & blue", would the green & blue be each side transfer and the purple a rear boost/C port?
Excellent question, John.
The purple (pink) one is responding directly to the pressure drop at the exhaust port.
The green & blue appear to be responding to the pressure in the crank-case.
Cheers, Daryl.
Rotax 128 John @12,000 rpm.
.........and that pressure comes from the cylinder not the decending piston. Here's an old MOTA shot showing velocity in a transfer port just after the transfer is open. Yes getting that mass in the transfers moving is the secret to a twostroke........... well one of them anyway.
(http://i155.photobucket.com/albums/s297/Lozza85_2007/IMG-4.jpg) (http://s155.photobucket.com/user/Lozza85_2007/media/IMG-4.jpg.html)
(http://i155.photobucket.com/albums/s297/Lozza85_2007/IMG_0001-1.jpg) (http://s155.photobucket.com/user/Lozza85_2007/media/IMG_0001-1.jpg.html)
(http://i155.photobucket.com/albums/s297/Lozza85_2007/IMG_0002-1.jpg) (http://s155.photobucket.com/user/Lozza85_2007/media/IMG_0002-1.jpg.html)
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Lozza's technical's make sense , perhaps reminding me of my wasted life opportunities.
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Hi Sleepy,
I love this stuff.
Do you have any ideas on what's driving the dramatic negative pressure in the intake AFTER TDC?
It'd suck the air cleaner through the carby.
Perhaps the graph is just a simulation, with some strange parameters.
Cheers DJ
PS. Get some Sleep, I only have to do work around home tomorrow.
I did get some sleep thanks.
The inlet port trace I think is showing the effect of harmonics. I haven't seen much on 2 stroke inlet harmonics but on a 4 stroke the lenght of the intake track can be designed to make use of these pressure pulses. getting the positive pulse to arrive just at the correct time during the inlet phase can push the volumetric efficiency well over 130%. I've got a book here somewhere call "Scientific design of intake and exhaust systems" you can borrow it if you like reading that sort of thing.
A lot more info on the motor in question would be needed before to much more could be thought about, things like intake type, size and RPM etc.
John. You asked about air flow causing pressure drop. Absolutely correct. The more you try and stuff through a hole or transfer port the greater the pressure difference will be at either end. The differing flow in the tranfers has a lot to do with the different shape coloured traces on the graph.
Got to get back to work now the cylinder rebore count is back up to 15 not to mention all the other work.
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Hi Lozza,
The quote from Bell proves the point that crankcase primary compression is a factor.
The factories moved away from it because it required very short transfer duration periods and very high reving engines with narrow powerbands.
You asked the question as to how the case pressure can increase with the transfers open, it can, the graph shows it can.
I think what you have been trying to suggest is that residual cylinder pressure can also pressurise the crankcase through the transfers.
I have no doubt that in some situations it probably can (especially off the pipe).
In this case, opening exhaust ports later and boost ports earlier (how about a tiny one before EPO) and significantly bigger case volumes should dramatically increase charge transfer, though I imagine the balance point is pretty delicate.
My problem with what you have presented so far is that you have still not explained how the residual pressure in the cylinder can be larger than the falling pressure recorded at the exhaust port which, even on your supplied chart, shows to be pretty well equivalent to crankcase pressure at effective TPO. Also the tiny pressure bump shown in the transfers is not even as high as the case pressure rise, the volume of the crankcase is far greater than the transfers, this is not going to produce the indicated case pressure rise. If it spiked significantly higher than max case pressure I would be more convinced.
I'll give you 1 point for the rotary valve Rotax. It was late & I didn't check the timing symmetry but the lack of response to the second harmonic peak eliminated a reed engine.
Cheers, Daryl.
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You asked the question as to how the case pressure can increase with the transfers open, it can, the graph shows it can.
Can I answer this for you.
That little thing called the piston is still on it's way down the bore and even though the transfers open what is in the crankcase can't get up into the cylinder fast enough to stop the pressure increase. I think a school kid doing science could even work that out.
Keep up the good work Daryl.
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Assumptions are mistakes awaiting discovery :) Would be the first piston port engine ever with asymetric inlet timing :D
They certainly are aren't they:
Probably not the 'first ever'.
A piston port engine fitted with a reed valve and NO compensating holes in the skirt can have asymmetric intake timing.
