How do you calculate carburettor CFM?

Multiply the engine displacement in cubic inches by the peak RPM, multiply by the volumetric efficiency as a decimal, then divide by 3456. For a 350 cubic inch engine at 6,000 rpm and 85% efficiency that is 350 x 6000 x 0.85 / 3456, which is about 516 CFM. The 3456 comes from 1728 cubic inches per cubic foot times two engine revolutions per intake stroke.

What volumetric efficiency should I use?

A stock street engine is usually around 80 to 85%. A well-tuned performance engine with good heads and cam can reach 90 to 100%, and a forced-induction engine can exceed 100%. Using a realistic figure for your build gives a more useful CFM number than assuming a perfect 100%.

Should I round the carburettor up or down?

Pick a carburettor rated at or a little above the calculated CFM. A slightly larger carb rarely hurts a performance engine, but a much larger one can hurt throttle response and low-speed driveability, while too small a carb caps top-end power. Match it to how the engine is actually used.

Carburettor CFM Calculator

This carburettor CFM calculator works out how much air, in cubic feet per minute, your engine needs at full noise, which is the starting point for choosing a carburettor (or carburetor) that is neither too big nor too small. You enter the engine displacement in cubic inches, the RPM you expect to peak at, and a volumetric efficiency figure that reflects how well the engine actually fills its cylinders, and it returns the airflow demand. The formula is the standard one: displacement times RPM times volumetric efficiency, divided by 3456. That constant comes from there being 1728 cubic inches in a cubic foot, doubled because a four-stroke engine takes in a full charge only every second revolution. A stock street engine usually sits around 80 to 85% volumetric efficiency, a sharp performance build can reach 90 to 100%, and forced induction can push past 100%, so the figure you choose matters. Pick a carburettor rated at or slightly above the result: a touch of extra capacity rarely hurts a performance engine, but a wildly oversized carb dulls throttle response and low-speed manners while an undersized one caps top-end power. If you think in litres, remember one litre is about 61 cubic inches, so a 5.0 litre V8 is roughly 305 cubic inches. Treat the number as a sizing guide rather than a final tuning spec.

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Engine displacement

cu in

Peak RPM

rpm

Volumetric efficiency

516 CFM

airflow your engine needs

At 100% efficiency608 CFM

Volumetric efficiency85%

Peak RPM6,000 rpm

A 350 cu in engine peaking at 6,000 rpm needs about 516 CFM at 85% volumetric efficiency. Choose a carburettor rated at or a little above this.

Airflow demand is a sizing guide, not a tuning spec. Real needs vary with heads, camshaft, intake and how the engine is driven. Match the carburettor to the build.

How it works

The airflow demand is CFM = displacement x RPM x volumetric efficiency / 3456, with displacement in cubic inches, RPM at the peak you plan to run, and volumetric efficiency entered as a percentage. The 3456 divisor is 1728 cubic inches per cubic foot multiplied by two, because a four-stroke engine draws a fresh charge once every two crankshaft revolutions. The "at 100% efficiency" figure above is the same sum with efficiency set to 100%, which is the theoretical ceiling and a common reference point for carburettor ratings. Your real demand is that ceiling scaled down by how completely the engine fills its cylinders.

Worked example

Take a 350 cubic inch V8 that you plan to spin to 6,000 rpm, running a realistic 85% volumetric efficiency for a mild street build. The airflow is 350 x 6000 x 0.85 / 3456. That is 1,785,000 divided by 3456, which comes to about 516 CFM. At a theoretical 100% efficiency the same engine would want 350 x 6000 / 3456, or about 608 CFM. So a common 600 to 650 CFM carburettor suits this engine well, giving a little headroom without being so large that it hurts throttle response.

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