OzVMX Forum
Clubroom => Tech Talk => Topic started by: John Orchard on January 28, 2015, 11:07:59 AM
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I did a bit of homework & thought regarding piston window's, thought I'd give it a try. Reports state an increase in 1 to 2 horsepower on a 500, plus a cooler running piston crown from escaping hot gases from under the crown.
Not worried about any more power but thought anything that may give cooler running is worth a try plus even a slight reduction in piston weight wouldn't be a bad thing?
To tell you the truth, I wouldn't notice any increase in power, I'm not getting what I've got to the ground now! It does seem a touch cooler in the combustion chamber; plug is a little darker. And my imagination is feeling there is less vibration (I'm a wanker lol).
Any thoughts?
(http://i1093.photobucket.com/albums/i423/JohnnyRacer89/CR500%20piston%20window_zpsofaiu3i7_1.jpg) (http://s1093.photobucket.com/user/JohnnyRacer89/media/CR500%20piston%20window_zpsofaiu3i7_1.jpg.html)
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Interesting stuff John. How did you cut the window? Would two windows work better?
Hopefully reliability isn't reduced. :D
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I drilled three 9mm holes next to each other, then dressed them with a couple of different shaped files.
Mx KDX400 piston is stock with one round 10mm hole in the same spot, the width of the rear boost passage on the KDX is about that width. The width of the CR500RE boost port is quite wider but I didn't open the hole to the full width of the CR port.
Most dyno reports I have read on the mod quote best results from not having too much height in the hole.
My KDX piston does not have reinforcing around the window; looks like it's just drilled? I monitor the piston condition regularly on both bikes (the benefits of an air-cooled bike ;-) ) but I don't think there'll be any issues. Plus I don't use the bikes a lot anyway. Not like when I was a kid; I'd go through tank after tank of fuel in a day lol.
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Probably 2 x 10mm holes might be just as good, and maybe less stress effected?
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as a matter of interest the Mugen piston in the 1980 Honda CR250 kit had additional ports added like this - 2 though I think.
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I'm assuming that's the intake side?
Where would the hot gasses escape to ? The only route would be the intake port, but by the time the piston got that low in the stroke to expose the back of the piston the hot gasses would have all travelled up through the transfer ports into the combustion chamber.
Would it also effect crankcase compression?
Lozza will know....
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Many modern reed-valve 2-strokes have a rear boost-port running up from the intake port to the same height as the transfers. Once the piston uncovers the side transfer and rear boost port, the arriving low-pressure (vacuum) pulse from the exhaust draws air/fuel up into the combustion chamber. And with it, hopefully, some of the mixture from under the piston :-)
If the mixture under the piston is getting displaced with each stroke, there would be a cooling effect under the piston with each new air/fuel charge.
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You can see in this pic that the intake port runs up to the transfer port height.
(http://i806.photobucket.com/albums/yy343/motorman2/1076%20yamaha%20tt500/honda%20cr500/SAM_1420.jpg) (http://s806.photobucket.com/user/motorman2/media/1076%20yamaha%20tt500/honda%20cr500/SAM_1420.jpg.html)
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This is a KX250A5 cylinder & piston.
(http://i1093.photobucket.com/albums/i423/JohnnyRacer89/CAM00227_zps5342a19e.jpg) (http://s1093.photobucket.com/user/JohnnyRacer89/media/CAM00227_zps5342a19e.jpg.html)
(http://i1093.photobucket.com/albums/i423/JohnnyRacer89/CAM00226_zps64904c25.jpg) (http://s1093.photobucket.com/user/JohnnyRacer89/media/CAM00226_zps64904c25.jpg.html)
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Reports state an increase in 1 to 2 horsepower on a 500, plus a cooler running piston crown from escaping hot gases from under the crown.
(http://i1093.photobucket.com/albums/i423/JohnnyRacer89/CR500%20piston%20window_zpsofaiu3i7_1.jpg) (http://s1093.photobucket.com/user/JohnnyRacer89/media/CR500%20piston%20window_zpsofaiu3i7_1.jpg.html)
If you are talking about hot gasses escaping out the window in the piston the only time its open would be when its lined up with the intake port, I'm thinking?
If a fresh charge is drawn in when that hole is open to the intake port it may cool the piston crown, but heading out?...everything seems to be going in the wrong direction.
Many modern reed-valve 2-strokes have a rear boost-port running up from the intake port to the same height as the transfers. Once the piston uncovers the side transfer and rear boost port, the arriving low-pressure (vacuum) pulse from the exhaust draws air/fuel up into the combustion chamber.
I thought the fresh fuel charge in the crank was moved up through the transfer ports to the combustion chamber by the descending piston increasing (compressing) the air pressure in the crank, not being sucked up by escaping exhaust gasses..
8 pages here we come.... ;)
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SFA pressure in the bottom end so anything that gets the charge to the chamber is a blessing. thats why they have those fancy turbo cranks and plugged counterweights that I think the jury is still out on if they do actually do anything exept lift the owners ego.The thing that does the most is a well designed expansion chamber.
The charge does get sucked up as well as pushed. when the sonic waves heads to the end of the pipe it creates a low pressure zone in the combustion chamber,. higher pressure in the bottom end moves the charge to the top. when the sonic wave hits the cones in the chamber it bounces back and pushes a bit of that fresh charge back into the combustion chamber to "supercharge "
and the cycle continues
Whatever you can do to get the charge to the top end better has got to be good, but its got to actually work
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The piston window is continually open to the intake/rear transfer.
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My limited knowledge of high performance two strokes indicates to me that with modern exhaust expansion chambers, crankcase compression considered so important in days gone by is now a non event.
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The piston window is continually open to the intake/rear transfer.
John, that's my point, or I've misunderstood your first post.
You've put the window in the piston to allow hot gasses to escape OUT through it.
So if it is open to the intake port aren't the escaping hot gasses from under the piston crown traveling in the opposite direction to the fresh charge coming in through the intake port?
If you want hot gases to escape from under the crown wouldn't you put it on the exhaust port side?
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seriously,you guys crack me up, :P
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The piston window is continually open to the intake/rear transfer.
John, that's my point, or I've misunderstood your first post.
You've put the window in the piston to allow hot gasses to escape OUT through it.
So if it is open to the intake port aren't the escaping hot gasses from under the piston crown traveling in the opposite direction to the fresh charge coming in through the intake port?
If you want hot gases to escape from under the crown wouldn't you put it on the exhaust port side?
Soon after the pistons starts its downward stroke, the reed petals close, stopping mixture coming in (or going back out) the inlet port, the piston hasn't uncovered the transfer/boost ports yet so it starts to compress the mixture in the crankcase, as the transfer/boost ports are uncovered in the combustion chamber the crankcase compression, combined with the suck from a properly tuned expansion chamber expel much of the mixture from the crankcase & under the piston up the transfer/boost ports into the combustion chamber.
In a modern reed valve engine with a rear transfer (or boost) port, the intake port becomes part of the transfers while the piston is basically on its downward stroke.
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The piston window is continually open to the intake/rear transfer.
John, that's my point, or I've misunderstood your first post.
You've put the window in the piston to allow hot gasses to escape OUT through it.
So if it is open to the intake port aren't the escaping hot gasses from under the piston crown traveling in the opposite direction to the fresh charge coming in through the intake port?
If you want hot gases to escape from under the crown wouldn't you put it on the exhaust port side?
