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Originally Posted by Larry Navarro
Very interesting info but not necessarily limited to "street" cars. 
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An example:
Theoretical cfm (total for engine) = (rpm x cid)/3456
Q (flow rate in cfm) = V (velocity in ft/min) x A (area of the inlet/port/carb in sqft.)
So theoretically (not taking in factors like ram tuning, friction etc.) for a 467cid engine with a 4.25in stroke will need at 6,000rpm a flow rate of,
Q = (6,000rpm x 467ci)/3456 = 811cfm total (carb size)
Now divide that by 8 and you need 101.4cfm(ft^3/min) per cyl.
Q=VA, 101.4 = VA.
At 6,000 rpm your piston velocity for a 4.25in stroke is equal to:
piston velocity(V) = (stroke(in) x rpm)/6
V = (4.25in x 6,000rpm)/6
V = 4250ft/min
Q = VA
101.4ft^3/min = 4250ft/min x A
A = 0.0238ft^2 = 3.42in^2
So you need an intake port 3.42 square inches to feed each cyl. at 6,000rpm. That's an intake port 1.848in x 1.848in.
That is just an example of the port size you need to get the right velocity to match the velocity needed to fill the void left by the piston at 6,000rpm for the above engine in theory. That doesn't say a thing about how well the port will work at lower or higher rpm or about friction, ram tuning etc, it's just straight flow rate at 6,000rpm but that is the basic relationship of flow to "volume" (port area) vs. cid and rpm.