Porting the head only increases air flow if the rpm calls for more air flow. If the heads have really large openings to gain air flow, at low rpm the engine becomes real sluggish and fuel hungry due to the low air velocity. A well ported head will keep the air velocity high at low lift and flow enough air at high lift to allow the engine to run reasonably well. I have killed enough heads porting on them trying to chase air flow on my air flow bench.
I made a crude gauge for my truck that hooked to the throttle pedal, it measured how much throttle I was using just to drive around town and out on the highway. 90% of all my driving was using 25% or less of the total throttle opening. So when you go figuring absolute cfm's needed, don't figure it for top rpm, you won't ever need it!