As the piston descends and pressurises the crank-case, the reed valve closes, the intake flow stops and the port becomes a transfer only.
Pressurised charge flows out the new hole in the back of the piston and up into the chamber. Movement of this charge past the underside of the piston crown ( inc. vapourisation) should remove some heat from this area.
Increases of transfer port area/flow can be beneficial to broadening mid range power.
None of the process is as simple and straight forward as above, but that's the basic idea.
I type too slowly... just read Johns reply above.
Cheers DJ
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At no stage ever does the decending piston 'pressurise' the crank case to any great degree or force the intake charge through the transfers.
All the work is done by the pipe at BDC, it pulls intake from the TRANSFER ports not the crankcase, it can open reeds but that only replaces the intake charge in the crankcase before it moves to the transfer ducts.
What the piston port does is what does it line up with at BDC, intake charge to travel from under the piston to the C port duct is the answer.
Its a very 80's idea that I suppose superceded now by case reed engines and a proper C port duct. Never found any real measurable gain on cylinder reeds with combined C port and inlet port. It can't hurt though.
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OOPS! I better explain to my Victa that it can't possibly start or run without an expansion chamber doing 'all the work' at BDC. :'(
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I did a bit of homework & thought regarding piston window's, thought I'd give it a try. Reports state an increase in 1 to 2 horsepower on a 500, plus a cooler running piston crown from escaping hot gases from under the crown.
Not worried about any more power but thought anything that may give cooler running is worth a try plus even a slight reduction in piston weight wouldn't be a bad thing?
To tell you the truth, I wouldn't notice any increase in power, I'm not getting what I've got to the ground now! It does seem a touch cooler in the combustion chamber; plug is a little darker. And my imagination is feeling there is less vibration (I'm a wanker lol).
Any thoughts?
I see the piston exhaust port helping refresh the under crown and improving the lube and cooling of the small end bearing.
I wonder about the effect of the under piston mixture exhausting from the new outlet into the scavenge stream from the standard boost ports.
Does the piston slot stream affect the aim and velocity of the standard boost ports mixture flow?
Back when I did the same mod to a YZ80C and lo and behold the following year the YZ80D incorporated the horizontal piston slot. It was very difficult to tell if there was any change in the relatively lack lustre performance these single transfer per side engines gave. When the design changed to the 4 transfer (then six like the CR) arrangement with the E? with the big side cavities the engines really came alive
At no stage ever does the decending piston 'pressurise' the crank case to any great degree or force the intake charge through the transfers.
All the work is done by the pipe at BDC, it pulls intake from the TRANSFER ports not the crankcase, it can open reeds but that only replaces the intake charge in the crankcase before it moves to the transfer ducts.
What the piston port does is what does it line up with at BDC, intake charge to travel from under the piston to the C port duct is the answer.
Its a very 80's idea that I suppose superceded now by case reed engines and a proper C port duct. Never found any real measurable gain on cylinder reeds with combined C port and inlet port. It can't hurt though.
(http://i1378.photobucket.com/albums/ah84/msfracingcomponentsmus/taipan_s67_lr_zpssjl9nh6c.jpg)
Aussie engineer Gordon Burford made thousands of these 2 strokes. They never had an exhaust pipe, much less a chamber. The inlet porting is rotary valve and there is no connection between the carburettor and crankcase at BDC, as with a piston port engine.
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I thought the fresh fuel charge in the crank was moved up through the transfer ports to the combustion chamber by the descending piston increasing (compressing) the air pressure in the crank, not being sucked up by escaping exhaust gasses..
8 pages here we come.... ;)
You are right , but the best answer to that is Yes & No,
while it is true that a good exhaust on a 2 stroke will pull mixture up from the crankcase through the transfer ports , an expansion chamber only works when the engine is firing and burning air/fuel mixture.
To get the engine to Start Running SOME crankcase compression is required to get some air/fuel mixture into the cylinder, after that
the exhaust can take over I suppose.
Also 8 pages is not enough , maybe 80 ...... :)
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OOPS! I better explain to my Victa that it can't possibly start or run without an expansion chamber doing 'all the work' at BDC. :'(
Box mufflers still provide (albeit very limited) a suction and plugging pulses. I order for the piston to push anything through the transfers, pressure in the crankcase would have to be higher than what is above the piston. That will only happen long after the transfers are open You are right , but the best answer to that is Yes & No,
while it is true that a good exhaust on a 2 stroke will pull mixture up from the crankcase through the transfer ports , an expansion chamber only works when the engine is firing and burning air/fuel mixture.
To get the engine to Start Running SOME crankcase compression is required to get some air/fuel mixture into the cylinder, after that
the exhaust can take over I suppose.
The exhaust only pulls mixture from the transfers, mixture in the transfers is replaced by what is in the crankcase. A air/fuel molecule will take 15-20 cycles to go through the engine not 1 or 2. No case compression is need to start an engine, fuel evaporisation is whats needed.
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I(n) order for the piston to push anything through the transfers, pressure in the crankcase would have to be higher than what is above the piston. That will only happen long after the transfers are open
Pressure above the piston is able to fall dramatically when the exhaust port opens.
If the engine is cold and not yet running it will return to atmospheric.
(With a running engine and a suitable pipe it can go negative).
The volume in the crankcase reduces due to piston displacement down from TDC.
Pressure WILL increase until the transfer ports open. (P1xV1xT1 = P2xV2xT2)
(no matter how negligible this may appear to be (to some), including losses due to back flow through the inlet port).
Once fully operating on a tuned pipe, a 2 stroke is more like a pulse jet engine, until it is, it's just a basic air pump.
I merely wished to provide a simple explanation to evo550 why the window was in the back of the piston not on the exhaust side.
I think we have managed that.
I am happy to agree to disagree on the rest.
Cheers, DJ
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Got to love some of the 2 stroke theory here. ::)
John I believe you have it correct in the cooling benifit for the piston crown. Quite a few good 2 stroke have been done that way in the past. 250 Maico's are a good example although they used the piston hole to feed the rear transfer port without an inlet port involvment. Not sure about the HP gain but the crown temp will help with detination control not to mention more consistent combustion.
Lozza has a unique theory, and if I get the gist of it we seem to not need a crankcase but should just plumb the carb straight to the tranfer port through a reed. Can't wait to see this new technology in action. :)
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My understanding is that it increases the inlet port duration and will only work with a reed valve motor.
It would bring the motor closer to case reed specs (inlet port nearly always open)
A piston port motor would suffer blowback into the carby which is why the Maico motors that Sleepy talks of would have the port holes lining up with the transfers.
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OOPS! I better explain to my Victa that it can't possibly start or run without an expansion chamber doing 'all the work' at BDC. :'(
Box mufflers still provide (albeit very limited) a suction and plugging pulses. I order for the piston to push anything through the transfers, pressure in the crankcase would have to be higher than what is above the piston. That will only happen long after the transfers are open You are right , but the best answer to that is Yes & No,
while it is true that a good exhaust on a 2 stroke will pull mixture up from the crankcase through the transfer ports , an expansion chamber only works when the engine is firing and burning air/fuel mixture.
To get the engine to Start Running SOME crankcase compression is required to get some air/fuel mixture into the cylinder, after that
the exhaust can take over I suppose.
The exhaust only pulls mixture from the transfers, mixture in the transfers is replaced by what is in the crankcase. A air/fuel molecule will take 15-20 cycles to go through the engine not 1 or 2. No case compression is need to start an engine, fuel evaporisation is whats needed.
