400 cfm is all?
The stock throttle bodies flow more than that (I saw a write-up with them tested at 950 cfm when ported).
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Originally Posted by Gravity Fed
i cant think of a way without knowing the velocity of the air moved.
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Velocity is not the point here, but i will explain how to calculate it later. The point is air flow. Each rotor requires 0.65L of air per revolution, if you crunch some numbers, you can see what the engine will require.
I would put a peripheral ported rotary at 110% VE Average (based on port timing and overlap). This means that each revolution of the engine will require 0.71L of air.
If you figure your rev limiter at 9500 rpm, that gives you 9500 rev/m * 0.71 = 6745 L/min which gives 238 cfm. According to this math, your ITBs are plenty large.
Unfortunately, this math is not exactly accurate, as it assumes sufficient plenum volume to allow constant flow through the throttle plates. This is not so, as you will be mounting your ITBs as close to your P-ports as possible.
The air flow into an engine can be approximated as offset-sinusoidal (we will make this assumption for simplicity).
Therefore, the minimum air flow is 0 and the maximum air flow is 2X the average air flow. This means that the port will require approximately 2 * 238 or about 480 cfm at peak flow. Basically, you will have some pressure drop across your throttle plate during the intake stroke, but not a lot.
The runner should be sized with a velocity stack for the inlet, gradually reaching the same diameter as the throttle body, then with a constant cross-sectional area all the way from the throttle plate to the rotor housing. A slight decrease in section height while passing through the housing should give good fuel mixing characteristics.
To compute the velocity is easy, however it has no real bearing on anything unless you care about Helmholtz tuning. Never the less, you simply take the peak air flow (480 cfm) divide by the cross-sectional area of the runner in ft2 and divide by 60 to give ft/sec. this can then be converted to mach easily by guessing Mach 1 at about 1070 ft/s. You want 0.3 Mach (about 300ft/s) peak intake velocity for best resonance tuning effectiveness.
Tuning for 9,000 rpm, throttle body/runner diameter should be about 2.144" (about 2 - 1/8" or 54mm) Runner length should be around 9 inches from port beginning (cross-sectional area stabilizes at bottom of velocity stack) to port end (the combustion chamber)
These are all rules of thumb, but they are good ones. I think that you will find that your intuition is a good as a calculator on this.