OzVMX Forum
Clubroom => General Discussion => Topic started by: TooFastTim on March 21, 2015, 07:53:29 am
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It looks like it's an expansion chamber. Would one of our more clued-up engine experts explain why it's needed?
(http://motocrossactionmag.com/wp-content/uploads/2014/12/pipefe450.jpg)
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Its just a resonance chamber. The pipe is still a header, but with some holes drilled in it. The 2 stroke part that you see is simply two pressed plates welded each side of the pipe where the holes are. A bit like a muffler. It changes the tune of the bike, but someone else on here may know how it does that.
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they used to have that little coke can welded on the top, this is a new flasher look
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Is it a new power bomb pipe . I have seen them done like this a few years back .
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I had a theory many years back that some four strokes may benefit from an expansion chamber.
A chamber on a 2 stroke ‘basically’ controls the flow of new fuel into the engine while all the ports are open.
I figured that a four stroke that has a valve overlap may benefit from a small chamber that may help control new fuel into the engine while all the valves are open.
Never got a chance to try anything so it was just one of many untried theories that bounce around inside my head.
Maybe KTM came up with the same idea?
Or maybe it’s just a gimmick ;D
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I had a theory many years back that some four strokes may benefit from an expansion chamber.
Never got a chance to try anything so it was just one of many untried theories that bounce around inside my head.
Maybe KTM came up with the same idea?
You should start wearing an alfoil hat to thwart any future industrial espionage. ;D
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may be similiar to this
http://www.fmfracing.com/Products/MX/44
http://www.motosport.com/gytr-fmf-powerbomb-head-pipe
Quote
""""The principal is this: What makes a 4-stroke exhaust "tuned" is the timing of pressure waves within the system, and having them synchronize with a certain range of engine speeds. The timing is controlled by varying the diameter and length (mostly the length, as the usable diameter is limited). The range tends to be narrowly focused, and you normally have to choose top end over low end, or vice-versa. The chamber in the 'Bombs has a damping effect on the pressure waves within the exhaust, and the result is that the engine "sees" the header as not having a fixed length. This in turn has the effect of widening and flattening the power curve, sometimes quite remarkably. Most such FMF exhausts will seldom produce as much peak power as other pipes, but quite often produce a wider power curve than most others. """
Or it could be a gimmick ::)
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The prayer of the salesperson:
If ya cant dazzle um with brilliance , Baffle um with bullshit. 8)
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The prayer of the salesperson:
If ya cant dazzle um with brilliance , Baffle um with bullshit. 8)
;D Seems FMF just copied someone else and marketing had to invent some schtick to go with it.
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One of the few attractions for me of modern 4T off road bikes is that the exhaust is out of the way and not exposed to rocks, stumps etc. If things carry on like this even that (slim) benefit that a 4T has over a 2T will evaporate.
Now, I'm reasonably clued up on the concept of resonance and how it turned the 2T motor from a stationary, generator motor into the thing of beauty that we all know and love and why we all worship at the alter of Kaaden, Degner and Blair but, damnit man a 4T's got valves! Why do you need a resonant pipe to duplicate what the valves are already doing?
and the result is that the engine "sees" the header as not having a fixed length.
Not arguing against you Noel but the whole point of an expansion chamber is that it is tuned for a fixed frequency (in the case of a 2T) how can a tuned cavity do the opposite?
As others have mentioned it looks like its all marketing BS. Gotta have one to match my new Oakley sunnies.
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"damnit man a 4T's got valves! Why do you need a resonant pipe to duplicate what the valves are already doing?"
Performance 4-strokes have a considerable overlap period where both the inlet & exhaust are open at the same time.
Just like a 2-stroke.
The first purpose of the 2-Stroke expansion chamber is to create a negative (sound/pressure) wave to extract exhaust gasses and to reduce pressure in the crankcase and inlet tract, allowing atmospheric pressure to push more air/fuel mixture into the engine than would occur otherwise.
The second purpose of the expansion chamber is to create a positive (sound/pressure) wave, timed to push fresh charge, that has entered the exhaust port & header pipe, back into the engine before the exhaust port closes.
A similar, but obviously smaller, effect can be produced for 4 stroke engines with an expanding & contracting chamber in the pipe.
As far as full expansion chambers for 4-strokes goes, I spent a fair bit of time with Lotus's Sim program, designing pipes for the mighty SL100/125.
