|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 & exhaust|
@ max power
|None (2-stroke & Wankel)||55%|
|Mild intake tuning (4-stroke)||80%|
|Mild intake & exhaust tuning (4-stroke)||90%|
@ 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%|