(Reed can close on reversion well before bottom of piston skirt closes the port).
Perhaps the worst of both worlds, but it would stop old Bully's loading up.
Cheers, Daryl.
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I think what you have been trying to suggest is that residual cylinder pressure can also pressurise the crankcase through the transfers.
I have no doubt that in some situations it probably can (especially off the pipe).
In this case, opening exhaust ports later and boost ports earlier (how about a tiny one before EPO) and significantly bigger case volumes should dramatically increase charge transfer, though I imagine the balance point is pretty delicate.
Now this has got me thinking, I might have to try this on the Victa.
A tiny hole (say 3mm) drilled above the exhaust port has been used to reduce compression for easier kick-starting of big bore, high compression 2-strokes. No 'seat of the pants' detection of any power loss when running.
Lets drill a hole on the inlet side, after the point of maximum con-rod/crank mechanical leverage and before EPO and pipe it into a transfer duct or the crank-case, a small volume of very hot, high pressure exhaust gas to directly pressurise the crankcase. A side benefit should be a reduction of NOx (from exhaust gas recirculation). That applies to the exhaust stroke, on compression it's just recycling a portion of the charge).
We should have plenty of crankcase pressure available at TPO, (Way in excess of any residual cylinder pressure), we can even delay the TPO to reduce charge losses, (though the engine might get a bit pipey).
We'll work on the chamber design, to get as much charge back into the cylinder as possible and get the 'effective' compression ratio as high as possible.
Now all we need.... is a solution to stop the 'Pinging'. :)
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I throught pinging was from to much compression I have found this on four strokes I was told to use higher octane fuels Av gas etc , but not always the case .
A few years back I was talking to a race engine engineer he said you can buy a 11 to one ratio piston off the shelf , but it won't be the case when fitted because all barrels are all different from the factory he has to machine some off the bottom of some barrels and some times put two base gaskets on or make a 2 / 4 mm alloy plate to get the right compression rate .
And then there is the head how many times has been skimed / decked over the last 40 years to these old engines.
Anyone can slap some new rings and piston in and thing it's 11 to 0ne or what ever it is but all this has to be measured my a pro .
This may be a bit of topic sorry .
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Is it pinging only under load like labouring it at times john .
I found this happening to me on one of my bikes years back .
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Is it pinging only under load like labouring it at times john .
I found this happening to me on one of my bikes years back .
It turned out to be sucking air through the cranks seals, at high rpm/small throttle openings the crank vacuum was drawing the main seals, all fixed now :-) Thanks.
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This may be a bit of topic sorry .
No...actually is the topic.
Pinging sound is due to cylinder pressure that is Too High Before TDC.
Many causes, including compression too high, ignition too advanced, lean mixture, low octane fuel.......Worse still, hot spots causing pre-ignition and/or detonation.
Experts can elaborate.
Cheers, Daryl
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Hi Lozza,
The quote from Bell proves the point that crankcase primary compression is a factor.
The factories moved away from it because it required very short transfer duration periods and very high reving engines with narrow powerbands.
You asked the question as to how the case pressure can increase with the transfers open, it can, the graph shows it can.
I think what you have been trying to suggest is that residual cylinder pressure can also pressurise the crankcase through the transfers.
I have no doubt that in some situations it probably can (especially off the pipe).
In this case, opening exhaust ports later and boost ports earlier (how about a tiny one before EPO) and significantly bigger case volumes should dramatically increase charge transfer, though I imagine the balance point is pretty delicate.
My problem with what you have presented so far is that you have still not explained how the residual pressure in the cylinder can be larger than the falling pressure recorded at the exhaust port which, even on your supplied chart, shows to be pretty well equivalent to crankcase pressure at effective TPO. Also the tiny pressure bump shown in the transfers is not even as high as the case pressure rise, the volume of the crankcase is far greater than the transfers, this is not going to produce the indicated case pressure rise. If it spiked significantly higher than max case pressure I would be more convinced.
I'll give you 1 point for the rotary valve Rotax. It was late & I didn't check the timing symmetry but the lack of response to the second harmonic peak eliminated a reed engine.
Cheers, Daryl.
That's pretty easy the small hump at TPO is because that is a VERY well developed engine engines that are not so well developed show much higher spikes and more pressure at the exhaust port. As I have said all along that small increase in pressure is from the residual pressure from the cylinder not from the decending piston.