Okay then so what you are saying is - crankcase compression is not required & has no effect {even for starting} ,
and the exhaust system is the only thing pulling air/fuel mixture up from the crankcase. Is that right ?
so a 2 stroke engine with the exhaust removed will never start & run ?
that's an interesting theory that I have never heard before, you learn something new every day , thanks :)
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Pressure above the piston is able to fall dramatically when the exhaust port opens.
If the engine is cold and not yet running it will return to atmospheric.
(With a running engine and a suitable pipe it can go negative).
The volume in the crankcase reduces due to piston displacement down from TDC.
Pressure WILL increase until the transfer ports open. (P1xV1xT1 = P2xV2xT2)
(no matter how negligible this may appear to be (to some), including losses due to back flow through the inlet port).
Once fully operating on a tuned pipe, a 2 stroke is more like a pulse jet engine, until it is, it's just a basic air pump.
I merely wished to provide a simple explanation to evo550 why the window was in the back of the piston not on the exhaust side.
I think we have managed that.
I am happy to agree to disagree on the rest.
Cheers, DJ
The pipe still 'works' without the engine running not to the degree when it is
The pressure gauges I connect to superkart engines show a massive +/- 1-1.5psi on the pulse pump connection. More than 10 psi in the cases you will blow the crank seals, compare that with maybe 1500psi at max compression and maybe 500psi at exhaust port open, then a mere 30deg of crank rotation later pressure above the piston is going to be less than 1.5psi?
Lozza has a unique theory, and if I get the gist of it we seem to not need a crankcase but should just plumb the carb straight to the tranfer port through a reed. Can't wait to see this new technology in action. :)
Just have a look at a 2015 Modena ICC kart engine that has a 24/7 inlet valve that does basicaly that. Unique in the sense that HRC, Jan Thiel, Jorge Muller and a host of kart engine manufacturers know what I'm talking about ;)
Okay then so what you are saying is - crankcase compression is not required & has no effect {even for starting} ,
and the exhaust system is the only thing pulling air/fuel mixture up from the crankcase. Is that right ?
so a 2 stroke engine with the exhaust removed will never start & run ?
that's an interesting theory that I have never heard before, you learn something new every day , thanks :)
atmospheric pressure is still the lowest regardless, even if the engine doesn't fire higher pressure is still in the combustion chamber until the exhaust port opens. Each time the piston ascends you fill the crankcase including the transfer ducts.
There is never going to be high enough pressure in the crankcase to 'push" intake out the transfers especialy on a running engine.
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I didn't get past "maybe 1 or 2 hp" with a 500 I thought why lessen the piston integrity for f*kall gain ? but im a lazy basket.... ;)
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I put fuel in the tank & it somehow magically gets to where it's supposed to go in the engine & my bike moves forward. That's all I need to know.
K
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There is never going to be high enough pressure in the crankcase to 'push" intake out the transfers especialy on a running engine.
If the pressure in the crankcase isn't greater than the pressure in the cylinder it is a pysical impossibility for the motor to ever run, simple physics. I didn't need to spend 4 years at UNI to work that one out.
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There is never going to be high enough pressure in the crankcase to 'push" intake out the transfers especialy on a running engine.
If the pressure in the crankcase isn't greater than the pressure in the cylinder it is a pysical impossibility for the motor to ever run, simple physics. I didn't need to spend 4 years at UNI to work that one out.
That makes sense ,
I have a lot to learn about 2 strokes , so far Sleepy & Daryl Jones explanations seem the most logical, but its all interesting anyway.
thanks
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There is never going to be high enough pressure in the crankcase to 'push" intake out the transfers especialy on a running engine.
I reckon any good 2 stroke on the bugle is going to have a net positive pressure in the cases and an inertia ram effect from column flowing through the carb and intake tract.
Of course the case pressure only has to be slightly higher than the depression in the cylinder caused by the exhaust to be net positive
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I'm loving this topic
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OK, I think I've finally got it.
Now, I just need some substantial funding for R&D, and the patent lawyers costs.
Four stroke style, lubricated bottom end, (No nasty 2 stroke smoke).
Simple barrel with an exhaust port and multiple reed valved inlets, (where the transfers used to be),
Multi-point Fueling system (Injection would be nice).
Inlet manifolds of appropriate length and volume to allow 'harmonic resonance'.
'Suitable' expansion chamber exhaust.
Oh! And a decent Starter, to wind it up to a minor harmonic speed, say 5,000 rpm, before introducing the fuel & ignition.
I have created a "pulse jet" engine which uses the piston to extract some energy from the pressure pulses above it.
The pressure pulses below it below it are sort of wasted, unless you direct the crankcase venting through another pair of reeds into the intake side.
(No patent for that bit, it's been done plenty of times already).
I really can't believe that no-one has done something like this already.
Okay, Detroit Diesel 2-strokes do, but then they messed up the beautiful simplicity of this idea, by using dirty great superchargers to Pump the air into the cylinders. Obviously, lacking in imagination.
Get in early, while the shares are still cheap.
Cheers, DJ.
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Get in early, while the shares are still cheap.
Ya.
What is Sarich worth these days; 700 plus million?
(http://i1378.photobucket.com/albums/ah84/msfracingcomponentsmus/Pulse%20350px-Puls1Motor_en_zpskccppkyl.gif)
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I try to be open minded, listen to new ideas & opinions,
but I am struggling to believe the LOZZA 2 stroke theory -
"there is never going to be high enough pressure in the crankcase to push intake out of the transfers especially on a running engine"
"no case compression is needed to start an engine"
"all the work is done by the pipe at BDC"
If all of these things are true then it must be IMPOSSIBLE for a 2 stroke to start & run with its exhaust removed.
well I just went out to the shed and took the complete exhaust off my old DT175, gave it a few kicks and it started up and runs !
surely this is impossible because "all the work is done by the pipe at BDC" and "there is never going to be enough pressure in the crankcase to push intake out of the transfers especially on a running engine"
can someone explain this :)
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Well on my CR500 you can feel the crankcase compression through the kickstart lever, take the head off and push the kick lever down, you can feel the kick lever getting over the primary compression, you can hear the 'pop' from the sudden pressure escaping from the crankcase to above the piston as the transfers open.
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There is never going to be high enough pressure in the crankcase to 'push" intake out the transfers especialy on a running engine.
If the pressure in the crankcase isn't greater than the pressure in the cylinder it is a pysical impossibility for the motor to ever run, simple physics. I didn't need to spend 4 years at UNI to work that one out.
connect a pressure transducer to the crankcase like Robert Fleck did and you see that the pressure in the case is only ever higher in a STATIC state. In a DYNAMIC ie the engine running pressure in the crankcase is never higher than the cylinder. Maybe yamaha got it all wrong when they realised that and changed the yzr250. Oliver Jacque is glad they did though.
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I try to be open minded, listen to new ideas & opinions,
but I am struggling to believe the LOZZA 2 stroke theory -
"there is never going to be high enough pressure in the crankcase to push intake out of the transfers especially on a running engine"
"no case compression is needed to start an engine"
"all the work is done by the pipe at BDC"
If all of these things are true then it must be IMPOSSIBLE for a 2 stroke to start & run with its exhaust removed.
well I just went out to the shed and took the complete exhaust off my old DT175, gave it a few kicks and it started up and runs !
surely this is impossible because "all the work is done by the pipe at BDC" and "there is never going to be enough pressure in the crankcase to push intake out of the transfers especially on a running engine"
can someone explain this :)
at no stage ever did I say an engine is impossible to start without the pipe
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Well on my CR500 you can feel the crankcase compression through the kickstart lever, take the head off and push the kick lever down, you can feel the kick lever getting over the primary compression, you can hear the 'pop' from the sudden pressure escaping from the crankcase to above the piston as the transfers open.
