Chrysler FirePower engine

426
Polished and chromed 426 Hemi engine in a 1971 Hemi 'Cuda
Overview
Manufacturer	Chrysler
Also called	Elephant engine
Production	1964–1971
Combustion chamber
Configuration	V8
Displacement	426 cu in (7.0 l)
Cylinder bore	4.25 in (107.95 mm)
Piston stroke	3.75 in (95.25 mm)
Cylinder block alloy	Cast iron
Cylinder head alloy	Aluminum Cast iron
Valvetrain	OHV
Compression ratio	10.25:1
Combustion
Fuel system	Carburetor
Fuel type	Gasoline
Oil system	Wet sump Dry sump
Cooling system	Water-cooled
Output
Power output	425 bhp (317 kW) (Gross) 350 bhp (261 kW) (Net)
Torque output	490 lb·ft (664 N·m)
Chronology
Predecessor	426 Wedge

The Chrysler Hemi engines, known by the trademark Hemi, are a series of I6 and V8 engines built by Chrysler with hemispherical combustion chambers. Three different types of Hemi engines have been built by Chrysler for automobiles: the first (known as the Chrysler FirePower engine) from 1951 to 1958, the second from 1964 to 1971, and the third beginning in 2003. Although Chrysler is most identified with the use of "Hemi" as a marketing term, many other auto manufacturers have incorporated similar designs.

During the 1970s and 1980s, Chrysler also used the Hemi name for their Australian-made Hemi-6 Engine and applied it to the 4-cylinder Mitsubishi 2.6L engine installed in various North American market vehicles.

A hemispherical cylinder head ("hemi-head") gives an efficient combustion chamber with an excellent surface-to-volume ratio, with minimal heat loss to the head, and allows for two large valves. However, a hemi-head allows no more than two valves per cylinder, and these large valves are necessarily heavier than in a multi-valve engine. The intake and exhaust valves lie on opposite sides of the chamber and necessitate a "cross-flow" head design. Since the combustion chamber is a partial hemisphere, a flat-topped piston would yield too low a compression ratio unless a very long stroke is used, so to attain desired compression ratio the piston crown is domed to protrude into the head at top dead center, resulting in a combustion chamber in the shape of the thick peel of half an orange.

The hemi-head design places the spark plug at or near the center of the chamber to promote a strong flame front. However, if the hemi-head hemisphere is of equal diameter to the piston, there is minimal squish for proper turbulence to mix fuel and air thoroughly. Thus, hemi-heads, because of their lack of squish, are more sensitive to fuel octane rating; a given compression ratio will require a higher octane rating to avoid pre-detonation in a hemi engine than in some conventional engine designs such as the wedge and bathtub.