Chrysler Hemi engine
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A Chrysler Hemi engine, known by the trademark Hemi, 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) from 1951 - 1958, the second from 1964 - 1971, and the third beginning in 2003.
A hemispherical (i.e., bowl-shaped) combustion chamber allows the valves of a two valve-per-cylinder engine to be angled rather than side-by-side. This creates more space in the combustion chamber roof for the use of larger valves and also straightens the airflow passages through the cylinder head. These improvements significantly improve the engine's airflow ("breathing") capacity, which can result in relatively high power output from a given piston displacement. With a hemi combustion chamber, there is minimal quench and swirl to burn the fuel-air mix thoroughly and quickly; the spark plug is frequently located at or near the centroid of the chamber to facilitate complete combustion. Engines with hemispherical combustion chambers often use dome-topped pistons to attain the desired compression ratio.
The hemi head requires intake and exhaust valve stems that point in different directions, requiring a large cylinder head and complex rocker arm geometry in both cam-in-block and overhead cam engines. Hemipsherical combustion chambers lack the quench area found in wedge combustion chamber designs, making the hemi more sensitive to fuel octane; a given compression ratio will require higher octane to avoid ping in a hemi engine than in a wedge engine.
Chrysler developed their first experimental Hemi for use in the Republic P-47 Thunderbolt fighter aircraft. The XIV-2220 engine was an inverted V16 rated at 2,500 hp (1,860 kW). 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
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 bhp (134.2 kW) Hemi V8 with a displacement of 331 cu in (5.4 L). The engine was not, however, marketed as a "Hemi". Each Chrysler division had its own unique version of the early Hemi engine, with different displacements and designations. Chrysler and Imperial called their versions the FirePower. DeSoto called theirs the FireDome. 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, did not have a hemispherical head design. Plymouth would not get a hemi engine until the 1960s.
As soon as this engine was introduced, Briggs Cunningham chose to use the Chrysler version in some of his race cars for international motor sports. A Chrysler-powered Cunningham C5-R won 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
The hemispherical head design was revived in 1964. These were the first engines officially designated Hemi, a name Chrysler trademarked. All Chrysler Hemi engines of this generation displaced 426 cu in (7 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 large dimensions. Its 10.72 in (272.3 mm) deck height and 4.80 in (121.9 mm) bore spacing made it the biggest engine racing in NASCAR at the time.
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 their 409 cu in (6.7 L) V8 in 1961, and other manufacturers were willing to build larger engines to remain competitive. In 1963 NASCAR limited engine displacement to 427 cu in (7 L), and the 1963 Chevrolet 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 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. With an oversquare 4.25 in (108.0 mm) bore and 3.75 in (95.3 mm) stroke like the big-block Chrysler RB, this new generation Hemi was an immediate success, earning recognition when it placed first, second, and third in the 1964 Daytona 500 race. This engine's dominance in a field artificially limited by the sanctioning body led the series organizers double the number of homologation engines required to be offered to the general public to qualify as a "stock" part, from 500 to 1,000. This eliminated the 426 Hemi's availability for the 1965 season, but Chrysler managed to sell enough Hemi engines to the public to regain use of the Hemi for NASCAR in 1966 in their new Dodge Charger. David Pearson, driving the #6 Dodge Charger, won 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. Its large casting allowed the engine to be overbored and stroked to displacements unattainable in the other engines of the day. Top-fuel racing organizers still limit the bore spacing and other dimensions to the 1960s hemi size, making it the de facto engine template. Engines with larger dimensions, such as Ford's 385-series, are banned under these dimensional restrictions. In NHRA racing, the Hemi was usually equipped with a large Rootes type supercharger and short individual exhaust pipes, and fuelled with nitromethane.
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-bolted main bearing caps and a different head bolt pattern. The street Hemi version was rated at 425 bhp (316.9 kW) with two Carter AFB carburetors, though on actual dynomometer test, it produced 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 engine speeds.
