An excerpt from the attached document:
Volumetric Efficiency (VE): The theoretical maximum volume of air that each cylinder can ingest during the intake cycle is equal to the swept volume (displacement) of that cylinder. The comparison of the actual volume of air ingested during an intake cycle with the theoretical maximum results in the volumetric efficiency, so a cylinder operating at “x-percent” VE is ingesting the equivalent of x-percent of its displacement every intake stroke.
VE Relationship to Torque: There are several factors which determine the torque an engine can produce and the RPM at which that occurs, however, the primary factor is the total Mass of air (not the volume) the engine can ingest into the cylinders at a given RPM, given the limitations of the intake tract, cam design, exhaust system, etc. The RPM at which the greatest mass of air is ingested per intake cycle will be the point of maximum torque.
This extreme dependent relationship between torque produced and VE is indicated by the remarkable similarity in shape between an engine's VE curve and its torque curve.
Realistic VE Considerations: For contemporary naturally-aspirated, two-valve-per-cylinder, pushrod-engine technology, a max-torque-RPM VE over 95% is excellent, and 100% is achievable but quite difficult. Only the best of the best can reach 110%, and that is by means of extremely specialized development of the complex system of intake passages, combustion chamber shapes, exhaust passages, and valve system components. The practical limit for normally-aspirated engines, typically DOHC layout with four or more valves per cylinder, is about 115%, which can only be achieved under the most highly-developed conditions, with precise intake and exhaust passage tuning.