See also:

Hemi (from "hemisphere") or "crossflow cylinder head" is a design of internal-combustion engines in which the cylinder head's combustion chamber is of hemispherical form. The term, "Hemi engine", is a trademark of Chrysler Corporation, though the concept is used by many manufacturers.

The BMW double push rod design, taken over by Bristol Cars, the Peugeot 403 and the Toyota T engine are other well known examples. Harry Arminius Miller racing engines were a more notable example. Stutz had built four valve engines, resembling modern car engines. Chrysler's main innovation was to build them in such large numbers.

The hemispherical combustion chamber design puts the intake/exhaust valves in-line, rather than side-by-side, allowing for better flow of air through the head (although the inlet and exhaust valves are not simultaneously open and there is no continuous flow). The spark plug in the center of the chamber makes for better ignition of the fuel/air mixture. These aspects help make the hemi-type engine more efficient and powerful, and less prone to engine knock.

The hemispherical cylinder head increases the engine's efficiency through reduced thermal energy loss and increased airflow through the engine. (A hemisphere has the lowest surface area to volume ratio, meaning the most space for combustion while losing the least amount of energy to the engine walls.) Drawbacks such as increased production cost have meant that it has been a rare design. Placing the intake on the opposite side of the engine also reduces the air intake temperature and increases efficiency.

Hemispherical cylinder heads have been used in some engines since they were first used by the Belgian car maker Pipe in 1905. Most applications have been in higher-priced luxury or sporting vehicles, because the hemi design is more expensive to build.

Perhaps the best-known proponent of the Hemi design has been the Chrysler Corporation, which has produced three generations of such engines: the first (the Chrysler FirePower engine) in the 1950s; the second (the 426 Hemi) from the mid 1960s through the mid 1970s; and finally in the early 2000s. Chrysler has used the word "Hemi" extensively in its advertising, to the extent that the word is indelibly associated with Chrysler in North America.

Porsche has also been a notable user of the Hemi design, generating up to 86 hp per liter displacement on production cars (1973 2.4 L 911S), and even more on racing engines (906 Carrera engine). Jaguar used this head design as well on the legendary XK engines, which powered cars ranging from the Le Mans winning D-Type to the XJ6 sedan.

Other manufacturers used the hemispherical design before World War II, including Daimler and Riley.

Chrysler Hemi engines

Main article: Chrysler Hemi engine

Chrysler's first experience with the Hemi design was during World War 2, in which it developed an experimental 2500 hp (1864 kW) Chrysler IV-2220 V16 engine for the P-47 Thunderbolt. Experience with this engine led to Chrysler using the Hemi design for their first overhead valve V8 in 1951. This design, the Chrysler FirePower engine, was used until 1959.

In 1964, Chrysler introduced a new 426 in³ (7.0 L) Hemi, designed to win at NASCAR racing and sold to the public to meet homologation requirements and to enable the public to buy the winning engine. It was based on the Chrysler RB engine big-block. The engine was available through 1971, and the DaimlerChrysler corporation still sells crate engines and parts. It was available in most Mopar muscle cars and pony cars of the period, although its high price and limited street tractability kept sales fairly low. Hemi blocks were traditionally painted orange to distinguish them from other V8s. The Hemi head design is so efficient and effective that it was, and is, a top performer in NHRA, IHRA, UDRA and other sanctioned drag racing events throughout the world. Racers like "Big Daddy" Don Garlits have set world records using Hemi power. Hemi.com provides information on three eras of Hemi power: 1951 to 1959, 1964 to 1971 and 2002 to current.

Chrysler introduced a modern Hemi in 2002. This engine is not a true hemispherical head engine; it has a polyspherical combustion chamber, but retains the Hemi's traditional inline perpendicular valves. This engine replaced Chrysler's large LA family of engines, particularly the Magnum 5.9, in the early 2000s. It is available in two sizes; 5.7 and 6.1 liters. Some versions of the 5.7L, including most 2006 production units, utilise a variable displacement technology called the Multi-Displacement System (MDS) to improve fuel economy. Also, at the 2005 SEMA show, Chrysler unvieled a 505-horsepower 6.4L HEMI which will be available as a crate engine and might find its way into production, perhaps with reduced horsepower.

Porsche Hemi engines

When Porsche introduced the 911, it had a 2.0 L flat-6 engine, with hemispherical heads. The basic design did not change much until 1996, when Porsche moved to water cooling.

some notable engines designed and used by Porsche in both customer production car and Race cars:

Year	Displacement	Power	Power per Litre	Notes
1964	2.0 L	130 hp (96 kW) at 6100 rpm	75 hp (56 kW)/L
1966	2.0 L	210 hp (157 kW)	105 hp (78 kW)/L	Carrera 6 race
1966	2.0 L	160 hp (119 kW) at 6600 rpm	80 hp (60 kW)/L	S
1970	2.2 L	180 hp (134 kW) at 6800 rpm	81 hp (60 kW)/L	S
1971	4.9 L	630 hp (470 kW)	128 hp (95 kW)/L	917 race
1973	5.4 L	1100 hp (820 kW) at 7800 rpm	203 hp (151 kW)/L	917/30 race, forced induction
1973	2.4 L	190 hp (142 kW) at 6500 rpm	79 hp (59 kW)/L	S
1974	2.2 L	500 hp (373 kW)	227 hp (169 kW)/L	RSR T race, forced induction
1976	3.0 L	260 hp (194 kW)	86 hp (64 kW)/L	turbocharged

The classic Hemi engines of the 1960s featured true hemispherical heads, and splayed (perpendicular) valves.

Mitsubishi hemispherical engines

Beginning production circa 1969 was the 4G13, a prototype engine used in the first Mitsubishi Lancer. Its displacement was very small, a 1.3 liter motor. The spark plugs were not in the centre of the dome, but slightly off to the exhaust side. In 1974, 4G14 (1,436 cc, 89.8 in³) unit (indeed, its entire driveline) was used by Hyundai Motor Company to power the Pony, until 1983 when the 4G16 (1,597 cc, roughly 99 in³) was used in the Stellar, and in 1985 on the PonyII. The "hemi" was used in other Chrysler cars (presumably in early model Dodge Lancers) as well as several mitsubishi vehicles. The largest being the 4G16 makes 101 hp (75 kW) @6000 rpm and 110 ft·lbf (149 N·m) @ 4000 rpm. They are I4, crossflow aluminum head, chaindriven 8 valve SOHC units. Perhaps the smallest hemispherical engine, the 4G12, a 1.2 L engine, was used in European and Middle Eastern Pony cars and pickups.