seiscanecos74
The good textbooks do explain how to measure and predict the volumetric efficiency of high performance racing engines. A good way to measure the volumetric efficiency is with a “nonintrusive” airflow meter in the engine’s air intake system, with the engine running at full, wide-open throttle. The measured airflow, converted to SCFM at a given rpm, is then divided by the airflow calculated from the basic rpm X displacement equation:
CFMgeometric = ((peak rpm * cid)/3456).
Volumetric Efficiency = CFMmeasured / CFMgeometric
Where one gets the nonintrusive airflow meter for an engine that did not come with one is another question. The high-tech dyno shops should have one.
Predicting volumetric efficiency is not easy. In addition to knowing the steady flow rate versus pressure drop table obtained from flow bench tests of the carburetors (or fuel injection throttle body), intake manifold, and heads, the effect of cam timing and the wave dynamics in the intake runners and exhaust headers is a big factor, as we all know. Volumetric efficiencies well above 100% can be obtained, even with flow losses, if the cam timing, valve lift, and wave dynamics are properly tuned.
A good example of how to predict volumetric efficiency for racing engines is in the textbook “Design Techniques for Engine Manifolds”, published by the SAE. One of the examples in this book shows performance data for a Formula I engine, where a volumetric efficiency of about 120% was predicted at 10,000 rpm, and 122% was measured. A link to the book’s cover and the example is below. The mathematics in the book can get pretty complicated and a computer is needed to actually make the predictions. It might be easier and more fun to do the tuning on the dyno.
Volumetric Efficiency example Wave Action textbook