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How to Calculate Engine CFM Intake

by John Stevens J.D.

As used in the automotive context, CFM, or cubic feet per minute, refers to the quantity of air and fuel the fuel delivery system can provide an engine through the intake manifold. Increasing the CFM of the fuel delivery system is commonly done to increase horsepower. However, equipping the engine with a fuel delivery system that provides too much CFM will have the opposite effect. Fortunately, calculating the optimum CFM for a given engine is simple.

Step 1

Multiply the engine’s highest expected revolutions per minute (RPMs) by the displacement of the engine, as measured in cubic inches. For example, if the engine is not expected to reach speeds in excess of 6,000 RPMs, and if the size of the engine is 350 cubic inches, then 6,000 x 350 = 2,100,000.

Step 2

Divide the result from Step 1 by 3,456. For example, 2,100,000 / 3,456 = 607.638.

Multiply the result from Step 3 by .85, which represents a standard street engine’s volumetric efficiency. For example, 607.638 x .85 = 516.49. Therefore, the engine should be equipped with a fuel delivery system capable of providing between 500 and 550 CFM.

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