<snip>
First a collector system won't work on a three rotor.
Tuned pipes will help of course.

The exhaust pulse travels down the pipe at the speed of sound.
As the RPM changes the distance between the pulse changes.
Higher RPM shorter distance. Optimum length occurs as one
pulse is leaving the collector and another is being emitted by
the engine.

The speed of sound is about 880 feet per second.
Each rotor fires once per rev. At 6000 RPM or 100 RPS
pulses are every 1/100 of a second apart or .01 second.
It takes the pulse about .003 seconds to travel 32 inches.
That is an exact multiple of .01 second. So the pressure
wave is bouncing back and fourth in the pipe three times
and the low pressure part of the wave arrives back at
the exhaust port just as the port opens again. This
also sync's in the collector enhancing the low pressure
part of the wave in the other pipe. Multiples of 32 inches
also work such as 16 inches at 6000 RPM.

Paul Lamar

Paul,

Attached is a quote from an old message dealing with optimum exhaust

runner

length.  I am beginning my 3-rotor exhaust system and would

appreciate a

clarification on a couple of points.  I will be using Tracy's 2.85:1

psru.

1)  A collector won't help any on a 3-rotor.  If this works on other
multi-cylinder engines, then why not on a 3-rotor wankel?

Long story. The pulses are 120 degrees apart and not 180 degrees as in

many

multi cylinder engines.

2)  If I understand your sound wave analysis correctly, then based

on

distance the sound wave will travel in .001 seconds would result in

a

runner length of 10-1/2".  From what I've read, this would be the

length

of

the initial 2" dia. runner tube, before it dumps into some larger

sized

pipe, 3" in my case?

3)  How much difference will the intake runner size make in

determining

the

optimum exhaust runner length?  The graph you refer to in the past

are

for

a 170mm intake runner length.

My head is beginning to throb!

Thanks,
Mark S.

As it should. Most of the exhaust data from Mazda was for a peripheral

port

engines. There is a strong interaction on PP engines. For a side port

engine you

can probably treat them as separate systems.

Paul Lamar

On the intake side the speed of sound is roughly the same as ambient air
or about 1100 fps - it increases with combustion temperature and

chemistry,

probably closer to 2000 ft /sec I could check references if you want
to pursue this - Vance

Yes I made a mistake in the message. Corrected in later messages
in that thread.  It varies widely in exhaust pipes.

So you are saying speed of sound at sea level is 750 MPH?

Paul Lamar

Yep!! at 68 degrees dry air the book says 1126fps or 343.3 m/sec
at 1340 F  2032 fps - Vance

For some reason or another I have thought for years it was lower
than that. You live and learn. I guess we are going to have to go
to thinner wings and area rule to get much past 400 mph or is it a prop limitation?

Paul Lamar
 
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