Volumetric efficiency is a term that’s tossed around quite a bit when talking about internal combustion engines. However, surprisingly few people understand what volumetric efficiency is and how it actually affects engine performance.
In this post, we’ll discuss volumetric efficiency, provide the volumetric efficiency formula, and show some examples of how it applies to specific engine examples.
What is Volumetric Efficiency?
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 percent VE means we’ve trapped 100 percent of the air a cylinder can theoretically hold by mass. Volume always measures the same, but restrictions lighten the mass of air/fuel compared to what a cylinder theoretically holds. Inertial tuning of the intake, heads, cam, and exhaust can make the volume heavier. The heavier it is, the more air/fuel it contains that can be burned to make power.
How is it calculated?
The Formula for Volumetric Efficiency Is:
VE = (CFM X 3,456) / (CID 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. This can be difficult to measure and requires special testing equipment.
How Does Volumetric Efficiency Affect Performance?
Volumetric efficiency can be a tricky topic. Air density changes with temperature and altitude. Therefore, VE can 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, turbos, 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 percent and make tons of power.
The chart below gives some general VE ratings for different types of engines:
Modern Stock Engine
Slightly Modified Engine*
Highly Modified Engine**
*Slightly Modified = performance cam, ported stock cylinder heads, performance intake manifold, headers, ignition upgrade
**Highly Modified = aggressive cam, performance intake manifold, headers, ignition upgrade, aftermarket configuration racing cylinder heads, 14:1 or higher compression ratio
How Volumetric Efficiency Relates to Your Engine’s Dyno Curve
VE is based on an engine’s horsepower dyno curve. The example in the graph below is a naturally aspirated 440 cubic-inch engine. We know it makes 592 horsepower and we know it makes it at 6,700 rpm.
Formula to estimate VE at different rpm from an engine’s dyno curve:
horsepower x 4235 / CID x rpm = VE
592 x 4,235 = 2,507,120
440 x 6,700 = 2,948,000
2,507,120 / 2,948,000 = .85 percent VE
Note that the engine peaked at 92 percent VE between 5,500 and 5,800 rpm—after peak torque began to fall. This number isn’t helpful in the carb cfm calculator, but is a good measure of performance. However, since we now know this engine has an 85 percent VE slightly below redline, we could use it to determine carb cfm rating using the standard cfm formula:
CID x redline rpm x VE /3,456
440 x 7,000 x .85 = 2,681,000
2,681,000/3456 = 757 cfm
A 750 cfm carb would be the smallest and generally most responsive that meets the horsepower peak airflow requirements. An engine wastes increasing amounts of air past peak power, so a slightly larger carb might be a benefit in a pure race engine.