A Chrysler Hemi engine, known by the trademark "Hemi engine", is an internal combustion engine built by Chrysler that utilizes a hemispherical combustion chamber. Chrysler built three generations of hemi engines for automobiles: the first (known as the Chrysler FirePower engine) in the 1950s, the second from the mid 1960s through the mid 1970s, and the third in the early 2000s.
A hemispherical (inverted bowl-shape) combustion chamber allows the valves of a two valve-per-cylinder engine to be angled rather than side-by-side. This design also allows the placement of the sparkplug nearer to the center. This is mandatory, given that there is minimal quench and swirl to burn fuel gasses thoroughly and quickly. A hemispherical combustion chamber often requires a dome-shaped piston crown to maintain the desired compression ratio.
The advantages of the hemi cylinder head come at the disadvantage of requiring intake and exhaust valve stems that point in different directions, requiring a much more complex rocker arm geometry in both cam-in-block and overhead cam engines. This also increases the space taken up by the cylinder head—hemi engines are not space-efficient. Also, the hemi design lacks the quench area that exists with wedge combustion chamber designs, making the hemi more sensitive to fuel octane. (A given compression ratio will require higher octane in a hemi engine than a wedged combustion chamber engine.)
There were several companies other than Chrysler that used some form of hemi engine with varying degrees of success before WWII, and no doubt this available data proved to be a useful starting point when Chrysler engineers were contracted to design their own for the war effort.
Chrysler developed their first experimental Hemi for use in the Republic P-47 fighter aircraft. The XIV-2220 engine was an inverted V16 rated at . The P-47 was already in production with a Pratt & Whitney radial engine when the XIV-2220 flew successfully in trials in 1945 as a possible upgrade, but the war was winding down and it did not go into production. However, the exercise gave Chrysler engineers valuable research and development experience with two-valve hemi combustion chamber dynamics and parameters.
FirePower with the OHV V8 Rectify
Using their military experience with the hemispherical combustion chambers, Chrysler decided to use this layout in their first OHV V8 in 1951, introducing a 180 hp (134 kW) Hemi V8 with a displacement of 331 in³ (5.4 L). The term Hemi was not applied to this engine by Chrysler, however. Different Chrysler divisions had their own versions of the early Hemi engine, all with different names and displacements. Chrysler and Imperial called their versions the FirePower. DeSoto, (Chrysler's now defunct medium class make), called theirs the FireDome (dome = hemi), Dodge had a smaller version, known as the Red Ram. Only Plymouth didn't have a version of the hemi. Their V-8 engine, introduced in 1955 as the "Hy- Fire", didn't have a hemispherical head design. Plymouth wouldn't get a hemi engine until the 1960s.
As soon as this engine was introduced, Briggs Cunningham chose to use the Chrysler OHV V8 in some models of his automobiles designed as race cars for international motor sports. A Chrysler-powered Cunningham C5-R won first in its class in 1953—and Team Cunningham automobiles using these engines finished as high as third place overall—at the 24 hours of Le Mans Grand Prix. Cunningham switched away from these designs in 1959 when Chrysler abandoned the hemispherical concept in favor of the wedge-head Chrysler B engine.
Hemi design reintroduced Rectify
The hemispherical head design was revived in 1964. These were the first engines to officially use the "Hemi" name, a word Chrysler trademarked. All Chrysler Hemi engines of the 1960s displaced 426 cu. in. (7.0 L). Although just 11,000 Hemi engines were produced for consumer sale due to their relatively high cost and poor street-use reputation, the engine became legendary, with "Hemi" becoming one of the most familiar automobile-related words in the United States. The 426 Hemi was nicknamed the "elephant engine" at the time, a reference to its far-from-compact dimensions. At a time when the largest engine from Ford, for example, in NASCAR, had a deck height of 10.17" and a bore spacing of 4.63", the elephant's 10.72" deck and 4.80" bore spacing made it literally, the biggest engine in the race (the tall-deck BBC had a 10.20" deck height and 4.84" bore-spacing).
The first 426 Hemi of the 1960s was the NASCAR stock car race engine, introduced in a Plymouth Belvedere in 1964. Chevrolet had been highly successful in NASCAR after introducing the famous 409 in 1961, and it became obvious that manufacturers were willing to build increasingly larger engines if necessary to remain competitive. In 1963 NASCAR limited the displacement of engines to 7.0 Liters (427 cu.in.), and the 1963 Chevy 427 "Mystery Motor" was very successful. Chrysler had to do something radical to regain their racing prominence.
There is an old racing expression: "Win on Sunday, sell on Monday", which alludes to when a particular make of car wins a popular race, the manufacturer will sell more of that car to the public. Although all manufacturers were familiar with multi-valve engines and with hemispherical combustion chambers, adding more valves per cylinder, or designing the complex valve train needed for a hemispherical chamber, were expensive ways of improving the high-RPM breathing of production vehicles. By canting the angle of the NASCAR-mandated two valves-per-cylinder, significantly larger valves could be used. This engine was an immediate success, and earned recognition when it placed first, second, and third in the 1964 Daytona 500 race. With an oversquare 4.25 in (107.9 mm) bore and 3.75 in (95.2 mm) stroke like the Chrysler RB engine big-block. This engine's domination in a field artificially limited by the sanctioning body, led the series organizers to dramatically increase the minimum number of homologation engines that were required to be sold to the public from 500 to 1,000 in order for it to qualify as a "stock" part. This eliminated the 426 Hemi's availability for the 1965 season, but Chrysler managed to sell enough Hemis to the public to regain use of the Hemi for NASCAR in 1966 in their new Dodge Charger with its low-drag "fast-back" body which had an extremely sloped rear window. David Pearson, driving the #6 Dodge Charger, went on to win the NASCAR Grand National championship in 1966 with 14 first-place finishes.
