Automotive Dictionary: Hemispherical Head

Throughout this site we use many technical terms, and given the breadth of readership our site enjoys, sometimes we are remiss and incorrectly assume everyone knows what we are referring to. For those that do not, here are some explanations of the technical terms use.

Hemispherical Head

A design of cylinder head having a hemispherical combustion chamber, the piston crown being the flat side. Especially advantageous on longer stroke and thus smaller bore engines, the hemispherical roof permits larger valves and ports, as the valves can be set at angles, thus promoting greater volumetric efficiency (better breathing). At the same time better thermal efficiency is attained due to this shape producing less surface area for a given clearance volume - less heat is lost to the metal and to the coolant.

Also see: How it Works - The Hemi-Head Engine