Depending on how your engine is set up 625 cfm may or may not be too much. The key is Volumetric Efficiency and your engine RPM range. If you are running a high RPM setup (cam, heads, intake) then 625 would be about right, but if you are running a street setup and the engine sees duty under about 6000 RPM, the 480 cfm would be about right.
I pulled this off of pony carburetor’s web site, (
http://www.ponycarburetors.com/default.asp?page=calculatingcfm) but you can find this formula in most good automotive publications and catalogs (it’s also on in the back of Edelbrock’s catalog for example).
"Calculating CFM
Actual Air/Fuel Mixture Intake = Volumetric Efficiency
Theoretical Air/Fuel Mixture
Engine/Carburetor Sizing Relationship
The larger the engine, the more air it's capable of "gulping". In math terms, it looks like this:
Cubic In. Displacement/2 X Maximum RPM/1728 = Maximum CFM
An Example:
289 Cubic Inches/2 x 8000 RPM/1728 = 669 CFM
This is still not the final answer for sizing a carburetor. The volumetric efficiency must be taken into consideration.
In our example a 289 @ 8000 RPM can use 669 CFM
however
660 CFM x .90 volumetric efficiency = 602 CFM
669 CFM x .85 volumetric efficiency = 568 CFM
Our example is based on a Hi-Po 289. Ford installed a 600 CFM Autolite 4100 on this engine. Did Ford engineers know what they were doing? You Bet!
A standard "A" code or "D" code 289 4 barrel with the factory valve train will not run much above 6000 RPM.
6000 RPM requires 501 CFM x .85 volumetric efficiency = 426 CFM. Ford installed a 480 CFM carburetor!"
For a 302 with max RPM of 6000 the numbers work out like this:
.90 VE = 471 cfm
.85 VE = 446 cfm
Hope this helps.