All case volumes fall into a very narrow range 1.25 at the low end and 1.32 at the high end (cyl to case including transfers and under the piston) , its a factor but not over riding.
The graph shows the cases getting pressure from the inlet AND from the cylinder, even the clunky old MOTA program shows reverse flow in the transfers at the point of opening.
Now this has got me thinking, I might have to try this on the Victa.
A tiny hole (say 3mm) drilled above the exhaust port has been used to reduce compression for easier kick-starting of big bore, high compression 2-strokes. No 'seat of the pants' detection of any power loss when running.
Lets drill a hole on the inlet side, after the point of maximum con-rod/crank mechanical leverage and before EPO and pipe it into a transfer duct or the crank-case, a small volume of very hot, high pressure exhaust gas to directly pressurise the crankcase. A side benefit should be a reduction of NOx (from exhaust gas recirculation). That applies to the exhaust stroke, on compression it's just recycling a portion of the charge).
We should have plenty of crankcase pressure available at TPO, (Way in excess of any residual cylinder pressure), we can even delay the TPO to reduce charge losses, (though the engine might get a bit pipey).
We'll work on the chamber design, to get as much charge back into the cylinder as possible and get the 'effective' compression ratio as high as possible.
Now all we need.... is a solution to stop the 'Pinging'. :)
I suspect what would happen would be the same as the starting hole and perforated rear cone pipes, that those holes would effectively disappear with rpm.
Here are traces from a optimised engine within the confines of the cylinder.
off pipe
(http://i155.photobucket.com/albums/s297/Lozza85_2007/pressure2_zpse0cbbdd3.png) (http://s155.photobucket.com/user/Lozza85_2007/media/pressure2_zpse0cbbdd3.png.html)
on pipe
(http://i155.photobucket.com/albums/s297/Lozza85_2007/pressure1_zps49128b7f.png) (http://s155.photobucket.com/user/Lozza85_2007/media/pressure1_zps49128b7f.png.html)
Pressure in the cases gets higher later when on the pipe.
http://www.thumpertalk.com/topic/881202-engmod2t-software-who-else-uses-it/
Pre-ignition and detonation are 2 different things, pre-igniton(as it suggests) happens before the spark event, detonation happens after the spark event from end gases self igniting. Pre-ignition is much more destructive and less under stood (it will wreck an engine in seconds) where detonation is measurable and controllable. In some cases a certain amount of detonation is desirable (about 3-5 dets per km) to make the best power
You asked the question as to how the case pressure can increase with the transfers open, it can, the graph shows it can.
Can I answer this for you.
That little thing called the piston is still on it's way down the bore and even though the transfers open what is in the crankcase can't get up into the cylinder fast enough to stop the pressure increase. I think a school kid doing science could even work that out.
Keep up the good work Daryl.
Intake charge does NOT go from the cases to the cylinder via the transfers in 1 transfer open period. Intake charge only moves to the transfer ducts replacing the charge already sitting in the transfer ducts. Then goes from the ducts into the cylinder. This was proven by using radioactive traces in fuel with an engine completing 20-25 cycles to move from carb to exhaust not 3-5. Jan Thiel knew that in the 60's but what would he know ::)
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A piston port engine fitted with a reed valve and NO compensating holes in the skirt can have asymmetric intake timing.
(Reed can close on reversion well before bottom of piston skirt closes the port).
Perhaps the worst of both worlds, but it would stop old Bully's loading up.
Cheers, Daryl.
Then it's a reed valve no longer piston port ;D, pistons don't need ports on reed engines case reed engines seem to run just fine. Yamaha in 1980..something the last year of their piston port OW-31?? YZR500 stopped the fueling up with what must have been a LOT of time spent on the size, shape and length of the inlet tract, that and King Kenny won a title on a 'old tech' bike.
