I've never heard of a magic number for the plenum volume.  The one I've constructed for my RX-8 is approximately 150 in cube.  It is about 4 inches wide, 2 1/2  in tall on average and 15 inches long.  The is measured from the throttle body to where the four runners leave the plenum.  The four runners are actually an original RX-8 intake cut off just below the first rotary valve.

Dennis H.

the mallorys wrote:

Is there a magical number for the size of the plenum?  Minimum or maximum?  I had heard something about 50% of the 1.5l for the engine, but don't know if this applies to the rotary.

 

Chris