The Chrysler Hemi engine, known by the trademark Hemi, is a series of V8 engines built by Chrysler with a hemispherical combustion chamber. Three different types of Hemi V8 engines have been built by Chrysler 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-valves-per-cylinder engine to face each other across the chamber, rather than opening side-by-side. This layout makes space in the combustion chamber roof for larger valves and straightens the airflow passages through the cylinder head. This creates what is known as a cross-flow head, where the intake charge flows directly across the chamber to the exhaust valve located directly opposite it. [Actually, a "cross-flow head" is only a term for an engine that has the intake manifold on one side, and the exhaust on the other, as opposed to both manifolds being located on the same side of the engine, or cylinder head. A cross-flow design allows the air to flow in one side and out the other, which adds efficiency and power. A "reverse-flow" engine requires the gases to flow in, stop, and reverse direction. A hemispherical engine spreads the valves apart and places them at an angle to decrease resistance, but it's only an improvement on a crossflow head]. These features significantly improve the engine's airflow capacity, which can result in relatively high power output from a given piston displacement. But the design can also significantly increase the flow of incompletely combusted air-fuel mixture straight out of the exhaust valve. 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 placed at or near the centroid of the chamber to facilitate complete combustion. Hemispherical combustion chambers, because of their lack of quench, are more sensitive to fuel octane rating; a given compression ratio will require a higher octane rating to avoid ping in a hemi engine than in a wedge engine. Engines with hemispherical combustion chambers often use dome-topped pistons to attain the desired compression ratio but this design only works best at high R.P.M.
The hemi head usually has intake and exhaust valve stems that point in different directions, requiring a large, wide cylinder head and complex rocker arm geometry in both cam-in-block and overhead cam engines. This adds to the overall width of the engine, limiting the vehicles in which it can be installed.