Page 83 Scientific Design for Exhaust & Intake Systems book
by Smith & Morrison Robert Bentley ISBN 0-8376-0309-9. 

Max torque for a four cylinder engine at 6000 RPM is a pipe
length of 34 inches. Sound familiar for the rotary? Mazda 
concurs with their dyno tuning as did Everett Hatch.

Makes sense as wave tuning deals the speed of sound in the pipe 
alone. In this case they claim 1700 foot per minute for the wave 
speed. Twice my initial guess. They go on to say it has been
measured at 1300 feet per second in other experiments.

A very simple formula from the book for a tuned intake pipe is:

L = (72 X C)/N

L = length of the pipe in inches.
N = RPM
C = the velocity of sound in feet per second.
Varies from 1100 to 1700 take your pick depending on temp.
The lower the temp the lower the speed.

Plugging our numbers in gives 72 x 1100 / 6000 = 13.2 inches.

This is the formula used by Chrysler with their ram tuned
V8 engines which were quite successful.
 
I highly recommend this book and I think 32 to 34 inches is 
the right place for the exhaust collector.

One of the things I would like to do with the dyno is measure the pressure
along the length of an exhaust pipe in real time. Low on the priority 
list right at the moment but us nerds have to have fun too :)
 
Paul Lamar
 
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