Volumetric efficiency. Volumetric efficiency (VE) of a compression cylinder may be influenced by a valve conversion. Clearance volume affects VE. Clearance volume is the volume in a cylinder end that is not swept by the movement of the piston. Depending on the size of the cylinder, clearance volume can range from 2% to as high as 60% of displacement, or more. It includes spaces between piston and head at the end of the stroke, space under the valves, and of course clearance within the piston side of the valve itself.
The higher the clearance the poorer the volumetric efficiency of the cylinder.