Volumetric efficiency is the percentage of the total piston displacement that actually flows through the engine at normal running speeds. Intake and exhaust restrictions, plus cylinder head design, valve timing, and a bunch of other things have an effect on how much air, fuel, and exhaust gas flows in and out.
Basically stock engines, without extensive porting and polishing, and with stock camshaft profiles and valve sizes usually run from 85% to 90% volumetric efficiency at normal running speeds, as measured by the flow meter on my dyno. With lots of head work, radical camshaft profiles, and high engine speeds, plus tuned headers, I can get slightly over 100% VE on my roundtrack race engines. That's all they're good for, though, "Go fast and turn left!"
Jerry
