Plenum Area

[bruce plecan]

Plenum does not include any area inclusive of the runner walls.
While others will cite tuned lenghts, for runner lenghts, I have a better
rule for use, the shorter the runner the more willing the motor will be
to change rpm, acclerate.  The greater the cross sectional the higher
the rpm range (peak torque).  Yes, Yes, Yes I can not offer any source 
other than a bunch of time I've spent trying different manifolds.  For
OEM, optimized BMFC, little old lady engines, you can apply your laws
about dual planes, runner lenghts, and min plenum areas.  But. in the
mild to real hot rod street engines, I'll stick with what I have said
in the above.  Now, if you take another few steps toward race only like
in Pro Stock then You might want a large plenum, or in the old Can Am
cars they used like 2"+ diam runners 14" long, but those were I.R., with
individual butterflies, and no plenum.  If you working with a high swirl
(turbo)and spinning the motor really high then large plenums have aplace.
HTH  Bruce