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Been thinking about this thread for a few weeks before weighing in my three cents worth, have been a bike mechanic since i started my trade way back in the seventies so have a small idea of things and i always tend to look for the easy things first. big bore twostrokes had few tuning problems a few used to ping a bit but mostly when something was wrong like a bad jetting job or various other problems generaly man made. i like the idea of the fuel foaming or airateing in the fulel bowl that john O came up with and think that he is right on the money someone else may have said this earlier but today i dont have time to read all the posts in this thread but keeping with the KISS principal i came up with this the other day just adjusting an air mixture screw, with my trusty 42 year old screw driver that i do the air screws and pilot jets i put it on the head of the air screw and couldnt keep it in the slot it kept vibrating out of the slot, not something that would normally happen but i put it down to me getting old and blind. a thought crossed my mind that i cant remember carbies haveing that much vibration and with a bit more thought i think that the fuel is foamed by vibration due to the carby and airbox rubbers being as hard as resin i feel that if the rubbers were soft and supple like on a new bike that they would isolate a fair bit of vibration from the carb. may be naybe not but i think this would be a good starting point i always tell my apprentices look for the simple things first as that is generally the cause of the problem.
Just a thought
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Been thinking about this thread for a few weeks before weighing in my three cents worth, have been a bike mechanic since i started my trade way back in the seventies so have a small idea of things and i always tend to look for the easy things first. big bore twostrokes had few tuning problems a few used to ping a bit but mostly when something was wrong like a bad jetting job or various other problems generaly man made. i like the idea of the fuel foaming or airateing in the fulel bowl that john O came up with and think that he is right on the money someone else may have said this earlier but today i dont have time to read all the posts in this thread but keeping with the KISS principal i came up with this the other day just adjusting an air mixture screw, with my trusty 42 year old screw driver that i do the air screws and pilot jets i put it on the head of the air screw and couldnt keep it in the slot it kept vibrating out of the slot, not something that would normally happen but i put it down to me getting old and blind. a thought crossed my mind that i cant remember carbies haveing that much vibration and with a bit more thought i think that the fuel is foamed by vibration due to the carby and airbox rubbers being as hard as resin i feel that if the rubbers were soft and supple like on a new bike that they would isolate a fair bit of vibration from the carb. may be naybe not but i think this would be a good starting point i always tell my apprentices look for the simple things first as that is generally the cause of the problem
could the carby be braced off the frame to stop vibration ?
if you don't use the org air filter box and use a uni filter this will make more carb vibration and movement ,stress on the rubber manifold over bumps and jumps etc , and could cause the manifold to crack and suck air .
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Why on earth would anyone consider using a pod uni filter? There is a good reason why manufactures designed and implemented air boxes. First, to allow the filter to be carried by something substantial. Second, to eliminate as much dust as possible. And third, so the bike can run and be ridden in the rain and mud. Solid mounting a carburetor is only going to make fuel frothing worse. All those lumps, jumps, stutters and everything else on the track is going to transfer to the carburetor if it is solid mounted to the frame.
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Hi Lozza,
More to consider:
Intake charge does NOT go from the cases to the cylinder via the transfers in 1 transfer open period. Intake charge only moves to the transfer ducts replacing the charge already sitting in the transfer ducts. Then goes from the ducts into the cylinder. This was proven by using radioactive traces in fuel with an engine completing 20-25 cycles to move from carb to exhaust not 3-5. Jan Thiel knew that in the 60's but what would he know ::)
All case volumes fall into a very narrow range 1.25 at the low end and 1.32 at the high end (cyl to case including transfers and under the piston) , its a factor but not over riding.
I do have a problem with this one.
If I need 250cc of charge and the case holds 250 X 1.32 = 330cc. I only have 80cc of charge left in the case which must be topped up by fresh intake. Turn over of charge cannot possibly take 20-25 cycles.
Are you perhaps suggesting that 'some' charge is trapped circulating with the crank in the case and fresh charge bypasses this by flowing directly from the intake through the transfers?
That's pretty easy the small hump at TPO is because that is a VERY well developed engine engines that are not so well developed show much higher spikes and more pressure at the exhaust port. As I have said all along that small increase in pressure is from the residual pressure from the cylinder not from the decending piston.
http://www.thumpertalk.com/topic/881202-engmod2t-software-who-else-uses-it/
That is an interesting reference as it supports my understanding of what is happening here:
dmcca:This one is the pressure trace at 8500rpm, just beyond peak power... you can see that as the transfers open (TPO) pressure in the cylinder is greater than in the transfers so they experience a reverse pressure spike, hence the reason power begins to drop at this point.
rayivers: Your results look great! I'm certainly no expert, but it looks to me like the boost ports may be stealing flow from the main transfers (maybe widen the mains?), and also that (a) faster blowdown (raised and/or wider EP, different pipe w/more aggressive initial flare from the manifold, etc.) might reduce that reverse pressure in the transfers and possibly result in more power.
