The equation is:
CFM = (RPM times CID) / 3456
This equation good for four cycle multi-cylinders engines with four or more cylinders at 100 percent volumetric efficiency.
CFM is required airflow in cubic feet per minute
RPM is revolutions per minute of the engine
CID is engine displacement in cubic inches
For what its worth, the constant 3456 is actually 12 times 12 times 12 times 2. The three 12's are necessary to convert cubic inches (on the right) with cubic feet (on the left). The 2 is necessary as the intake valve opens once for 2 revolutions of the crankshaft.
Volumetric efficiency of a stock 307 is probably around 70 - 75 percent.
Most apply the equation only to WOT (wide open throttle) which is the right approach if always driving at wide open throttle.
However, the equation can be applied for any RPM. If one attempts to maximize runner velocity at cruise RPM, one will normally find that Wrenchbender's suggestion of a Q-Jet (spread-bore) carburetor is quite accurate. While most Q-Jets are 750 CFM (more than you need total, but 150 CFM on the primary), the "on-demand" feature of the airvalve on the secondary side allows the carburetor to only deliver that amount of CFM needed by the engine.
Oh, and that 1250 CFM the parts guy was trying to sell you??? Just wrap that 307 to about 14,000 RPM and the carb should be just right!
Jon.
