A hemispherical combustion chamber is a combustion chamber in the cylinder head of an internal combustion engine with a domed "hemispheric" shape. An engine featuring this type of hemispherical chamber is known as a hemi engine. In practice, shapes less than a full hemisphere are typically employed, as are variations (or faceting in parts) of a true hemispheric profile. The primary advantage of such shapes are increased compression (leading to greater power) and very large intake and exhaust valves (allowing better flow of intake and exhaust gasses, also resulting in improved volumetric efficiency and greater power); the primary disadvantages are complex valve trains (caused by valves being placed opposite one-another in a head) and expense (of machining the heads and pistons, and additional valve train components).
While hemispherical combustion chambers are still found in the 2000s multi-valve arrangements (of four and even five valves per cylinder) and the popularity of overhead cam (including double overhead cam) arrangements have altered the traditional trade-offs in employing "hemi heads".