My thoughts too, Over-speed for the Ex dur., Restricted Blow Down and/or Transfer timing a bit long.
I did look at the timing & value of cyl. pressure & reversion pulses in the transfers and at best you get a positive pulse (restricting flow for say, 7deg), a a smaller negative pulse (charging the transfer from the cyl. or case, which ever pressure is greater) finally a very much smaller positive pulse, but the transfer is near fully open. Key initial transfer time & flow is restricted with the initial positive pressure pulse.
Pre-ignition and detonation are 2 different things, pre-igniton (as it suggests) happens before the spark event, detonation happens after the spark event from end gases self igniting. Pre-ignition is much more destructive and less under stood (it will wreck an engine in seconds) where detonation is measurable and controllable. In some cases a certain amount of detonation is desirable (about 3-5 dets per km) to make the best power
In between, there are plenty of other things to cause 'pinging' noise.
Ignition may be occurring exactly where the designer intended but the engine might be producing too much cylinder pressure, before TDC.
Plenty of different factors (man-made & natural) can speed up combustion to the point where it is having a negative effect.
I once had to diagnose a bike that was 'pinging' at certain revs. Turned out to be a chamber retaining spring that occasionally vibrated enough to touch the front downtube!!
cheers, Daryl
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Here is another opinion on the transfer port pressure bump effect:
It's by Trevor Amos, who is the 2T "subject matter expert" at the UK Bantam Racing Forum, and a friend & colleague of the much quoted Jan Thiel.
"The pressure wave picture clearly shows the trans. port reverse flow with a spike at the A port, the first to open, and the higher the port goes the more the cylinder pressure impacts upon the flow. If the port is open when residual cylinder pressure exceeds scavenging pressure, flow cannot take place and will not do so until the ratio of trans. to cylinder pressure reduces to at least ONE. The real downside to this is that the trans ports offer another potential escape route, in addition to the ex. port, and spent gas enters the transfer ducts, this will reduce transfer time/area before fresh, meaningful, gas flow can take place . Subsequent combustion will have to commence with a contaminated charge and power will be compromised, and will do so at higher initial temperature, extreme cases could promote deto which will then be combatted with a rich mixture and/or lower comp. ratio, in both cases reducing power.
As far as gas flow is concerned, the number of degrees the port is open is secondary to the actual, real time, the port is effectively open, reverse flow from excessive trans. timing reduces this time. As engine revs reach a peak above the blowdown available, the transfer time/area suffers further and power drops steeply . This may seems counter intuitive but it happens in reality and the sim image reveals the phenomenon quite clearly".
http://bsabantamracing.editboard.com/t1076p90-timetimingtime-area
Well worth a look at the site, there's heaps of interesting tech stuff here, they are still modifying & developing BSA Bantams for racing, including case & side reed engines.
http://bsabantamracing.editboard.com/f1-bantam-racing-forum
Cheers, Daryl
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Looking at 1 thing in isolation is not quite right, because at any 1 time in a 2T engine there are 2 things happening. Flow needs a a pressure differential as soon as that difference is equalised flow stops(yes that flow wil have inertia and continue) but effectively it is zero. From the graphs you can see case/tx pressure is equalised not long after BDC only the pipe is pulling on the cylinder. Soon after BDC case pressure starts to drop and the disc starts to open we haven't even got the full duration of the tx from a theoretical maximum. Even at a modest 6000 rpm (100Hz) and 120 deg tx duration 0.0033sec is the maximum time the transfers are open - reverse flow, time to start flow + the inertia there isn't much time to jam 250cc from the cases through ducts. Even if 250cc or more goes through the cases while that is happening the cases are being filled. Two strokes don't have clear defined phases unlike a 4T (suck, squeeze, bang ,wheeze) everything is intertwined and connected. A single unit or block of intake charge does not travel through carb,cases, transfers combustion then out the pipe. It takes 3-5 cycles to go from cases to cylinder.
Before and after peak power the pipe influence isn't as strong, look at a 2 stroke road bike black soot in the transfers everywhere, from being ridden at not an optimum rpm.
