Dave said:

One final point that I dont know the answer to.  I think that the tuned
length will be shorter for a 20B because the effective rate at which the
ports are opening  would be increased, much like increasing the engine

RPM by

50%.  (and RPM has the most important effect on tuned length.)  If I am
guessing right, this would mean that a 20B's tuned length is 1/3 shorter

for

a given engine RPM.

Al Gietzen wrote:

I think Dave raises a good point.  Most of what is done with runner lengths
is guess work because of adapting what worked in one case or one engine, to
another.  Reusults for a 6-cylinder piston engine has little to do with a
rotary, other than to demonstrate the effect.  In considering "tuning" of
runner length; a configuration with a throttle body (butterfly) for each
runner is different than the case where the runner from one rotor
communicates with the other.   And effects we see with runner length may be
as much a result of chnages in the charge inertia in the runner as it is any
"tuning" of the compression waves.

As I understand it; the curve that Paul likes to show for the Le Mann engine
is for a TB for each runner (also, incidentally; a straight-in runner on a
peripheral port).  What Tracy and others have is the rotors communicating
with each other via a 'dynamic chamber' of sorts.  Case 1 reflects the
compression wave (pulse) from the closing of the port to the TB (or sudden
expansion at TB exit) and back, giving optimum runner length 1.  Case two
uses the pulse from one rotor to enhance the pressure into the other
resulting in optimum length 2; which is roughly 1/2 the runner length of
case 1.

Where the rotors 'communicate' with each other; the optimum length for a
3-rotor is obviously different than for a 2-rotor.  I did a fairly detailed
analysis for a 3-rotor (using a rotor timing chart like the excellent chart
the Marvin Kaye did for the 2-rotor); from which  I computed the optimum
length to be about 13".  But the analysis also indicates that it was not at
all critical because in the 3-rotor, the compression pulse from one rotor
will effect both other rotors; one just when the port is opening, and the
other later in the intake stroke; and the varying of length simple effects
the timing of the point in the stroke where the pulse occurs.  Basically it
just becomes to complex to handle analytically, and you have to go to the
dyno.

Also consider that when you want the performance is when the butterfly is
fully (or close to fully) open; so depending on where your TB is, the
configuration (and length) upstream from the butterfly matters.  And for a
aircraft flying at altutude, it is likely that your cruise power may be at
WOT.

Let's see, what was the point of all this.  Oh, yeah; optimum runner length
is not obvious; and it is very likely going to be different for a 3-rotor
than a 2-rotor.

Al Gietzen

Come on guys. That is a secondary effect. Perhaps the size
of the plenum changes with the number of rotors butnot the pipes. 
Think of three one rotor engine connected together with a long shaft. 
It is quite obvious every Mazda rotary since about 1985 has had long 
pipes all about the same length. If you are thinking there 
is some sort of magic to somehow make a short pipe manifold work 
as well as a long pipe manifold that is pure wishful thinking.

Paul Lamar
 
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