Ken Powell wrote:

It's been 10 years since I last modified an expansion chamber, but I
think this post has it backwackwards.  A short / fat pipe is for high
RPM, and a long skinny pipe is for low RPM.  If you will remember, the
trials bikes did not appear to have an expansion chamber at all, 125CC
motocross pipes were short and fat, and Open class bikes had pipes
with a thin center section.  Could be wrong, but this is how I
remember it.

Ken Powell

That's the way we do it with 4 cycle engines
on the dyno, Ken.
Archie

Funny, I can remember reading that book that Paul is speaking of, "Scientific Design of Exhaust and Intake Systems" (specific parts) many, many years ago.  You guys are into the practical part, not the theory, but it seems to agree pretty well as I recall.

I know this sounds weird, but if I remember correctly, maximum negative pulse at the exhaust valve (or port in our case) was achieved (other than by varying cross sectional size and length) by using a megaphone (to strengthen the wave) at the end of the pipe.  'Megaphoning" out and then back down is essentially your "expansion chamber", right?

What do you estimate could be gained by proper design of an expansion chamber at the end of the pipe of one of our aero rotaries?  Sound amplification by use of a megaphone seems awfully counterproductive considering everybody's quest for a better muffler, but isn't a properly designed expansion chamber really no louder than a straight pipe?  With one, can't the pipe be shortened appreciably and still maintain the same tuning effect?   Perhaps on a harmonic?  Maybe a muffler could be designed that utilizes the effect.

Noticed recently that my RX-7 has bell mouthed mini-megaphones hiding under the chromed "exhaust tips".  What did Mazda know, that we don't?

Just curious.

Dale Smith

 
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