Chrysler Hemi engine

A Chrysler Hemi engine, known by the trademark Hemi, is an internal combustion engine built by Chrysler that utilizes a hemispherical combustion chamber. Chrysler built three generations of hemi engines for automobiles: the first (known as the Chrysler FirePower engine) from 1951 - 1958, the second from 1964 - 1971, and the third beginning in 2003.
A hemispherical (i.e., bowl-shaped) combustion chamber allows the valves of a two valve-per-cylinder engine to be angled rather than side-by-side. This creates more space in the combustion chamber roof for the use of larger valves and also straightens the airflow passages through the cylinder head. These features significantly improve the engine's airflow ("breathing") capacity, which can result in relatively high power output from a given piston displacement. With a hemi combustion chamber, there is minimal quench and swirl to burn the fuel-air mix thoroughly and quickly; the spark plug is frequently located at or near the centroid of the chamber to facilitate complete combustion. Engines with hemispherical combustion chambers often use dome-topped pistons to attain the desired compression ratio.
The hemi head requires intake and exhaust valve stems that point in different directions, requiring a large cylinder head and complex rocker arm geometry in both cam-in-block and overhead cam engines. Hemipsherical combustion chambers lack the quench area found in wedge combustion chamber designs, making the hemi more sensitive to fuel octane; a given compression ratio will require higher octane to avoid ping in a hemi engine than in a wedge engine.