Thats not with the engine running is it
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There is never going to be high enough pressure in the crankcase to 'push" intake out the transfers especialy on a running engine.
If the pressure in the crankcase isn't greater than the pressure in the cylinder it is a pysical impossibility for the motor to ever run, simple physics. I didn't need to spend 4 years at UNI to work that one out.
connect a pressure transducer to the crankcase like Robert Fleck did and you see that the pressure in the case is only ever higher in a STATIC state. In a DYNAMIC ie the engine running pressure in the crankcase is never higher than the cylinder. Maybe yamaha got it all wrong when they realised that and changed the yzr250. Oliver Jacque is glad they did though.
Sounds like BS to me. Prove it by posting a link to their research paper or publish your own finding from all the dyno work you have done with pressure transducers so that myself and the other mechanical engineers on this forum can re-learn all the thermodynamic theory that we have been taught.
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I try to be open minded, listen to new ideas & opinions,
but I am struggling to believe the LOZZA 2 stroke theory -
"there is never going to be high enough pressure in the crankcase to push intake out of the transfers especially on a running engine"
"no case compression is needed to start an engine"
"all the work is done by the pipe at BDC"
If all of these things are true then it must be IMPOSSIBLE for a 2 stroke to start & run with its exhaust removed.
well I just went out to the shed and took the complete exhaust off my old DT175, gave it a few kicks and it started up and runs !
surely this is impossible because "all the work is done by the pipe at BDC" and "there is never going to be enough pressure in the crankcase to push intake out of the transfers especially on a running engine"
can someone explain this :)
at no stage ever did I say an engine is impossible to start without the pipe
Lozza , YOUR THEORY says its impossible for the engine to start without the pipe.
Because you state that " ALL the work is done by the pipe at BDC "
if you had said "SOME of the work is done by pipe at BDC" or even "MOST of the work is done by the pipe at BDC" then it would still be possible for the engine to run without the pipe.
But if the pipe is doing "ALL the work" then surely the engine cant run without the exhaust fitted,
because you also state that " there is never going to be high enough pressure in the crankcase to push intake out of the transfers"
without the pipe fitted how then does any air/fuel get into the cylinder if its not from crankcase compression ?
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This is from the DYNOmite Forum. They make engine transducer and data recording kits.
I suspect that they know more than just a little bit about these measurements!
The coloured and resized text is my work.
Link here: http://forums.land-and-sea.com/showthread.php?t=64 (http://forums.land-and-sea.com/showthread.php?t=64)
2-stroke crankcase pressures
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How should I set up to measure pressures in my snowmobile’s crankcase? I already have the -12 to 22 PSI manifold pressure sensors but it seems to read rather erratically.
#2
07-05-2005, 08:49 AM
Tech
Land & Sea Engineering Join Date: Mar 2004
Location: Concord, NH
Posts: 210
Re: 2-stroke crankcase pressures
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1) If you are trying to map the cyclical crankcase pressure changes that occur during each revolution of the crankshaft, you need more than just your sensor. This requires the same type of equipment and software Option Pack as combustion chamber pressuring mapping.
Our DYNOmite-Pro boards collect data at either 200 Hertz or 1,000 Hz (in USB mode). This is ideal for traditional dynomometer testing - where you are collecting data on Hp, Torque, RPM, fuel flow, etc. But, to map what goes on during a single revolution of the engine, you'll need individual data readings taken consecutively at each degree of crankshaft rotation. At high (two-cycle range) RPM's we are talking about board sets that can handle over 100,000 Hertz - like the 500,000 Hz board found in our Combustion Pressuring Mapping Kit.
You also need an encoder that provides an index for each pressure reading (to the exact degree position of the crankshaft when that data is captured). Again, the encoder in the Combustion Pressuring Mapping Kit is appropriate.
Note: It's mounting (which is designed for an automotive crankshaft dampener) would have to be custom adapted to your magneto housing.
Lastly, you will want to custom order the kit with a much lower range pressure transducer. Instead of the normal 2,000 PSI range, you will want one that is appropriate for the -14 to +30 PSI readings of a normally aspirated two-stroke's crankcase.
2) If instead all you want is to monitor the average crankcase pressure (a relatively low positive number), then your existing sensor can do that. However, you can not just screw the sensor into the crankcase. Those erratic readings are caused by the arbitrary synchronization aliasing) between your DYNOmite's 200 Hz sampling rate and the position of your engine's crankshaft. You're never taking the pressure readings at the same index twice.
Instead, plumb it to your engine's crankcase using a restricted dampening orifice. A hose with about a 0.030" fuel jet stuck in it works well as the orifice. Also, put a small dampening chamber (plenum) between the orifice and your pressure sensor. A small in-line fuel filter makes a convenient chamber. Once your have the restrictor and plenum in place I think you will find your erratic readings cured.
Note: We recommend that the pressure sensor be oriented so any residual fuel droplets will gravity drain back into the plenum and crankcase between tests. That greatly extends the sensor's life.
__________________
Land & Sea Engineering
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positive/ negative pressure, even minimal, is needed to move fresh charge/mixture through the engine.
block your intake, pressure drops/stops fresh mixture is not geting into combustion chamber- engine will stop
block your exhaust ,pressure builds up and over power intake pressure and stops fresh mixture from reaching combustion chamber- engine will stop
so, yes there is pressure inside crankcase/motor that moves fresh fuel mixture around
various modifications of engine parts can improve/change this.
you can not start an engine simply by spraing fuel mixture into it. pressure in various form is is needed to start it.
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Woo. seems like the hard way of going about something when you can use what you already know
if a bottom end has bad seals it runs like crap. the fact it has seals says it has some pressure but just how much?
When you pressure test a bottom end most manufacturers state it needs to hold the pressure of 6 - 15 psi for 5 minutes(most say 6). and vacuum of 5-7 inches of mercury for the same time..
To my mind thats sfa in the bottom end and its not rocket science.
ergo the crank case does play a bit of a part but something else is doing the big work.
try sticking a CR125pipe on a bog stock MT125 and it will give you the clues.
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Any-one who would like to read what Emeritus Professor Gordon P. Blair's thoughts on how 2-Stroke engines work, might like to download this:-
http://www.dragonfly75.com/motorbike/2StrokeDesign.pdf (http://www.dragonfly75.com/motorbike/2StrokeDesign.pdf)
Cheers, DJ
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Interesting reading guys with a lot of differing points on how the 2 smokers work.
As Pokey said, a 2 stroke will not run properly with buggered crank seals. In some cases, like a rooted gear side crank seal, the motor won't run at all. It will fire, but will not run as there is no crank case pressure to help get fuel to the cylinder. Conversely, a leaking ignition side seal will make the Air/Fuel mixture way too lean and the engine will run erratic with no top end power at all.
There MUST be some positive pressure in the crank case or there would be no need for pressure type seals because the piston and pipe would be scavenging the fuel/air mix faster than any pressure build up. There also MUST be negative pressure in order for vacuum to take effect to enable more Air/Fuel mixture to be dragged into the crank case.
