From:   Richard C Oeftering <Richard.C.Oeftering@nasa.gov>

>  When the exhaust valve opens a
> pulse of exhaust gas enters the exhaust pipe.  It's a sound wave, a POP,
> which travels down the pipe.  When the pulse hits the tail end of the pipe
> there is a reflection of the sound.  As it turns out, the reflected POP is
> inverted. That is, the reflected pulse is a POP of LOW pressure gas.
> This pulse travels back up the pipe to the exhaust valve and gets there
> (assuming a 65 inch pipe and 2600 rpm) while the valve is open for the
> next exhaust stroke. The returning pulse actaully sucks the exhaust gas
> out of the cylinder.

Jeff has it right.
The low pressure wave reflecting back toward the engine is the secret to
its operation.  The length of the pipe controls the timing of this wave and
thus the "tuning"  for a certain RPM.
BTW this low pressure wave is called a "rarefraction wave" by the folks who
work with shocks waves and acoustic waves.

Rich Oeftering

[ yea, that's what I was missing, thanks Jeff... using sound
energy to make a low pressure is a very cool thing.  -Rob ]