Yes a spring ringing can even fool a det counter, spent a full day chasing that one :D, combustion can never happen fast enough, the faster the burn the more power you will make from a) not having to run as much advance b) peak cylinder pressure at or about 15deg ATDC.
Yes deto can occur in the situation Trevor says(I've seen his name on the ESE tuner thread) less likely to happen when the throttle is open. Which bring a neat circle back to Johns origninal problem, which never happened with the throttle open, reeds didn't open cases didn't fill -faster and greater pressure drop was enough to pull on the seal.
Never heard of that one before
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Never heard of that one before
If you have never heard of a 2 srtoke bike ping on closed throttle before because of a leaking seal I think you should stick to your computer sims and stop waisting our time.
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lets throw manners out the window and forget what your mum said :D
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lets throw manners out the window and forget what your mum said :D
Sorry if I seem rude but the last 4 pages of this thread has just been about Lossa disagreeing with everyone else.
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I feel like we are starting to approach "harmonic resonance".
The 'point' gap, at least, seems to be closing.
My own opinion (and the others quoted) is that a transfer pressure bump is detrimental to the 2T process.
A small bump certainly indicates that TPO is at the very maximum position (well, just beyond it) for the exhaust timing (blow-down).
I am prepared to consider, that in some 'very specific' engine configurations, a small burst of hot exhaust gas into the transfer port might compress the colder transfer charge and then cause it to spring back out of the port.
(Still needs the bleed-down/cylinder pressure to have dropped below the transfer port pressure)
This is more a "compressible fluid" effect, rather than a "sonic pressure wave" effect.
It might allow the power range to extend a little bit beyond the geometric limit.
It will be very engine specific and a delicate balance, getting it wrong or going too far will have all the negatives spelled out by Trevor Amos.
On the other hand:
There is not enough energy in this 'bump' to produce any significant pressure rise in the crank-case.
Cheers, Daryl.
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Another wonderful example of how friendly, honest, open discussion can resolve conflicts & issues.
Look at all the Yes agreement points.
Flow needs a a pressure differential as soon as that difference is equalised flow stops(yes that flow wil have inertia and continue) but effectively it is zero.
Yes
From the graphs you can see case/tx pressure is equalised not long after BDC only the pipe is pulling on the cylinder.
Yes
Soon after BDC case pressure starts to drop and the disc starts to open
Yes
we haven't even got the full duration of the tx from a theoretical maximum.
Yes, We need to modify this pipe to delay the pressure rise and keep it below transfer pressure until TPC.
Then it would be truly "Optimised"
Even at a modest 6000 rpm (100Hz) and 120 deg tx duration 0.0033sec is the maximum time the transfers are open - reverse flow, time to start flow + the inertia there isn't much time to jam 250cc from the cases through ducts.
Yes, If we remove the reverse flow with sufficient/efficient blow-down we would save a lot of the effective transfer flow time.
Even if 250cc or more goes through the cases while that is happening the cases are being filled.
Yes? 250cc out - replaced by 250cc in.
Two strokes don't have clear defined phases unlike a 4T (suck, squeeze, bang ,wheeze) everything is intertwined and connected.
Well, Four Stroke racing engines are commonly recognised to have 5 cycles, with the processes during overlap behaving much like a 2T.
....and then there is this one:
(https://s3.amazonaws.com/WEBPOSTS/OZVMX/2-4+sight.JPG)
A single unit or block of intake charge does not travel through carb, cases, transfers, combustion then out the pipe.
It takes 3-5 cycles to go from cases to cylinder.
What? Not 20-25 1960's Radioactive cycles? And why do modern 2T race engines lower the intake port, cant the cylinders forward, and even relocate gearboxes to provide the most direct flow from intake to transfer ports to the cylinder? Not much point doing all this, if the charge is only sitting/bouncing around for a few cycles, heating up and waiting for it's turn to transfer and burn.
Cheers, Daryl.
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Never heard of that one before
If you have never heard of a 2 srtoke bike ping on closed throttle before because of a leaking seal I think you should stick to your computer sims and stop waisting our time.
It wasn't a 'leaky' seal at all was it. I think if you don't like the thread because you don't want to or can't understand stop reading. Simple.
I feel like we are starting to approach "harmonic resonance".