Therefore, the theory of having the boost port window on the piston is sound to my way of thinking. It has to help drag more Air/Fuel mixture into the cases, thereby helping with not only more fuel into the cylinder, but also keep the piston crown cooler through there being more mixture in the case.
That's my take on it anyway.
When all else fails...... get a 4 stroke ;D
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Therefore, the theory of having the boost port window on the piston is sound to my way of thinking. It has to help drag more Air/Fuel mixture into the cases, thereby helping with not only more fuel into the cylinder, but also keep the piston crown cooler through there being more mixture in the case.
That's my take on it anyway.
When all else fails...... get a 4 stroke ;D
That makes much more sense to me. In Johns initial post, he said he cut the hole in the piston to allow hot gasses from under the crown to escape OUT the window, that seemed to me to "go against the flow"
That combined with gasses escaping out the window on a descending piston would reduce any pressure created in the crankcase to assist with delivering the fuel charge to its ultimate demise in the combustion chamber.
Scratching my bald head I was....
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This is from the DYNOmite Forum. They make engine transducer and data recording kits.
I suspect that they know more than just a little bit about these measurements!
The coloured and resized text is my work.
Link here: http://forums.land-and-sea.com/showthread.php?t=64 (http://forums.land-and-sea.com/showthread.php?t=64)
2-stroke crankcase pressures
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How should I set up to measure pressures in my snowmobile’s crankcase? I already have the -12 to 22 PSI manifold pressure sensors but it seems to read rather erratically.
#2
07-05-2005, 08:49 AM
Tech
Land & Sea Engineering Join Date: Mar 2004
Location: Concord, NH
Posts: 210
Re: 2-stroke crankcase pressures
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1) If you are trying to map the cyclical crankcase pressure changes that occur during each revolution of the crankshaft, you need more than just your sensor. This requires the same type of equipment and software Option Pack as combustion chamber pressuring mapping.
Our DYNOmite-Pro boards collect data at either 200 Hertz or 1,000 Hz (in USB mode). This is ideal for traditional dynomometer testing - where you are collecting data on Hp, Torque, RPM, fuel flow, etc. But, to map what goes on during a single revolution of the engine, you'll need individual data readings taken consecutively at each degree of crankshaft rotation. At high (two-cycle range) RPM's we are talking about board sets that can handle over 100,000 Hertz - like the 500,000 Hz board found in our Combustion Pressuring Mapping Kit.
You also need an encoder that provides an index for each pressure reading (to the exact degree position of the crankshaft when that data is captured). Again, the encoder in the Combustion Pressuring Mapping Kit is appropriate.
Note: It's mounting (which is designed for an automotive crankshaft dampener) would have to be custom adapted to your magneto housing.
Lastly, you will want to custom order the kit with a much lower range pressure transducer. Instead of the normal 2,000 PSI range, you will want one that is appropriate for the -14 to +30 PSI readings of a normally aspirated two-stroke's crankcase.
2) If instead all you want is to monitor the average crankcase pressure (a relatively low positive number), then your existing sensor can do that. However, you can not just screw the sensor into the crankcase. Those erratic readings are caused by the arbitrary synchronization aliasing) between your DYNOmite's 200 Hz sampling rate and the position of your engine's crankshaft. You're never taking the pressure readings at the same index twice.
Instead, plumb it to your engine's crankcase using a restricted dampening orifice. A hose with about a 0.030" fuel jet stuck in it works well as the orifice. Also, put a small dampening chamber (plenum) between the orifice and your pressure sensor. A small in-line fuel filter makes a convenient chamber. Once your have the restrictor and plenum in place I think you will find your erratic readings cured.
Note: We recommend that the pressure sensor be oriented so any residual fuel droplets will gravity drain back into the plenum and crankcase between tests. That greatly extends the sensor's life.
__________________
Land & Sea Engineering
Pity old mate didn't stick a transducer in the transfer duct and the cylinder, with a 360 tooth wheel and an encoder to graph crank degrees all at the same time. That would have showed the pressure in the transfer duct highest AFTER the transfer port opens. Which is still higher than the crankcase.
Any-one who would like to read what Emeritus Professor Gordon P. Blair's thoughts on how 2-Stroke engines work, might like to download this:-
http://www.dragonfly75.com/motorbike/2StrokeDesign.pdf (http://www.dragonfly75.com/motorbike/2StrokeDesign.pdf)
Cheers, DJ
That book isn't about how a engine works it's more about mathmatical models and making test rigs to prove the model. I can post plenty of screen grabs from simulators (loosely) based on Blairs work.Lozza , YOUR THEORY says its impossible for the engine to start without the pipe.
Because you state that " ALL the work is done by the pipe at BDC "
if you had said "SOME of the work is done by pipe at BDC" or even "MOST of the work is done by the pipe at BDC" then it would still be possible for the engine to run without the pipe.
But if the pipe is doing "ALL the work" then surely the engine cant run without the exhaust fitted,
because you also state that " there is never going to be high enough pressure in the crankcase to push intake out of the transfers"
without the pipe fitted how then does any air/fuel get into the cylinder if its not from crankcase compression ?
Without the pipe you have a greater cylinder purity ie no exhaust gas to mix with the fresh charge, the piston going up and down, the rod and crank shaft spinning together with the crankcase getting filled with dense inake charge each time the piston goes up will create enough turbulence to get fresh charge into the cylinder.
If you look at say a DT 175 cylinder how does all that black carbon get into the transfer ducts? On rgv engines it is possible to see carbon shadow of the C or boost port duct on the reed stops.
This isn't my theory at all first person to highlight this to me was Wayne "Wobbly" Wright from NZ, as confirmed by Jan Thiel, Frits Overmars and the late Calvin Pollet of aftermarket CPI Banshee cylinder fame. Again plain to see on even the most basic engine simulator.
Twin, triples and inline 4 cylinder engines have labryinth seals on the crankshaft between the webs that bleed off and allow pressure to pass either way (decreasing as it passes) they seem to run just fine ;)
The window is to allow cool gas out from under the piston
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Lozza.
I'm not going to go into this anymore as you are clearly talking around in circles contradicting yourself along the way.
I will point out a fact about labryinth seals as you have obviously never seen one in real life. They are a very close fitting to the the main shaft of the crank with radial groves on the inside. There is a very small amout of mixture that leaks from crankcase to crankase but it is such small amount there is no effect in a running engine.
Just thought I'd point that out as your computer simulation may not tell you that.
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Without the pipe you have a greater cylinder purity ie no exhaust gas to mix with the fresh charge,
One quick negative sound wave, back into in the cylinder. No extended extraction from a diffuser cone, to get residual gas out.
the piston going up and down, the rod and crank shaft spinning together with the crankcase getting filled with dense inake charge each time the piston goes up will create enough turbulence to get fresh charge into the cylinder.
Without the heat & pressure of combustion, the charge in the cylinder is compressed & decompressed and goes nowhere.
Without discharge from the crankcase to the cylinder, the charge in the crankcase is compressed & decompressed and goes nowhere
If you look at say a DT 175 cylinder how does all that black carbon get into the transfer ducts? On rgv engines it is possible to see carbon shadow of the C or boost port duct on the reed stops.
Two examples of bikes with exhaust gas reversion problems, do not support the case for "Case Pressure Always Less than Cylinder Pressure".
