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Variable exhaust backpressure.
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HeadTrip






PostPosted: Tue Mar 07, 2006 4:00 am    Post subject: Variable exhaust backpressure. Reply with quote

This may have been asked before, but I couldn't find it. Has anyone tried something like the veriable exhaust valve on the GSX-R1000s. pretty much a butterfly valve in the exhaust pipe to dynamically alter the backpressure. I know it really makes a diffrence in these bikes.

http://venus.13x.com/roadracingworld/issues/feb01/gsxr1000.htm
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PostPosted: Tue Mar 07, 2006 4:05 am    Post subject: Reply with quote

That's interesting. I've been researching exhaust for a short while now, and from all sources I've read, backpressure = BAD. As I understand it, increased exhaust gas velocity from smaller tubes increases low end torque. Which is why you lose some low end when getting a bigger exhaust or header.
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budiceman0






PostPosted: Tue Mar 07, 2006 4:06 am    Post subject: Reply with quote

I always wondered the same. Im my jetski I have water injection to add and remove volume.. But I think backpressure will add torque what we dont need. But maybe we'll see some other feedback
CHRIS
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HeadTrip






PostPosted: Tue Mar 07, 2006 4:10 am    Post subject: Reply with quote

If I can find a 2" high temp butterfly valve for a good price I will give it a shot. worst case I'm out a little change, I'm getting used to that with this car.
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PostPosted: Tue Mar 07, 2006 4:55 am    Post subject: Reply with quote

See my posts here at the bottom of the second and top of the third pages:

http://TSN.SaturnWiki.net/viewtopic.php?t=27192&postdays=0&postorder=asc&start=20

I would assume that maybe having the butterfly partially closed on the bike exhaust could reflect pressure waves back into the primaries at lower RPM, similar to the way a two stroke engine's "tuned pipe" works.

The valve probably never closes enough to make significant backpressure, per say, but maybe just far enough that part of the acoustic energy is reflected back through the system.

Two strokes use this principle as a significant amount of air/fuel mixture blows through the cylinder and into the exhaust pipe before the piston can cover the exhaust port. The reflected pressure (or acoustic) wave forces alot of that air/fuel back into the cylinder just as the piston is covering the exhaust port back up, effectively "supercharging" the engine.

These "tuned pipes" only work over a narrow RPM range, which is one of the biggest reasons high-output two strokes have such a peaky torque band. Once the engine revs into the tuned pipe's frequency range it's Volumetric Efficiency increases dramatically, kinda like a turbo "spooling", but for totally different reasons.

The "average" backpressure in a tuned pipe is probably quite low as to allow good overall flow, but the pressure wave reflecting back into the cylinder would be quite high.

I would assume that's the effect of that butterfly in the bike exhaust and could possibly force small amounts of air/fuel back into the cylinders due to the overlap in the cams, but I'm no expert, just throwing ideas around.
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