[Trackrash]

Originally Posted by Bill Verburg

You'll notice that 8k a 2.7 processes the same volume of intake air as a 3.8 @ 5.5k under theoretical condition of 100% volumetric efficiency

the 2.7 operating up to 8k may want the same exhaust as a 3.8 operating to 6.5k

general rule of thumb for the eigen values of stockish street motors all ~31" primaries +/-
to 3.0 1 3/8" ID
to 3.4 1 1/2" ID
to 4.0 1 5/8" ID

That makes sense. Although I have seen formulas that predict flow and velocity related to RPM and cylinder volume.