DT175's have marginal exhausts and excessive silencing, they generally ridden by riders who would be unlikely to remain in the power band for any length of time.
The RGV had an effective power band between 8,000 - 11,000 RPM and top speed of 209 kph.
They need to be ridden at 152kph just to touch 8,000 RPM. Most road ridden ones would rarely achieve that.
My old Pursang has never shown any signs of carbon in the transfers in the 35 years I have owned and raced it.
This isn't my theory at all first person to highlight this to me was Wayne "Wobbly" Wright from NZ, as confirmed by Jan Thiel, Frits Overmars and the late Calvin Pollet of aftermarket CPI Banshee cylinder fame. Again plain to see on even the most basic engine simulator.
Twin, triples and inline 4 cylinder engines have labryinth seals on the crankshaft between the webs that bleed off and allow pressure to pass either way (decreasing as it passes) they seem to run just fine ;)
The -14 to +30psi pressure fluctuations in the crankcase take around 0.006 secs @ 10,000 RPM. The spirals of the labyrinth are there to increase the distance so escaping gas will stop, and come back, before it gets out.
The window is to allow cool gas out from under the piston[/quote]
Sure, Gas that has been in contact with the underside of the piston, is obviously cooler than the fresh charge ???
Particularly if, as claimed, it's there for 15 revolutions
Like Sleepy.. I'm done with this too. Life's too short, I'm happy to agree to disagree with this interpretation of some-one else's theory.
Cheers DJ
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Lozza , YOUR THEORY says its impossible for the engine to start without the pipe.
Because you state that " ALL the work is done by the pipe at BDC "
if you had said "SOME of the work is done by pipe at BDC" or even "MOST of the work is done by the pipe at BDC" then it would still be possible for the engine to run without the pipe.
But if the pipe is doing "ALL the work" then surely the engine cant run without the exhaust fitted,
because you also state that " there is never going to be high enough pressure in the crankcase to push intake out of the transfers"
without the pipe fitted how then does any air/fuel get into the cylinder if its not from crankcase compression ?
[/quote]
Without the pipe you have a greater cylinder purity ie no exhaust gas to mix with the fresh charge, the piston going up and down, the rod and crank shaft spinning together with the crankcase getting filled with dense inake charge each time the piston goes up will create enough turbulence to get fresh charge into the cylinder.
If you look at say a DT 175 cylinder how does all that black carbon get into the transfer ducts? On rgv engines it is possible to see carbon shadow of the C or boost port duct on the reed stops.
This isn't my theory at all first person to highlight this to me was Wayne "Wobbly" Wright from NZ, as confirmed by Jan Thiel, Frits Overmars and the late Calvin Pollet of aftermarket CPI Banshee cylinder fame. Again plain to see on even the most basic engine simulator.
What you call "Turbulence", 99% of other people call this "Crankcase Compression"
had a look at the DT175 cylinder, there is no carbon in the transfers.
You keep name dropping Jan Theil, Fritz Overmars , etc. etc. etc. etc. but I have never read anything by these people where they deny that Crankcase Compression exists.
In fact I was reading an article the other day by Jan Theil where he was discussing "Crankcase Compression Ratios."
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Well now I have the carburation operating cleanly; no rich blubber, no lean ping anywhere through the various circuits, I am finding the combustion temps are too cool, ha go figure! :-) So I have dropped from the stock NGK '8' plug to a '7'; was running a '9' when I had the part-throttle pinging (which wasn't helping). Getting a much cleaner, dryer burn in the combustion chamber now.
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Tried dropping the needle or 1 size less on the main?
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Hi John,
Did you do the holes in the piston? Where is your carby up to re settings and mods etc, do you still have the tube on the pilot tube where the jet goes?
The only thing I am aware of re holes in pistons, is YZ 490 does not have them, and IT490 does.
Cheers
Kim :D
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Well now I have the carburation operating cleanly; no rich blubber, no lean ping anywhere through the various circuits, I am finding the combustion temps are too cool, ha go figure! :-) So I have dropped from the stock NGK '8' plug to a '7'; was running a '9' when I had the part-throttle pinging (which wasn't helping). Getting a much cleaner, dryer burn in the combustion chamber now.
Pretty much proves the theory of cooling the underside of the piston crown. Now you can bump up the compression ratio a little and go back to the 8 plug. That will be just the ticket more power for the weak CR500!
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Hmmm just went testing today, man that thing has some power !!!!! Trenches everywhere.
If I bump up the compression any more I would not be able to kick it over, plus that crankcases might break?
I've ground in some exhaust bleed ramps as per the later CR500's, it has helped with kick starting but not lost any punch (well not that I can feel!).
Lozza, the needle is full lean clip now, plus once I get off the straight section of the needle (1/4 throttle onwards) I feel it is very clean. The black plug colour is on the pilot jet/straight section of the needle, it is now running sweet with virtually no ping.
The pilot jet is down from a 68 to a 50.
Might even go back to a '6' plug?!!!
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Hi John,
Did you do the holes in the piston? Where is your carby up to re settings and mods etc, do you still have the tube on the pilot tube where the jet goes?
The only thing I am aware of re holes in pistons, is YZ 490 does not have them, and IT490 does.
Cheers
Kim :D
G'day Kim :-)
For sure the hole in the piston has cooled combustion chamber temps. I haven't even fitted my airscoops back on yet.
Not sure if my jetting specs would work for anyone else but I'll list all that I have done ....
Cyl head squishband narrowed to 14mm wide.
Cyl head mating surface machined .7mm
Dual stage Boyeson reeds (will be replacing them with my own cut-out full carbon reeds down the track).
Reed cage has 4 bridges cut out, just left the centre bridge top & bottom for reed support.
Main jet: 165
Needle: DEF
Pilot jet: 50
Air screw 1 1/2 turns.
DG pipe & muffler.
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Picking-up fuel from the bottom of the fuel bowl initially cut my part throttle ping by 50%, the needle cleaned-up the rest.
The only problem is, drawing fuel from the bottom of the bowl means ANY crap in there WILL block the pilot jet.
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Thanks John,
Sounds like you are making real progress mate that I am sure many other big bore owners will be learning from and applying your research and learning's to their bikes, I certainly will to my YZ490...yes yes I know, its a 490...
Cheers John,
Kim
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Need the next leanest needle I don't know the Keihin numbering.
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The pilot jet is down from a 68 to a 50.
:o What increments are those jets in? If 2's that a massive jump.
How does a cooler piston crown stimulate a jetting change ? My understanding is a optimum fuel to air ratio is around the 14:1(correct me if I'm wrong) this ratio is altered by the amount of fuel being delivered through changing jets. So unless you've had a serious change in atmospheric conditions, why the dramatic jetting change?
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Yep I agree it is a massive jet change! That's why I've mentioned it. And the plug is still black !!!! And the bike starts cold without the choke. I just think Honda got it so wrong in the first place?
But I have compensated with a richer needle, as mentioned earlier in my ramblings, the big engine, combined with the odd PE38 carb, draws fuel from both the pilot and the needle jet with the twist-grip closed, and up to 1/8th throttle (straight section of the needle).
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More likely would be increased air speed past the slide. AFR 12:1 makes the best power on a 2T not that AFR is a reliable guide on a two stroke
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With regards the cooler running crown, my thoughts are to use a hotter plug, hence my drop from the stock 8 to a 7 to keep the plug burned clean. :-)
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More likely would be increased air speed past the slide. AFR 12:1 makes the best power on a 2T not that AFR is a reliable guide on a two stroke
If its increased air speed wouldn't that require richer brass not leaner?
