Volumetric Efficiency Calculator
Work out the airflow your engine needs at a given rpm, or the volumetric efficiency implied by an airflow figure you already have.
Engine and airflow
- Volumetric efficiency peaks at the torque peak and falls away either side. A single number describes one rpm point, not the whole engine.
- Carburettor and throttle body ratings are quoted at a test pressure drop. Match the rating standard to the application.
- Airflow figures here are at standard conditions. Hot air is less dense and worth less power.
What volumetric efficiency measures
An engine is an air pump. In theory a four stroke ingests half its displacement every revolution. Volumetric efficiency is how much of that theoretical volume it actually draws in.
A stock engine manages 75 to 85% at its best. A well developed naturally aspirated engine with good heads and intake tuning can reach or slightly exceed 100% at its torque peak, because intake momentum keeps packing the cylinder after the piston has stopped drawing. Forced induction pushes it well past that.
The formula
Airflow in CFM equals displacement times rpm times volumetric efficiency, divided by 3,456. The constant folds in the fact that a four stroke fires every other revolution, and the conversion from cubic inches to cubic feet.
It is a single point, not a rating
Volumetric efficiency changes constantly with engine speed. It peaks wherever the intake and exhaust tuning line up, which is the torque peak, and falls away on both sides. Quoting one figure for an engine only makes sense if you also say at what rpm.
Frequently asked questions
How do I calculate CFM for my engine?
Multiply displacement in cubic inches by rpm, multiply by volumetric efficiency as a decimal, then divide by 3,456. A 350 cubic inch engine at 6,000 rpm and 85% VE needs about 517 CFM.
Can volumetric efficiency exceed 100%?
Yes. Naturally aspirated engines can exceed 100% in a narrow band where intake runner tuning and exhaust scavenging work together to keep filling the cylinder after the piston stops drawing. Any boosted engine exceeds it across most of the range.
What is a good volumetric efficiency?
For a stock engine, 80% at the torque peak is normal. A well developed naturally aspirated engine reaches 95 to 105%. Below about 75% suggests either a serious restriction or that you are looking at an rpm well away from the torque peak.
Does a bigger carburettor always help?
No. Oversizing kills intake velocity, and velocity is what carries fuel and helps fill the cylinder at low and mid rpm. An engine with too much carburettor is soft off idle and only wakes up near peak rpm, which is worse for almost every road application.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.