Chrysler Hemi engine
 Hemi engine history
Few innovations in automotive technology have had as great an impact on hotrodding as the Chrysler Hemi engine.
Chrysler first produced hemispherical cylinder heads on an experimental engine design for World War II fighter planes. In 1951, Chrysler would draw on this experience to produce its first Hemi engine for the consumer market, known later as the "Early Hemi". The hemispherical-shaped combustion chamber design allowed for increased airflow and better combustion efficiency. However, this came at a price of increased complexity and cost, eventually forcing Chrysler to temporarily discontinue the design after 1958. This early motor was truly gigantic, measuring a full yard wide at the valve covers and weighing in at around 750 lbs. Noted race car designer and builder Briggs Cunningham use the 331 cubic inch Hemi from 1951 to 1954 in his C2 through C4 cars.
This early Hemi would also become the favorite of drag racers in the top ranks of NHRA, IHRA and AHRA fuel racing such as Don Garlits and others. It would prove to be such an excellent design that even today, aftermarket aluminum cylinder blocks and heads used in Top Fuel and Funny Car racing are patterned after the same design. No matter what the body of the car is patterned after, the powerplant is a 16-valve, 500 cubic inch Hemi.
Six years later, in 1964, Chrysler would produce another engine design featuring hemispherical cylinder heads: the iconic 426 Hemi. The 426 Hemi dominated NASCAR racing, significantly increasing Hemi engine sales, cementing the old automotive adage: "What wins on Sunday, sells on Monday." The word "Hemi" would eventually become a Chrysler trademark, and a household name. Chrysler's 426 Hemi design proved so efficient that, for the past 50 years nearly all major top fuel and funny car drag racing teams have used derivatives of it. Actually the "late" Hemi was not a true Hemi like the "early" edition of the '51 through '58 motor. This later incarnation had the cylinder heads angled up by 10 degrees to the centerline of the cylinder bore to provide more engine bay space for items such as brake boosters.
Despite the higher cost and decreased parts availability, Hemi engines remain a favorite rebuild candidate for many hotrodders.
 Early use of hemispherical combustion chambers
Hemispherical combustion chambers were first incorporated into automobiles in the early 1900s. Hemi engines were being used experimentally in motorcar events, including the Grand Prix, and hemi or hemi-like engine designs were sporadically available in high-end sports cars of the time. Historical records point to a Welch Motor Car Company, incorporated in 1904 in Chelsea, Michigan, as being the first to bring a hemi engine design to market. Welch engines had hemispherical combustion chambers, overhead valves, and a single overhead camshaft. Welch was eventually purchased by General Motors in 1910, who discontinued the hemi engine design. Peugeot also had a hemispherical design motor in production in 1912.
 Chrysler IV-2220 fighter plane engine
Chrysler's first hemispherical-head engine was the IV-2220, a 2,500 horsepower, supercharged, turbocharged V-16. Chrysler had begun work on the engine around 1940, and presented their idea to the United States Army Air Corps, a World War II era predecessor to the modern-day US Air Force. The engine was intended to be installed in Republic P-47 fighter planes. It first flew in 1945, but it never went into production. By that time the war was ending, and the need for such an engine was dwindling as the jet age approached. One of these engines is on display at the New England Air Museum in Bradley Airport, Windsor Locks, CT.
 Chrysler "Early Hemi" engine
In the late '40s, Chrysler realized that its current line of flat head 6 and 8 cylinder power plants weren't going to make the grade when matched against the new more powerful V8 designs Cadillac and Oldsmobile were planning to introduce during the 1949 model year. They also knew larger engines capable of producing more horsepower would be required to haul around the new heavy weight barges Chrysler currently had on their drawing boards. Chrysler needed an engine capable of leveling the playing field.
Chrysler's engineers began to research possible V8 engine designs, looking for something that might be capable of producing real horsepower. Eventually, the decision was made to go with a conventional V8, similar to those being developed by Ford and GM. However, Chrysler would tap its previous military design experience for a unique twist on the conventional V8. In 1951, Chrysler introduced the FirePower engine, their first hemispherical-head engine for the consumer market, fondly referred to by many enthusiasts as the "Early Hemi" and by some as the "Whale" motor. Though Chrysler made reference to the engine's "hemispherical" combustion chambers, the word "Hemi" was not overtly used to market the engine. The term "Hemi" was more frequently applied by the late 1950s. This was most likely due to its colloquial use by enthusiast magazine writers and drag racers of the era.
