Carburetor CFM Calculator

Background.

Is a 600 CFM carburetor too much for a mild 355? The way to find out is to work out how much air the engine can actually swallow at the top of the rev range it uses, then hold that figure up against carburetor ratings. Engine airflow comes from three numbers: displacement, maximum rpm, and volumetric efficiency, the share of its swept volume the engine really fills on each intake stroke.

The formula is the one Edelbrock and Holley both publish for four-stroke engines: CFM = CID × RPM × VE ÷ 3,456. A 355 cubic-inch engine turning 4,500 rpm at 105% volumetric efficiency comes to about 485 CFM.

That number is the engine's demand, not a part number. Carburetor ratings are measured on a flow bench at a fixed pressure drop, and an oversized carburetor moves air too slowly through its venturis at low speed to meter fuel well, so check the estimate against the carburetor maker's application guidance.

What is carburetor cfm calculator?

Engine airflow in CFM is the volume of air, in cubic feet per minute, that an engine draws through its intake at a given speed. Each cylinder of a four-stroke engine takes one breath every two crankshaft turns, so at perfect filling the engine inhales half its displacement per revolution. Volumetric efficiency scales that ideal: below 100% when restrictions stop the cylinders filling completely, above 100% when tuned intake and exhaust pulses or a supercharger pack in more. A carburetor's CFM rating is a separate figure, the flow it passes on a test bench, and sizing means matching the two.

How to use this calculator.

1. 01Enter total engine displacement in cubic inches. To convert from litres, multiply by 61.0237; a 5.7 L engine is about 347.8 in³.
2. 02Enter the highest rpm the engine will actually run at under load, such as its power peak or shift point, rather than a rev limiter it rarely reaches.
3. 03Enter volumetric efficiency as a percentage, from dyno or flow data if you have it. Untuned street engines usually fall short of 100%; use a figure above 100 only with evidence for it.
4. 04Compare the result with carburetor ratings taken on the same test standard, and confirm the choice with the manufacturer's application guide.

The formula.

CFM = CID × RPM × (VE ÷ 100) ÷ 3,456. CID × RPM is the volume the pistons sweep in a minute, in cubic inches. A four-stroke cylinder fills on only every second revolution, so half that volume is drawn in, and dividing by 1,728 cubic inches per cubic foot converts it to cubic feet; the 2 and the 1,728 multiply to give 3,456. Volumetric efficiency then scales the ideal figure up or down.

Every input enters in direct proportion, so 10% more rpm or 10% more efficiency means 10% more air. The divisor is specific to four-stroke engines; a two-stroke, which fills once per revolution, would use 1,728 instead. The result is a volume flow at whatever density the air happens to have, with no correction for altitude or temperature.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Edelbrock, Performance Tech Discussion: gives the four-cycle CID × RPM × VE / 3456 airflow equation, worked examples and selection limitations.
2. [2]Holley, How to Choose Your Next Holley Carburetor: presents the same engine-airflow calculation and explains application factors.