"the effective rate at which the ports are opening would be increased"

 Why???

 I think you are off on this one Dave. Think of it as three one rotor

engines

 flying in formation :-)

 Paul Lamar

:-)  It is very possible that I dont fully understand tuning.  The way I
picture it goes like this.  Air is rushing into one port and momentum gets
built up.  Then the port slams closed.  The air then slams into the closed
port and bounces back.  Now there is a pressure wave traveling back along the
intake.  The idea in tuning is to time it so that this pressure wave arrives
at the next port just before it closes to slam in that last little bit of
'air'.

In a 2 rotor, the ports are closing 180 deg apart.  This means that at 6000
RPM there is 0.5 ms between port closings so you need to set the tube length
so this pressure wave will travel out and back in 0.5 ms.  (or just less than
.5 ms, allowing some time for the wave to "bounce" and arrive at the next
port before it closes)

Each rotor has individual pipes. Any one rotor is 6000 RPM. 
The light bulb will come on any second Dave :-) PL

In a 3 rotor, the next port closes in 120 deg.  This gives the pressure wave
only 0.33 ms to make the trip so the distance needs to about 1/3'd shorter.

If you tell me that my pressure wave theory is all wrong and that tuning
works because of a harmonic oscillation of the air within each individual
tube not involving the other ports, then I am wrong.  But why then do Tracy
and Mazda go to the trouble of rounding out the plenum?  Seems to me they are
redirecting the energy back around into the ports of the other rotor.

Sound would travel about 66 inches in 0.5ms (66.3" at 70 deg F, 64.8" at
32F).  I assume a pressure wave would travel about as fast (slightly slower
because of drag).  Leaving some time for the "bouncing" and the distance
traveled in the plenum, this puts our runner length in the high to mid 20's
for a 13B at 6000rpm.

Sound would travel about 44 inches in 0.33ms.  The dynamics of a 20B plenum
make the situation a little more difficult.  Largely because this pressure
wave is now being split between two rotors, one of which isn't ready yet.
Also, unless you made a very fancy plenum, the distance between rotors 1&3 is
different than between 2&3 and 1&2.  But assuming "bounce" time and plenum
travel is worth about 12", that leaves the tuned length of a 20B intake at
6000rpm at about 16".

It doesn't hurt much for the wave to get there a little too early, the port
is still open.  But if it gets there late, it does no good at all.  So
slightly too short would be better than slightly too long.

All this is my own conjecture and will likely promptly be proven wrong by
experiment or wiser people.

Does anyone know what the length of the stock 20B intake was?

Dave Leonard

Long.

Paul Lamar
 
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