The 'point' gap, at least, seems to be closing.
My own opinion (and the others quoted) is that a transfer pressure bump is detrimental to the 2T process.
A small bump certainly indicates that TPO is at the very maximum position (well, just beyond it) for the exhaust timing (blow-down).
I am prepared to consider, that in some 'very specific' engine configurations, a small burst of hot exhaust gas into the transfer port might compress the colder transfer charge and then cause it to spring back out of the port.
(Still needs the bleed-down/cylinder pressure to have dropped below the transfer port pressure)
This is more a "compressible fluid" effect, rather than a "sonic pressure wave" effect.
It might allow the power range to extend a little bit beyond the geometric limit.
It will be very engine specific and a delicate balance, getting it wrong or going too far will have all the negatives spelled out by Trevor Amos.
On the other hand:
There is not enough energy in this 'bump' to produce any significant pressure rise in the crank-case.
Cheers, Daryl.
On the contary every powerful competition two stroke engine uses that to it's advantage. Four strokes are nothing like a 2 stroke if it was 2T engines would not still be very reluctant to accept EFI . Again you can't modify pipe design for 1 very tiny part of the phase without throwing all the aspects of a pipe design that work away. A 2T engine can never have enough blow down angle area, the more your output goes up the more blown down is required (and vise versa). The amount the cases fill is dependent on how much the throttle is open case pressure is lower without the cases filled. The amount the cases fill has a direct bearing on the crankcase pressure. Then the better the pipe works the better the blow down and the better the plugging pulse which means higher cylinder pressures the more critical the blow down angle area.
Single cylinder engines have the cylinder canted over for vibration reasons mainly, there is a flat stone skipping effect off the spinning crank (loose hp if you reverse the direction of the crank against flow) perhaps why cylinder reeds are near universal angle to the crank. 'Stacked' gearboxes are so you can have a longer swingarm ( as was done on the Aprilia RSA 250).
The radioactive test was made in Aprilia Corse racing dept at Noale in Italy in 97 (I think) with a full time staff of 50 with 4 or 5 GP engineers working under the best two stroke tuner ever. Working 8-9 hours a day 5 days a week with near unlimited budgets with only 1 goal......... more hp. Nothing was left to chance nothing was assumed, everything was trialed and tested. That test showed that it took 20 cycles for the radioactive fuel to appear in the exhaust after introduction. Maybe all those blokes got it all wrong but judging by the results they got I very much doubt it ;)
What their conclusions were was what happened between the intake going out the transfers and the squish band starting to work was the most important bit, but then again what would Jan Thiel know ;)
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Again you can't modify pipe design for 1 very tiny part of the phase without throwing all the aspects of a pipe design that work away.
Hardly a Tiny part of the 2T process though!
The principle object of an expansion chamber exhaust is to generate a reversion pressure pulse to 'stuff' fresh charge that has been drawn into the exhaust, back into the cylinder.
Timing of that pulse can be modified by changing the length of the centre section of the pipe, without throwing the rest out of the design away.
Strength & Duration of the pulse is determined by the cone angle. Pressure in the pipe by stinger diam. Extraction by the header and diffuser will also be unchanged.
If this modification is successful, and we delay our positive pulse until TPC we could increase the quantity charge transferred and captured for combustion. We could make significantly more power.
Other things will undoubtedly change and of course we will need to adjust.
We might decide to reduce the static compression ratio or change the ignition timing. We might need to modify the exhaust port to allow for increased blow down.
But, of course, this is what tuning for improved performance is all about...
Cheers, Daryl.
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Single cylinder engines have the cylinder canted over for vibration reasons mainly, there is a flat stone skipping effect off the spinning crank (loose hp if you reverse the direction of the crank against flow) perhaps why cylinder reeds are near universal angle to the crank. 'Stacked' gearboxes are so you can have a longer swingarm ( as was done on the Aprilia RSA 250).
A 'vibrating' single is still going to vibrate if you cant it forwards, lay it down, or tip it upside down???
HRC was quoted previously as knowing more than a little bit about this:
Lovely 'balance' job in the layout of the RS cases. Rolling the ass of the gearbox forward didn't change the sprocket position much.
