Figuring CFM Requirments For An Engine

[68 Ford Stepside]

I have shoe boxes full of tidbits of info I saved over the years. Some of it is helpful, some outdated, some trash. Every now & then I'll post anything I think would be helpful to members here.


Air Flow

Determining an engine’s airflow requirements is fairly easy if you know its volumetric efficiency (VE). A good running 2-valve pushrod V8 is probably in the 85-95% VE range. A highly tuned race motor may exceed 100% VE. To simplify the formula most people use this formula that solves for 100% VE and then adjust the final flow number up or down.


CFM = CID (displacement) X RPM
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For example our 600 hp hypothetical engine has 400 cid and revs to 7,000 rpm. When these numbers are plugged into the formula we come up with an airflow requirement of 810 cfm. If you believe your engine has an 85% VE you multiply 810 X .85 = 788 cfm. That is why a 750 cfm carb works so well on this extremely popular engine combination.