How to Calculate Volumetric Efficiency

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Engine performance is generally a reflection of the ability to intake air flow. Theoretically, there are many factors that can influence the torque an engine can produce. However, according to EPI, a manufacture of piston engines, the fundamental determinant is the mass of air the engine can ingest into the cylinders. There are two factors to be considered in this process -- the theoretical flow and the actual flow -- and it is the comparison of the two that determines the ratio known as volumetric efficiency.

  • State the formula. Volumetric efficiency is actual volumetric air flow rate divided by theoretical air flow rate times 100.

  • Calculate actual volumetric air flow rate (AVF). Determine AVF by dividing the mass flow rate by the density of intake air.

  • Determine the theoretical air flow rate (TVF). Calculate TVF by multiplying the engine displacement times maximum RPM times volumetric efficiency divided by the engine stroke times the conversion factor. (Fahrenheit is not used.)

  • Compute volumetric efficiency. Using the formula, consider if you wanted 300 HP from a 540-cubic-inch engine at 2700 RPM. Compute the volumetric efficiency as (9411 x HP x Fahrenheit conversion) divided by (displacement x RPM) x 100, i.e. (9411 x 300 x 0.45) divided by (540 x 2700) x 100, which equals 0.87 x 100, which yields a volumetric ratio of 87 percent.

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