(http://i1095.photobucket.com/albums/i471/EdwardPickering/ccr2.jpg)
(http://i1095.photobucket.com/albums/i471/EdwardPickering/ccr2-1.jpg)
Cheers, Daryl
PS. thanks to Trevor Amos and the Bantam Forum for the pics. http://bsabantamracing.editboard.com/t925p30-crankcase-compression-debate?highlight=crankcase
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Again you can't modify pipe design for 1 very tiny part of the phase without throwing all the aspects of a pipe design that work away.
Hardly a Tiny part of the 2T process though!
The principle object of an expansion chamber exhaust is to generate a reversion pressure pulse to 'stuff' fresh charge that has been drawn into the exhaust, back into the cylinder.
Timing of that pulse can be modified by changing the length of the centre section of the pipe, without throwing the rest out of the design away.
Strength & Duration of the pulse is determined by the cone angle. Pressure in the pipe by stinger diam. Extraction by the header and diffuser will also be unchanged.
If this modification is successful, and we delay our positive pulse until TPC we could increase the quantity charge transferred and captured for combustion. We could make significantly more power.
Other things will undoubtedly change and of course we will need to adjust.
We might decide to reduce the static compression ratio or change the ignition timing. We might need to modify the exhaust port to allow for increased blow down.
But, of course, this is what tuning for improved performance is all about...
Cheers, Daryl.
Change the belly length and that throws out the % lengths of the rest of the pipe. Crude and not effective. Change the heat in the pipe and the gas slows down or speeds up. Change the stinger dia you will notice the increase in exhaust gas temp before the pressure, changing the baffle cone angle will change the power characteristics and delivery and more importanly over rev. All well and good hotting up a TS185 but another world on a proper competition engine.Single cylinder engines have the cylinder canted over for vibration reasons mainly, there is a flat stone skipping effect off the spinning crank (loose hp if you reverse the direction of the crank against flow) perhaps why cylinder reeds are near universal angle to the crank. 'Stacked' gearboxes are so you can have a longer swingarm ( as was done on the Aprilia RSA 250).
A 'vibrating' single is still going to vibrate if you cant it forwards, lay it down, or tip it upside down???
HRC was quoted previously as knowing more than a little bit about this:
Lovely 'balance' job in the layout of the RS cases. Rolling the ass of the gearbox forward didn't change the sprocket position much.
(http://i1095.photobucket.com/albums/i471/EdwardPickering/ccr2.jpg)
(http://i1095.photobucket.com/albums/i471/EdwardPickering/ccr2-1.jpg)
Cheers, Daryl
PS. thanks to Trevor Amos and the Bantam Forum for the pics. http://bsabantamracing.editboard.com/t925p30-crankcase-compression-debate?highlight=crankcase
Angle of the cylinder changes you perception of the damaging vertical vibrations, Manx's G50's have near verticle cylinders for their balance factors. ALL single cyl two strokes are inclined 20-25deg(I'd have to check that) from vertical.
HRC did by putting a balance shaft into the NX4(pretty sure they are the pictured cases). HRC did that to make a better handling chassis by changing to a rising rate rear suspension(from cantilever NF4) and a longer swing arm. Engine length was reduced making more space for a better pipe route. More power better handling= Lower lap times
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Well, weren't they Lucky then, all those major engine redesign mods for bike balance & handling.... and the reed port ends up where it is.
(Sounds like PR trying to hide the Engineering)
And there was me.... thinking you would point out the soot in the transfers!!
It's been Fun, but it's 'Back to the Shed' for me, and the TS185's, the Bully's, the Bantams, the mighty Victa, the B31/3 and the 5-cycle SL125.
You can have the World Beater's, I've just got beaters.
Cheers DJ :D
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Yes heaps of fun DJ. Stick to the TS185's the fancy ones are 20 times easier to loose HP than make more.
Looking again I think that could be a NF4 engine same as a CR125 basicaly, if there is soot in the transfers there point the finger at the rider. This is the NX cases shorter and taller. The NSR/RS 250's , various kart engines and the ADM engines all had the reed pisitioned verticaly theory is that was much easier for intake charge to negotiate it's ways past the spinning crank and the rod.
(http://cdn3.volusion.com/jpojk.qdcfg/v/vspfiles/photos/11200-NX4-780-2.jpg?1407062024)
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i never new Bert Flood worked for Aprilia! Bawhahahahaahh