In my attempts, the full expansion chamber did not ever produce a higher output than a tuned length pipe.
What it could do for the little Honda was produce an almost completely flat torque curve across the entire rev range.
Might be good for 4-Stroke trial engines, but not MX.
Cheers, DJ
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and the result is that the engine "sees" the header as not having a fixed length.
Not arguing against you Noel but the whole point of an expansion chamber is that it is tuned for a fixed frequency (in the case of a 2T) how can a tuned cavity do the opposite?
Noel's answer is pretty good, and is backed up with my attempts to create a full 4-Stroke chamber producing a flat torque curve..
IF KTM designed this, it is possible that it has been 'tuned' to be effective below the optimal engine speed.
This would allow a 'wilder' cam, with greater overlap for peak HP, but then allow the engine to operate below that RPM.
Effectively producing more peak HP & widening the power-band. Paradise.
Don't know, just throwing it in the mix.
Cheers, Daryl.
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These have been around for years as aftermarket systems, fmf has a patent out on the powerbomb which must be about 15 yrs old by now. Looks like KTM have decided to fit them standard.
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Maybe KTM have come up with a new type of engine. A hybrid 2 stroke / 4 stroke engine with valves and port. It would be a 3 stroke or 2x4 = 8 stroke.
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Maybe like this one?
(https://s3.amazonaws.com/WEBPOSTS/OZVMX/2-4+sight.JPG)
DJ
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Years of formulating and testing of modified SL125 pipes has led to this being the best all round system.(http://i951.photobucket.com/albums/ad360/tdee754/104_0343_zpsbbd41556.jpg) (http://s951.photobucket.com/user/tdee754/media/104_0343_zpsbbd41556.jpg.html)
or perhaps it was just shite hanging around the shed I cut and shut for looks..... ;)
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Check out these old SL125 "racing" bits from some bloke called Pops Yoshimura or something like that. What would he/they know, No fancy pants foofer valves or malignant fungus growths on their pipes... .
(http://i951.photobucket.com/albums/ad360/tdee754/Yosh_Catalog_125_3_zpszqh2whri.jpg) (http://s951.photobucket.com/user/tdee754/media/Yosh_Catalog_125_3_zpszqh2whri.jpg.html)
(http://i951.photobucket.com/albums/ad360/tdee754/Yosh_Catalog_250_6_zpsvci7fpz8.jpg) (http://s951.photobucket.com/user/tdee754/media/Yosh_Catalog_250_6_zpsvci7fpz8.jpg.html)
Look at the prices...... :-[
1974/80 advertisement....
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I'm definitely in the market for at least 3 Yoshimura racing crankshafts at $27.50 ea.
The other bits look reasonably priced as well, (Japanese recession/stagflation?)
I 'lost' the ball cage from one of my (44 y.o.) main bearings at the Masters Games.
Here is my MX/Flat Track pipe.
It has a small "finger proof" baffle as an "effective silencer."
It has also Passed an MQ sound test at Nudgee flat track.
(https://s3.amazonaws.com/WEBPOSTS/OZVMX/SL!%40%25+MX+pipe.jpg)
The expanded chamber pipe was built for the CMX Nationals at Barrabool in 1996, Cherabah in 1994, and so preceded FMF's patents by a fair bit.
Cheers, DJ
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looking at taht KTM thingy makes me think we need to keep the same volume of pipe so if we make this thing we wont have a pipe sticking to far out past the engine where it runs past the cylinder and everyone will think its a real trick bit but more just to stop the rider getting burnt anymore than buying a KTM. well really thats my thinking.and no diagrams explaining the suck squeeze bang and blow.. Lol
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......and no diagrams explaining the suck squeeze bang and blow.. Lol
Chris, you could be right about the flattened chamber for leg clearance, with the side benefit of a "trick" look.
Don't discount, and Lol, the Ricardo diagram, though.
Look closely and you will (could/should) see that it is a 2-Stroke cycle (with valves).
The whole process is completed in 1 revolution, not 2.
Cheers, Daryl.
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Most of the power bombs and cans, create a longer header volume at low RPM giving better performance down low while retaining the benefit of a short header at higher RPM.
This KTM example is looking more like a 2 stroke chamber; which is a little more complex and more effective.
The same theory applies to 2 strokes and 4 strokes,
all about timming pressure pulses at the speed of sound to assist extraction of spent gasses and cylinder filling of fresh charge,
only happens half as often with a 4 stroke for the same RPM, so gains will be half of a 2 strokes gain of the same capacity.
