Jerry Hey wrote:

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

Questions:  at any particular RPM is the exhaust temp constant and
predictable?  It seems like the exhaust pipe length would have to
vary with the mixture.  Tracy is operating his engine one or two
hundred degrees below peak and most of  us want to do likewise.  How
does this reality interface with the ideal exhaust pipe length?  If
you tune you engine for 6000 rpm, is it then out of tune at 5500 rpm
or 6500 rpm?   Does a pipe become gradually more in tune as it
approaches the ideal length or is it suddenly in or out of tune.
Example,  I was trying to attain 32inch pipe length but had to settle
for 28 inches.  Does this mean that to have a tuned engine all I
would have to do is operate it at a higher rpm where 28 inches was
the perfect length?        Should the engine be tuned for max
performance at 6000 rpm or should it be tuned for best cruise
performance?    And so on.......Jerry

True but it does not seem to be that critical. +/- 500 RPM 
will not make that much of a difference.

Take your pick. If you tune for cruise you might be able
to reduce the fuel burn.

Paul Lamar
 
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