In engineering terms, volumetric efficiency (VE) is a measure of the engine’s ability to pump air. On the performance level, our only exposure to this ability is on the dynamometer. A simplified definition of VE would then become the amount of filling that the cylinder experiences. Many engine builders brag about VE greater than 100 percent with a normally aspirated engine, which would mean that they are actually filling the cylinder beyond capacity and creating a positive pressure above atmosphere while the piston is still at or near BDC. This is usually accomplished through matching the cam profile with the design of the intake manifold and allowing the laws of physics and resonant tuning to come into play. However, most engineers agree that values over 100 percent are not possible without forced induction. Since most engineers experience only 80- to 85- percent VE at peak torque, a lot of tuning would be needed to find the additional 15-20 percent of cylinder fill to reach that perfect goal, let alone surpass it. When scanning a dyno sheet for peak torque, it is easier to follow the VE column, since when the cylinder is the most filled, the amount of work the engine can do is the greatest.

    So who’s right? Both. Most engine dynos in the performance industry determine VE through a data-acquisition system that calculates total volume, and then compares this to the incoming air volume as measured by a flow meter attached to the engine’s inlet tract. If the cylinder volume is less than the incoming air volume, then the VE exceeds 100 percent. The problem is that you must measure more than incoming air; you must also measure the amount of exhaust volume and subtract that from the first value to calculate the internal losses.

    We must remember that when the engine is running, it is not completely sealed. There is a leakage past the rings and valve seals, as well as what is scavenged during overlap of the cam. These values are not taken into account when the amount of incoming air is all that is being measured. But don’t be misled into thinking that VE values on a dyno are useless. They are good indicators, and any increase will return higher output regardless of whether your engine is really seeing 100 percent or greater cylinder fill rates.