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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cu in
rpm
%
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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Frequently asked questions

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.