A guy named SnowBum gives a very good, semi-technical essay on his website about intake tuning, velocity, air flow, etc...
You can find it here
He's a BMW guy, but the principles are obviously the same.
It is lenghty, but a good read nonetheless

'05 America - Tec 2-1 (from a Thruxton)/ 4 pot caliper/ Kurt's Rearsets /