I'm finding out it isn't all that simple. I'm learning that it has much to do with the different cylinder heads and intake manifolds. Presently I've gained about 45 cfm by porting the intake runners and gained very little with the intake/head combination. I've got to get the carburetor base and plenum done and see what happens.
Until you test the specific head and intake, you are not going to know what is needed. That's my opinion anyway.
What is it again?.........intake runner volume should be the same as the engine cubic inch size, or is it plenum size the same as the engine cubic inch size? Or plenum and runner combined? Or is it as simple as X(head port cfm) = intake manifold runner cfm? I don't know!
Humbug, I'm not going to know anything but what this engine does with what we give it.