During the '50s, Chrysler Corporation had a decentralized approach to engine building. Each division (Chrysler, Dodge, and DeSoto, and Plymouth) had its own engine manufacturing facility. Given the freedom to do so, each adopted a slightly different variation of the basic Hemi design. Chrysler's FirePower engine was accompanied by DeSoto's FireDome in 1952, and Dodge's Red Ram design in 1953. Plymouth would use polyspherical-shaped cylinder heads, also known as "semi-hemis".
This decentralized and inefficient manufacturing approach, coupled with the expense of manufacturing a complicated design like the Hemi, eventually spelled the downfall of this mighty powerplant. Production ceased with the completion of the 1958 model year in favor of a new simplified corporate-wide wedge head design, known as the Chrysler "B" engine.
 The 426 Hemi
In 1964, Chrysler introduced their new 426 Hemi. The 426 had not been designed primarily for consumers, but for NASCAR racing. Car manufacturers were well aware that racing victories bolstered their high-performance images, and significantly increased sales -- hence the saying: "What wins on Sunday, sells on Monday". So, they had been making engines specifically designed to win NASCAR races. NASCAR officials countered by requiring manufacturers to sell a certain number of a model to the public before they could race them, a process known as homologation. In 1964, if a car was to be entered in a NASCAR race, 500 cars of that same model had to be sold to the public. At the time, this ensured that stock car racing used only "stock" cars.
The 426 Hemi was a clear success; A Hemi powered the 1st, 2nd, and 3rd place race cars in the 1964 Daytona 500. This instant domination led NASCAR to increase the homologation requirement from 500 to 1,000. So, Chrysler initiated the "Street Hemi" program. A detuned version of the 426 engine became a publicly-available production option in certain Dodge and Plymouth models. The program was successful; Chrysler was able to race the Hemi again in the 1966 season. That year, a Hemi-powered Dodge Charger would rack up 14 first-place finishes and clinch the NASCAR Grand National championship.
The 426 Hemi was discontinued after 1971. The 426 Hemi was often referred to as the "Elephant" motor.
 Modern Hemi engine design
Chrysler reintroduced the Hemi design in 2002, primarily for trucks and sport utility vehicles. However, Hemi purists protest that the slightly flatter and less-hemispherical shape of the combustion chamber means the engine is not a "true" Hemi.
Despite this, the design has shown to have potential as a performance platform and is fairly well supported by the aftermarket; there are turbocharger and centrifugal supercharger kits, camshafts, exhaust headers and systems, etc. available.
 Hemi engine advantages and disadvantages
The Hemi engine design offers a high degree of efficiency both in breathing capability and in heat dissipation. This comes at a price of complexity, size, and increased manufacturing costs.
- The valves are placed on opposite sides of the combustion chamber, allowing larger valves, very short and nearly unrestricted intake and exhaust ports, making the airflow in and out of the cylinder extremely efficient. It also allows the valves to run cooler because of improved air movement around the valve pockets
- Since a hemisphere has the lowest possible surface area to volume ratio, there is more space for combustion, and less loss of energy to the cooling system
- Spark plugs are located in the center of the cylinder, creating a very even flame front, increasing combustion efficiency (somewhat offset however, by large domed pistons which shrouded the spark plugs)
- Because of these things, Chrysler Hemi engines could run higher compression ratios without the fear of detonation
- Complex valve train geometry, with dual rocker shafts in each head
- Expensive to produce
- Larger cylinder heads were heavy, not space efficient
- Large combustion chambers required the use of large domed pistons to increase compression to nominal levels. These pistons were heavy (high reciprocating mass) and the large dome shrouded the spark plugs and valves, requiring more ignition timing advance and increasing hydrocarbon emissions
 Hemi engine identification
For identifying early Hemi engines, see Early Hemi engine specifications.
 Rebuilding Hemi engines
For information on rebuilding early Hemi engines (1951-1958) see: Rebuilding an early Hemi engine.
 Hemi websites
- Polyspherical Engine Association
 Hemi articles
- The Early Hemi Guide of Death HR mag article by Marlan Davis
- The Early Hemi Guide Of Death, Part 2 HR mag article by Marlan Davis
- 426 Hemi
- Chrysler FirePower engine
- Chrysler Hemi engine
- Chrysler IV-2220
- Early Chrysler 331, 354, and 392 Hemi motors
- Early Hemi motor
- Hemi milestones
- Hemi overview
- Hemi rebuild
- Mopar Hemi engine
- What is a Hemi head?
 Hemi references
- Early Hemi engine history question