Development & Evolution of the Hemi Engine

Alan R. Welch and his brothers, owners of the Welch Motor Car Company, are credited for pioneering the first hemispherical engine. The power plant was a simple 20-horsepower, 2-cylinder engine with a single overhead camshaft. Although the General Motors discontinued the HEMI in 1910 when it bought Welch, Peugeot developed its own hemispherical engine. BMW mass-produced a HEMI version before Chrysler engineered it for modern use.

Three generations of Hemi engines have been built by Chrysler for automobiles: the first (known as the Chrysler FirePower) 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.

The Hemi engine is named for its hemispherical cylinder head. It provides 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 a wedge.

The hemi head always 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 single overhead cam engines (dual overhead cam engines may not have rocker arms). This adds to the overall width of the engine, limiting the vehicles in which it can be installed.

The Welch design served as a blueprint for Chrysler's design of the modern Hemi engine. In 1945, it was used in the P-47 Thunderbolt fighter plane. The engine was an inverted V16 rated at 2,500 hp. Although it did not go into production, it gave Chrysler engineers valuable research. In addition to the aircraft engine, Chrysler and Continental worked together to develop the air-cooled V12 Hemi engine used in the M47 Patton tank.

Chrysler applied their military experience with the hemispherical combustion chamber to their first overhead-valve V8 engine, released under the name FirePower, for the 1951 model year. The first version of the FirePower engine had a displacement of 331 cu in. and produced 180 hp. Eventually, three of the four Chrysler divisions had its own versions of the FirePower engine, with eight different displacements and designations, and having almost no parts in common. This lack of commonality was due in part to the three engine versions using different bore pitches (the center-to-center distance between adjacent cylinders). Chrysler and Imperial called their versions the FirePower. DeSoto called theirs the FireDome. Dodge had a smaller version, known as the Red Ram.


331 c.i. FirePower (180 hp.)

Collectively, the 1951-'58 Hemi engines are now commonly referred to as first-generation Hemi engines, and the group can be identified by the rear-mounted distributor and the spark plugs in a row down the center of wide valve covers.

The hemispherical head design was revived in 1964. These were the first engines officially designated Hemi, a name Chrysler had trademarked. Chrysler Hemi engines of this generation displaced 426 cu in (7.0 L). Just 11,000 Hemi engines were ultimately produced for consumer sale due to their relatively high cost and the sheer size of the engine bay required to fit it in. The 426 Hemi was nicknamed the "elephant” at the time, a reference to its high power, heavy weight and large physical dimensions.

The 426 Hemi of the 1960s was an engine produced for use in NASCAR. The 426 Hemi was not allowed to compete in NASCAR's 1965 season due to its unavailability in production vehicles sold to the general public and because of complaints by Ford regarding its power. However several special production cars were produced and sold with the 426 Hemi. These were the Dodge Dart, Dodge Coronet, and Plymouth Fury. Chrysler introduced the "Street" Hemi in 1966 for its intermediate range of cars and sold the required number of Hemi engines to the public to legitimize its use for NASCAR in 1966. The 425 hp. "Street Hemi" was the same as the racing Hemi but with lower compression (10.25:1 from 12.5:1) and lower-lift camshaft, with iron headers instead of lighter steel long tube headers.



426 Hemi (425 hp.)

The 426 Hemi also was used in NHRA and AHRA drag racing. Its large casting allowed the engine to be overbored and stroked to displacements unattainable in the other engines of the day. The engines based on the old Chrysler design dominate nitro-burning Top Fuel and Funny Car classes due to plentiful parts, large amount of research and development, as well as decades of experience with the problems of the engine's design.

The 426 Hemi, in "Street Hemi" form, was produced for consumer automobiles from 1965 through 1970. Here is a list of the models that had the engine:

· 1966–1970 Dodge Coronet/Plymouth Belvedere

· 1966–1971 Plymouth Satellite

· 1966–1971 Dodge Charger

· 1967–1971 Plymouth GTX

· 1968 Dodge Dart Super Stock

· 1968 Plymouth Barracuda

· 1968–1971 Dodge Super Bee

· 1968–1971 Plymouth Road Runner

· 1969 Dodge Charger Daytona

· 1970 Plymouth Superbird

· 1970–1971 Plymouth Hemi 'Cuda

· 1970–1971 Dodge Challenger R/T

· 1970 Monteverdi Hai 450

The current, third generation, of production "HEMI" engine heads are flatter and more complex than the 1950s–'70s Hemi V8 chamber. The combustion chambers are no longer truly hemispherical. It uses a coil-on-plug distributorless ignition system and two spark plugs per cylinder to shorten flame travel leading to more consistent combustion and reduced emissions. A new 6.1L (425 hp.) Hemi engine was first made available on the 2008 Challenger SRT. It was followed in subsequent years, with a 5.7L (375hp.), 6.4L (485 hp.), 6.2L (707 hp. and 808 hp. supercharged versions), in SRT, R/T, Scat Pack, Hellcat and Demon models.


6.1 Hemi (425 hp.)



5.7 Hemi (375 hp.)


6.4 Hemi (485 hp.)


6.2 Hemi Hellcat (707 hp.)


6.2 Hemi Demon (808/840 hp.)