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John just curious as to the fuel oil ratio you are using. Perhaps a bit more oil in the fuel may be enough of a change to clean up the mixture with the added benifit of cooling the piston even further. Some years ago as an experiment I got a guy with WR490 (I think thats what it was) with a very rich bottom end to go from 50:1 to 20:1. He was very reluctant to try it cause he had been told to much oil would destroy his engine or some such BS.
It made a dramatic improvment to the light throttle running and the transition onto the main. He went back to the 50:1 on the advise of the local bike shop sales idiot.
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Yes more oil would lean it out more, I am using 28:1 at the moment but the bike's not dribbling splooge from the pipe joins. I'll try running it as it is for a couple of rides and see what thoughts enter my head :-)
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More likely would be increased air speed past the slide. AFR 12:1 makes the best power on a 2T not that AFR is a reliable guide on a two stroke
If its increased air speed wouldn't that require richer brass not leaner?
No higher air speed drags more fuel out of a circuit.
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The way I recall it Sleepy ..is yes..the presence of more oil will lean off the mixture.. but the fuel does the cooling .. the oil does the lubricating.. so more oil will not make the piston any cooler..
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The way I recall it Sleepy ..is yes..the presence of more oil will lean off the mixture.. but the fuel does the cooling .. the oil does the lubricating.. so more oil will not make the piston any cooler..
Everything that goes in cool will take away some heat but the extra oil means less frictional losses which in turn means less heat. I recall seeing dyno tests years ago on oil ratio/head temp and from it was quite clear that less oil meant higher head temp which would equate to piston crown temps as well.
Although in this case with a 28:1 mix there would only be a small gain as it is already more oil than most run.
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Cutting the iddy biddy little bit of glass to go in the windows a prick... still that's easier than makin the catch to slide it open and closed.
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Cutting the iddy biddy little bit of glass to go in the windows a prick... still that's easier than makin the catch to slide it open and closed.
Lexan ;-)
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I know it has been done to death a million times on here.. but I can't for the life of me, with modern synth oils , work out why anyone would want to run 28:1.. even on an old banger aircooled engine..40:1 is fine..
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A hotter than standard plug might keep the plug cleaner & firing,
but may not have too much effect on the total combustion process.
Maybe Honda didn’t get that bit of the engine design wrong.
i.e. perhaps they didn’t put in the piston window boost ports for a reason.
The performance aim is to achieve maximum possible cylinder pressure after TDC.
(We are burning fuel and oxygen to heat and expand the Nitrogen in the air.)
If the piston crown is now running much cooler, then the produced pressure will be lowered.
Increasing the compression ratio will produce quicker & hotter combustion, but if this is not desirable,
for mechanical or physical starting issues, you might need to look at the head and upper cylinder finning.
Perhaps there is now too much, for the boost port cooled piston?
I wouldn’t start chopping fins off (yet) but you could conduct some tests with airflow ‘reduced’ to the head fins
and see what effect that has on combustion chamber temp (use the std. plug) and the resulting power output.
Have fun, DJ
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My understanding is a optimum fuel to air ratio is around the 14:1
AFR 12:1 makes the best power on a 2T not that AFR is a reliable guide on a two stroke
Lots of factors, inc. fuel quality, temperature, humidity, air pressure, combustion time, engine load & throttle opening affect the best AFR, in any situation.
Carby's do an amazing job, (Fuel Injection is a topic for another thread)
Best power AFR is when you have 'enough' fuel to use up every available O2 molecule in the charge. (A Bit Rich)
Best economy is when you have 'enough' air (O2) to burn up every fuel molecule in the charge. (A Bit Lean)
The stoichiometric ratio for complete combustion of petrol is 14.7 parts of air to one part of fuel,
but it's really hard to manage because of all the variable conditions listed above.
Best to be A Bit Rich. A Bit Lean is really hard on engines & if your looking for economy you're in the wrong forum.
Cheers, DJ
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I know it has been done to death a million times on here.. but I can't for the life of me, with modern synth oils , work out why anyone would want to run 28:1.. even on an old banger aircooled engine..40:1 is fine..
The difference between 28 and 40:1.
28 will put a bit of black goup out the pipe.
28 will make more power.
28 will run cooler.
28 will make bigends and pistons last longer.
40 will cost less per tank but more in rebuilds.
I recommend 25:1 for air cooled MX motors. Kawasaki KX125 2000 model, factory recommend 25:1 with modern oils.
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The energy density of oil (mineral or vegetable) is greater than that of petrol.
If your burning Most of it, you will be making Power from it.
A Bit Rich (oil) is better than A Bit lean (oil) even if you get a goupy pipe.
Cheers, DJ
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I know it has been done to death a million times on here.. but I can't for the life of me, with modern synth oils , work out why anyone would want to run 28:1.. even on an old banger aircooled engine..40:1 is fine..
If your running 40:1 in an air cooled engine it's more about the load and the tune of the engine. An air cooled 100cc kart engine would last a matter of laps with that ratio. Spooge out the pipe is jetting and ignition rather than oil ratios.
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I know it has been done to death a million times on here.. but I can't for the life of me, with modern synth oils , work out why anyone would want to run 28:1.. even on an old banger aircooled engine..40:1 is fine..
If your running 40:1 in an air cooled engine it's more about the load and the tune of the engine. An air cooled 100cc kart engine would last a matter of laps with that ratio. Spooge out the pipe is jetting and ignition rather than oil ratios.
I can remember when 2 stroke lawn mowers n whipp snippers etc used to run a recommended ratio of 15:1. Now with better metallurgy and oil quality it's 50:1. 2 stroke MXers were 20:1 ratios and then the research about best fuel/air/oil ratio took on another logic and came to the conclusion that too much oil does infact cause a lean situation because the oil isn't being burnt efficiently. I'm with Watto here. 40:1 is fine for my 2 strokers. Even after an entire season on an Elsinore, 6rounds x 3 races x 3 laps (54 laps) with the odd ten lapper thrown in, there was no evidence of piston or bore wear, very minimal carbon build up (wiped off with a rag and petrol) and I only changed the rings because I thought I should while I had the top end off. That's all the proof I need.
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I know it has been done to death a million times on here.. but I can't for the life of me, with modern synth oils , work out why anyone would want to run 28:1.. even on an old banger aircooled engine..40:1 is fine..
If your running 40:1 in an air cooled engine it's more about the load and the tune of the engine. An air cooled 100cc kart engine would last a matter of laps with that ratio. Spooge out the pipe is jetting and ignition rather than oil ratios.
I can remember when 2 stroke lawn mowers n whipp snippers etc used to run a recommended ratio of 15:1. Now with better metallurgy and oil quality it's 50:1. 2 stroke MXers were 20:1 ratios and then the research about best fuel/air/oil ratio took on another logic and came to the conclusion that too much oil does infact cause a lean situation because the oil isn't being burnt efficiently. I'm with Watto here. 40:1 is fine for my 2 strokers. Even after an entire season on an Elsinore, 6rounds x 3 races x 3 laps (54 laps) with the odd ten lapper thrown in, there was no evidence of piston or bore wear, very minimal carbon build up (wiped off with a rag and petrol) and I only changed the rings because I thought I should while I had the top end off. That's all the proof I need.