In theory there should be no reason a 2 stroke style chamber wont work on a 4 stroke if you dont mind the volume of the chamber.
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Fark, it is a 4-choke pipe ..... any old crap pipe laying around that is the right diameter will do the trick.
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I had a theory many years back that some four strokes may benefit from an expansion chamber.
Never got a chance to try anything so it was just one of many untried theories that bounce around inside my head.
Maybe KTM came up with the same idea?
You should start wearing an alfoil hat to thwart any future industrial espionage. ;D
I have such a hat but the voices inside my head told me not to put the hat on :o
Do you think it's too late?
Performance 4-strokes have a considerable overlap period where both the inlet & exhaust are open at the same time.
Just like a 2-stroke.
As far as full expansion chambers for 4-strokes goes, I spent a fair bit of time with Lotus's Sim program, designing pipes for the mighty SL100/125.
In my attempts, the full expansion chamber did not ever produce a higher output than a tuned length pipe.
What's your definition of a "full chamber"?
As the valve overlap is only short compared to the open ports of a 2 stroke I had imagined a chamber being short and small for a 4 stroke not unlike the KTM version.
Also on the KTM above the pipe does appear to be an expansion chamber, as in an open chamber like a 2 stroke.
The FMF power bombs have a tube running all the way through the 'bulb' with just a 10mm hole into the cavity part.
So the big question is what's inside this KTM pipe?
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That hole would 'disappear' over a certain rpm, there would be the similar in the KTM bulge, the 4T engine has no benefit from an expansion chamber when a simple step will do the same thing
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"What's your definition of a "full chamber"?
As the valve overlap is only short compared to the open ports of a 2 stroke I had imagined a chamber being short and small for a 4 stroke not unlike the KTM version".
My definition of a "full chamber" was a Two-stroke type chamber with header, diffuser, centre section, baffle cone, and restricted exit (stinger) to maintain a working pressure in the pipe.
Lengths varied between about 300mm to 1.2m. & max diam 100mm (the practical limits for fitting on a SL100/125 VMX bike).
Yes, the transfer (overlap) is shorter but the time span from exhaust valve open to exhaust valve closed is a bit longer
Exhaust pulse strength & duration is different, as the exhaust valve 'window' is smaller and more restricted and after initial "blow-down" the
exhaust flow is provided by the rising piston.
What I was looking for, was a long tapered diffuser to create & maintain a long duration negative pulse from ExVO to IVO.
This negative pulse was also intended to draw fresh charge into the cylinder and into the exhaust port at IVO.
As with a Two-stroke, it was then the job of the baffle cone to generate a pressure pulse to push the escaped charge back into the cylinder before EXVC.
Not too dissimilar to a reverse cone megaphone, but with pressure to create more sound wave strength.
(In the vacuum of space, no-one can hear you scream).
Where things get more complicated is during the remainder of the induction & compression strokes (c. 360deg.).
The exhaust pipe is only communicating with the back of the closed exhaust valve.
Waves are bouncing and reflecting and inverting, up and down the pipe.
Pressure is bleeding out, with no new pulse to top it up.
AND, If we produce a positive pressure wave at next EXVO, we have taken a big step backwards.
I only tried what I could at the time, within the limits of my understanding and the capabilities of a "free" down-loaded Sim program.
THE ANSWER is still Out There.:o Put on your tin-foil hats, and get into it.
cheers, DJ
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The FMF power bombs have a tube running all the way through the 'bulb' with just a 10mm hole into the cavity part.
That hole would 'disappear' over a certain rpm, there would be the similar in the KTM bulge
My old SR500 had a chamber, off the side of the pipe, under the engine, the communication hole was also about 10mm.
Back in those days, it was generally considered that this was to smooth-out the exhaust note, by reducing the strength of the pulses.
Of course, we cut them off, to make the bikes FASTER (noisier). :P
Cheers, DJ.
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Sounds like one of these
(http://www.pakwheels.com/forums/attachments/wheels-fan-clubs/261716d1309430095-yamaha-rx115-owners-fan-club-yeis1.jpg)
or is it more like one of these?
(http://resources0.news.com.au/images/2013/08/09/1226694/550208-peter-brock.jpg)
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"Now hold still. This is gonna hurt"
Oww Ouch! Nice one Pokey ;D