Engine

 Power Brake Mean Effective Pressure Volumetric Flow Rate Indicated Mean Effective Pressure Friction Mean Effective Pressure Compression/Expansion Ratio Combustion Pumping Losses Mean Piston Speed Total Bore Area Volumetric Efficiency Gas Dynamics

As seen in volumetric flow rate, the volumetric efficiency is defined this way:

VE =
 Qm Qth
(1)

Where:

 Qm = Measured volumetric flow rate (m³/s) Qth = Theoretical volumetric flow rate (m³/s)

The theoretical volumetric flow rate (Qth) is more or less chosen arbitrarily, based on atmospheric air density. Because air is compressible, a volume does not limit the quantity of air that can be pushed in, especially when gas dynamics are considered.

So volumetric efficiency is not a true «performance» criteria for an engine because what counts is the air mass flow rate. With everything else equal, an engine with a large VE burns the same amount of fuel and gives the same power output than a larger engine with a smaller VE. But values do compare with similar engines.

Mathematically, only a complex gas dynamics analysis can give a good approximation of the volumetric efficiency, but here are some ballpark values:

 intake & exhausttuning VE@ max power None (2-stroke & Wankel) 55% None (4-stroke) 75% Mild intake tuning (4-stroke) 80% Mild intake & exhaust tuning (4-stroke) 90% Tuned 95% Fully tuned 100% Best 110%

Note: Add 5% @ maximum torque.

 forcedinduction VE@ max power Street (10 psi) 135% Racing (20 psi) 165% Top fuel dragster (45 psi) 230% Tractor pulling − Pro stock (115 psi) 360% Tractor pulling − Super stock (200 psi) 485%

Note: For a forced induction, it is a quasi-adiabatic compression that can be approximated by
VEboost = VEun-boost (
 boost gauge pressure atmospheric pressure
+ 1 )
 1 / 1.7

Sources: