Intake runner sizing..

[Edward Hernandez R]

"If that's the case should the primarys be long and the secondaries 
short?"
snip
"That is basically true.  If you are using the secondary control 
valves, then, yes you could tune the secondaries for high rpm use 
(3500 and up) and tune the primaries for low rpm use (less than 3500 
rpm)."

This is exactly what we do in the 1996 3.8L and 4.2L Split Port 
engines we put in the new F-150 and Windstar. RPM switchpoint is 
entirely dependent on the design of the intake, so it doesn't have to 
be 3500rpm. This is extremely effective. SAE paper coming shortly. I'm
glad someone pointed out the Mazda race engine. That was brilliant, 
the ulimate execution of a variable resonance induction system.

"converts the equations into something useable by rotaries."

Somone later posted the correct equation(referred to in the industry 
as the Helmholtz method). For mean cylinder displacement, divide the 
cylinder displacent in half regardless of whether it's a piston or 
rotor. CAUTION: This only works for 4 cylinders or less, and will
 approximate the rpm where volumetric efficient will peak, not how 
high the peaks will be. If you're good and select the right
combination of lenghts and areas, you will get two peaks. The SAE 
paper where this can be found is 900680. The original which this paper
 is based on is an ASME paper by H.W. Engelman "Design of a Tuned 
Intake Manifold". The second is a great tuned intake primer for this
 list, the first has a sample calulation.

"Why use RPM sensing? The tuned length should change with air temp, 
pressure, etc."

 No, the character of the curve you get with a given intake geometry 
changes with RPM only. Air temp and pressure can be considered DC 
offsets for that curve.

"I would assume that greater air flow would lead to  higher MAP values
and the ECM would adjust the fuel mixture  accordingly."

Greater airflow does not necessarily relate to greater MAP pressure 
because most MAP sensors are located in the plenum of an intake. If 
you improve the airflow downstream of the plenum, the MAP sensor will 
never know it. Furthermore, a nicely tuned intake will not affect MAP 
yet can really boost engine performance, particularly at low rpm where
 airflow is low and corresponding flow losses are negligible.