Volumetric Efficiency Calculator
Works out volumetric efficiency from airflow, or the airflow an engine needs at a given VE — the figure that sizes carburettors, throttle bodies and intake systems.
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The formula
theoretical CFM = displacement (ci) × rpm ÷ 3,456 VE = actual airflow ÷ theoretical airflow 3,456 = 1,728 in³/ft³ × 2 revolutions per intake
A four-stroke engine inhales its displacement once every two revolutions, so theoretical airflow is displacement times rpm over two, converted to cubic feet. Volumetric efficiency is how completely the cylinders actually fill compared with that ideal.
What to watch out for
Typical VE at peak torque:
- Stock road engine: 75–85%.
- Tuned naturally aspirated: 95–110%, using intake and exhaust resonance to pack the cylinder.
- Forced induction: over 100% by definition — the pressure ratio does the packing.
VE varies with rpm. It peaks near peak torque and falls either side, which is why the torque curve looks the way it does. Sizing an intake for 100% VE at redline on a stock engine over-sizes it and costs low-rpm response.
Frequently Asked Questions
What is volumetric efficiency?
How completely an engine fills its cylinders compared with the theoretical volume — actual airflow divided by displacement × rpm ÷ 2. A stock road engine is 75–85% at best; a tuned one can exceed 100% at its resonant rpm.
What size carburettor or throttle body do I need?
Work out the airflow at peak rpm and realistic VE, then add 10–15%. A 2.0 litre at 6,000 rpm and 85% VE flows about 180 CFM; a carburettor rated 200–210 CFM suits it, and a 600 CFM one would ruin the throttle response.
Can volumetric efficiency exceed 100%?
Yes. With a turbo or supercharger the cylinder holds more than its own volume at atmospheric density. Naturally aspirated engines manage it too, briefly, at the rpm where intake and exhaust pulses are tuned to ram the cylinder.