The 426 Hemi also proved to be an immediate success in NHRA drag racing due to its large casting. The elephantine casting allowed the engine to be bored and stroked to displacements impossible in the other engines of the day. To this day, top-fuel racing organizers limit the bore-spacing and other dimensions to the 1960s hemi size, making it the defacto 'top-fuel' engine template. Engines with larger dimensions, such as Ford's 385-series (429,460...), are banned under these dimension rules. In this venue it was usually paired with a large Roots type positive displacement supercharger, and used nitromethane fuel with short, open, individual exhaust pipes. Current rules in the top fuel classes limits nitromethane to 90% of the fuel blend, the balance typically being methanol or ethanol.
The 426 Hemi was produced for consumer automobiles from 1965 through 1971, and new crate engines and parts are available today from Chrysler. There were many differences between the Hemi and the Wedge-head big-block, including cross-bolting and a different head bolt pattern. The street Hemi version was rated at 425 hp (317 kW) with two Carter AFB carburetors though in actual dyno testing, it made 315 rear-wheel HP in purely stock form which would be approximately 360 gross HP at the flywheel using generally accepted conversion formulas. The engine could produce much higher HP figures with relatively few modifications, but those modifications drastically affected the engine's drivability on the street as they usually were made to take advantage of the free-breathing nature of the heads at high rpms.
This engine was used in the following vehicles:
- 1966–1970 Dodge Coronet/Plymouth Belvedere
- 1966–1971 Plymouth Satellite
- 1966–1971 Dodge Charger
- 1967–1971 Plymouth GTX
- 1968 Dodge Dart
- 1968 Plymouth Barracuda
- 1968–1971 Dodge Super Bee
- 1968–1971 Plymouth Roadrunner
- 1969 Dodge Charger Daytona
- 1970 Plymouth Superbird
- 1970–1971 Plymouth Barracuda
- 1970–1971 Dodge Challenger
- 1970 Monteverdi Hai 450
The current-production "HEMI" combustion chamber is not truly hemispherical; it is flatter and more complex than the 1950s-'70s Hemi V8 chamber. It uses a coil-on-plug distributorless ignition system and two spark plugs per cylinder to shorten flame travel leading to more consistent combustion which helps reduce emissions. There is one coil for every two cylinders which allows the engine to continue running with one failed coil. Like most of Chrysler's past-model hemi-head engines, it is rated at approximately one horsepower per cubic inch.
The '5.7 L Hemi' was released in 2002 on the Dodge Ram 2500 and 3500 pickup trucks to replace the Magnum 5.9 engine. Chrysler has since made the 5.7 L Hemi available in the 2003 Dodge Ram, the 2005 Chrysler 300C, Dodge Magnum R/T, Jeep Grand Cherokee and the 2006 Dodge Charger R/T.
The 5.7 L Hemi in the Ram delivered 345 hp (257 kW) and 375 lb·ft (508 Nm), but 340 hp (254 kW) and 390 lb·ft (529 Nm) for the 300C and Magnum R/T. It is a 90-degree V8, 2-valve pushrod design displacing 5654 cc (345 in³), with a bore of 99.5 mm (3.92 in) and a stroke of 90.9 mm (3.58 in).
The Hemi was on the Ward's 10 Best Engines list for 2003 through 2007.
This engine is used in the following vehicles:
- 2003-present Dodge Ram
- 2004-present Dodge Durango
- 2005-present Chrysler 300 300C
- 2005-present Dodge Magnum R/T
- 2006-present Dodge Charger R/T
- 2005-present Jeep Grand Cherokee
- 2006-present Jeep Commander
- 2007-present Chrysler Aspen
The Hemi is also available in a 6.1 L (370 in³, 6,059 cc) version. The engine's bore is 4.1 in (103 mm), and many other changes were made to allow it to produce 425 hp (317 kW) at 6,200 rpm and 420 lb·ft (569 Nm) at 4,800 rpm. The engine block is different from the 5.7, with revised coolant channels and oil jets to cool the pistons. A forged crankshaft, lighter pistons, and strengthened connecting rods add durability. A cast aluminum intake manifold is tuned for high-RPM power and does not include variable-length technology. Chrysler's Multi-Displacement System is not used on the 6.1.
- 2005-present Chrysler 300 SRT-8
- 2005-present Dodge Magnum SRT-8
- 2006-present Dodge Charger SRT-8
- 2006-present Jeep Grand Cherokee SRT-8 (420 hp/310 kW)
Chrysler displayed a larger 6.4 L (390.6 in³) Hemi in 2005 and claimed that the engine puts out 505 hp (377 kW).
Hemi-powered cars as collector's itemsRectify
Dodge & Plymouth Hemi-powered cars produced between the model years of 1965 and 1971 have become desired collector's items (as have other muscle cars manufactured during this era). At auctions like the Barrett-Jackson Car Collector's Show, it is common to see restored & mint condition Hemi-powered Dodges and Plymouths command bids of over US$100,000.