You say that to much oil causes a lean situation. Almost all the air cooled 2 strokes in the 70's came from the factory jetted to suit 20:1 so to go to 40:1 would require richer jetting. I bought a new RM250A in 76 and raced it for a hole season which include practicing for an hour every weekend when not racing. It only ever had 20:1 R30 and at the end of the season still didn't need a ring. It did get decoked a couple of time though.
Could you post a link to the research that shows the benefits of using 40:1 over 25:1. Things like dyno comparisons or technical papers.
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Less oil in the mix does make the jetting richer.
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VERY VERY relevant statement of FACT from Lozza "Spooge out the pipe is jetting and ignition rather than oil ratios."
Learn and practice tuning and maintenance of your engine, follow manufacturers recommended ratios for the oil you wish to use. (ask them to find out...)
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Less oil in the mix does make the jetting richer.
I had a bit of brain fade there. It should be leaner jetting required.
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Could you post a link to the research that shows the benefits of using 40:1 over 25:1. Things like dyno comparisons or technical papers.
I have no research papers or dyno reports to speak of but I don’t dispute the power claims of the more oil brigade.
My experience from early days was running mineral oils at 20:1 and constantly suffering fouled plugs if I let my bike idle too long or rode slowly too long (no I was talking about riding in tricky terrain)
Then I moved to Synthetic oils and started running at 40:1 and hardly ever fouled a plug again.
Most of the reports I see about running more oil are usually about road race bikes or Karts that are nearly always running at full throttle as they are when they are on a dyno, most off road stuff is far removed from that. I have no doubt that more oil would be beneficial if running flat out all day.
I will take a bit more notice when a dyno simulates riding single track for 20 minutes or dropping down some knarly valley with the throttle chopped most of the way.
Bulk horsepower isn’t always necessary for off road stuff which is why it appeals to me as you can use many other skills to overcome obstacles and other competitors.
To finish first, first you must finish
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A 15yr old on 125 will be flat knacker on what ever terrain is in front of them, likewise when Shayne Watts smashed everyone in a 4 day on a KTM125 on1 set of tyres. ;)
The Gordon Jennings oil ratio test was completed on a PE250 with load applied to the dyno drum with the engine held at 5500rpm. Closed throttle is when you need the oil the most, reeds are not really opening and the rear wheel is driving the engine.
http://www.bridgestonemotorcycle.com/documents/oilpremix6.pdf
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Could you post a link to the research that shows the benefits of using 40:1 over 25:1. Things like dyno comparisons or technical papers.
I have no research papers or dyno reports to speak of but I don’t dispute the power claims of the more oil brigade.
My experience from early days was running mineral oils at 20:1 and constantly suffering fouled plugs if I let my bike idle too long or rode slowly too long (no I was talking about riding in tricky terrain)
Then I moved to Synthetic oils and started running at 40:1 and hardly ever fouled a plug again.
Most of the reports I see about running more oil are usually about road race bikes or Karts that are nearly always running at full throttle as they are when they are on a dyno, most off road stuff is far removed from that. I have no doubt that more oil would be beneficial if running flat out all day.
I will take a bit more notice when a dyno simulates riding single track for 20 minutes or dropping down some knarly valley with the throttle chopped most of the way.
Bulk horsepower isn’t always necessary for off road stuff which is why it appeals to me as you can use many other skills to overcome obstacles and other competitors.
To finish first, first you must finish
If you fouled plugs it was the tuning not the mix ratio. My first trials bike was a 1976 bultaco sherpa 250 which would idle all day with a 20:1 mineral oil and never foul a plug also got a new PE175 in 78 and it ran 20:1 R30 without oiling up plugs either.
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Ive run 75 to 100 to 1 in my 010 kato 300 and my wr 400. I only use motorex as it has a deep base construction allowing it to maintain the required film strength at these ratios.
r30 is a real light base oil and needs to be at 20: 1 to lube. don't use normal oils at leaner ratios than 40:1 other than full synthetics that have a deep base.
the bikes have run fine, start very well and the kato has just had a piston kit thru it for the same reasons ol mate changed his rings..cos I had the top off for a look...
the husky has done 3 Vinduro seasons and other riding on the piston and rings it had in it when I bought it. it runs very nicely at 200 rpm when you fluff a hill or a tricky bit and pulls out of trouble cleanly. I have the needle jet just on the lean side and run the leaner fuel to compensate. works well.
also note being water cooled helps. I would not be keen on going past 75:1 on an aircooled bike due to heat dispersion wanting more oil (in my tiny mind)
a few mates went to a kato ride in wa with leisky and he told them they were running 100:1 when they were fanging across some salt lake in top gear. spooked em a bit but it started us all running those style ratios
At end of the day its a personal choice and if the big end seizes out from lack of oil, I will go back to higher ratios but so far so good
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I don't have any of the dyno reports or any of the real time test reports that I had saved on my last PC. It all vanished when the lap top went to computer heaven. I will say however, that attempting to compare what fuel/oil ratios we all used to run our 2 strokes on back in the late 70's and 80's and what we do today isn't really comparing apples with apples. Modern synthetic oils are much better and actually cling to parts even at extreme high temps. And the metallurgy that comes with our replacement pistons and rings these days is so much better than it was back then. I'm not trying to convince anybody that they need to run leaner oil ratios just because I do and have positive experiences from it. Run what you like. It's your bike. I'm only talking about my experience.
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Here is some information from a Company that spent a lot of time & money (dyno & racing) researching this stuff.
Yamaha did as much as (or more than) anybody to develop the 2T engine technology that is relevant to our VMX era.
They determined that 2 stroke engines require different Fuel:Oil ratios at different engine speeds, loads & throttle openings.
This is the chart they released with the new Autolube system back in 1968.From Digga's article link.
(http://i1043.photobucket.com/albums/b431/DoggyDigger/autolube_zpscqfuz5r5.jpg) (http://s1043.photobucket.com/user/DoggyDigger/media/autolube_zpscqfuz5r5.jpg.html)
see the whole DTI thread here: http://forum.ozvmx.com/index.php?topic=37632.0
From this chart it is simple to see that 40:1 or even leaner, is more than sufficient for many riders.
Just locate your own "successfully tested" F:O ratio in the chart and then read off your average throttle setting & speed.
Of course We All Knew Better than Yamaha, and it was the first thing pulled off a DT or MX. ::)
Before we hear the argument; that technology & manufacturing & metallurgy has improve greatly since then, of course it has.
Modern Go-Kart engines have the full benefit of all off that and more but as Lozza said. they still need a lot of oil, just to survive.
In 1968, Yamaha were first and second place in 125cc & 250 cc GP championships, in 1970 they held the first 5 or 6 championship places in the 350cc.
Their technology was world leading then and was way beyond normal production standards for the era.
I might argue about lubrication improvements, as much of the 'progress' has been to designed to minimise environmental impacts (eg. reducing smoke levels and providing biodegradable emissions). From a 'pure lubrication' point of view we haven't bettered Castor Oil.
Better engineering has allowed oil manufacturers to get away with lesser grade base oils for their retail products.
No hard recommendations for any-one here, you are free to use what 'works', for you.
(But an honest look at the chart provides some clues).
Cheers, DJ
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Yep when synthetics stop working and break down film strength castor starts working. I only know of 1 engine where a specially developed full synthetic oil is specified over a castor/synthetic blend. With the engine probably worth 60,000Euros and fighting for a world title not many were prepared to experiment. From memory the Elf oil was $65 a litre.