What is volumetric efficiency (VE)?

Volumetric efficiency (VE) is the actual amount of air flowing through an engine, compared to its theoretical maximum. Basically, it is a measure of how full the cylinders are.

VE is expressed as a percentage. An engine operating at 100% VE is using all of its volumetric capacity.

How is it calculated?

The formula for VE is:

VE = (cfm x 3,456) / (c.i.d. x rpm)

In this formula, cfm is the amount of air (in cubic feet per minute) being pumped through the engine. This is NOT the CFM Rating of the Carburetor. VE is difficult to measure and requires special testing equipment.

How does it affect performance?

VE is a tricky topic. Air density changes with temperature and altitude. Therefore, VE will change based on the environment.

At sea level, the air is more dense. This means that there are more air molecules inside the cylinder. In the mountains, the opposite is true. The same engine would operate at a higher VE at sea level than in the mountains.

You can improve VE by making it easier for air to flow. This is the idea behind aftermarket intake manifolds, cold air kits, porting and polishing cylinder heads, and headers.

Pumping more air is also the idea behind superchargers, turbo's, and nitrous. These power adders force more air into the cylinder. When the fuel and ignition systems are properly tuned, this can raise VE over 100% and make tons of power.

The chart below gives some general VE ratings for different types of engines:

*Slightly Modified = performance cam, performance intake manifold, headers, ignition upgrade.

**Highly Modified = aggressive cam, performance intake manifold, headers, ignition upgrade, aluminum or ported iron cylinder heads, 11:1 or higher Compression Ratio.