To avoid confusion with earlier (1951-'58) and current Hemi engines, the 426-based Hemi is sometimes called the "2G" or "Gen 2" Hemi
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
|This section may contain original research or unverified claims. Please improve the article by adding references. See the talk page for details. (August 2008)|
The current-production "HEMI" engine does not employ truly hemispherical combustion chambers; they are 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 cylinder, and early versions[vague] had a wasted spark ignition arrangement which would permit the engine to continue running on all eight cylinders with one failed coil. Like most of Chrysler's past-model Hemi-head engines, the 5.7 version is rated at approximately one horsepower per cubic inch, however the updated 2009 model produces even more than that.[vague]
A new variable displacement technology called Multi-Displacement System (MDS) is used in some versions which can shut off two cylinders on each bank under light load to improve fuel economy. For the 2009 model year power has been bumped up to as much as 390 horsepower (290 kW) and 404 lb·ft (548 N·m) depending on application. It also gets 4% better fuel economy. Variable valve timing (VVT) was also introduced.
The '5.7 L Hemi' was released for model year 2003 on the Dodge Ram 2500 and 3500 pickup trucks to replace the Magnum 5.9 engine. It also replaced the 8.0L V-10 engine in the heavy duty Ram. As of 2004 it was the only available gasoline engine in the Ram Heavy Duty. Chrysler has since made the 5.7 L Hemi available in all models of the 2004 Dodge Ram, the 2005 Chrysler 300C, Dodge Magnum R/T, Jeep Grand Cherokee and the 2006 Dodge Charger R/T. Dodge has also announced that the 5.7L Hemi will be available on the 2009 Dodge Challenger R/T; the 6 speed version will not feature MDS.
The 5.7 L (345 cu in) Hemi in the Ram delivered 345 hp (257.3 kW) and 375 lb·ft (508 N·m), but 340 hp (253.5 kW) and 390 lb·ft (529 N·m) for the 300C and Magnum R/T, which is exactly 100 hp (74.6 kW) more than the old 5.9 engine. It is a 90-degree V8, 2-valve pushrod design like the past LA engines, displacing 5,654 cc (345 cu in), 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
- 2005–2008 Dodge Magnum R/T
- 2006–present Dodge Charger R/T
- 2005–present Jeep Grand Cherokee
- 2006–present Jeep Commander
- 2007–present Chrysler Aspen
- 2009–present Dodge Challenger R/T
Chrysler has made various revisions to the 5.7 L for the 2009 model year. The first for all applications is what Chrysler calls Variable Camshaft Timing or VCT. VCT uses a valve to control oil flow to a unique camshaft sprocket containing a phasing device, which advances or retards camshaft timing according to oil volume provided by the control valve.
Cylinder heads have been revised to increase flow. New model-specific intake manifolds are used in all applications. Dodge Ram, non-Hybrid Electric Vehicle (HEV) Chrysler Aspens, and non-HEV Dodge Durango utilize an active intake manifold with a short runner valve to optimize torque and horsepower. At lower engine speeds the valve is closed, resulting in improved low-end torque from the longer runners. At higher engine speeds the valve opens diverting the incoming air into the center of the manifold, and the shorter runners results in improved horsepower. Passenger cars, Jeep vehicles, and HEV Chrysler Aspen & HEV Dodge Durango do not use this manifold. Instead, these vehicles utilize a plain intake manifold without a short runner valve.
Six-speed manual transmission applications will differ by not having the Multiple Displacement System (MDS). The new version of the 5.7L has five different camshaft profiles. All will have VCT.
- Active intake with MDS
- Active intake without MDS
- Passive intake with MDS
- Passive intake without MDS
- HEV Application (modified version of passive intake with MDS)
2009 Power Numbers
- 300C: 360 hp (268 kW), 390 lb·ft (529 N·m)
- Charger R/T: 368 hp (274 kW), 395 lb·ft (536 N·m)
- Challenger R/T 5 Speed Automatic: 372 hp (277 kW), 401 lb·ft (544 N·m)
- Challenger R/T 6 Speed Manual: 375 hp (280 kW), 410 lb·ft (556 N·m)
- Ram Truck: 390 hp (291 kW), 407 lb·ft (552 N·m)
- Jeep Grand Cherokee and Jeep Commander: 357 hp (266 kW), 389 lb·ft (527 N·m)
- Chrysler Aspen and Dodge Durango non-HEV: 376 hp (280 kW), 401 lb·ft (544 N·m)
- Chrylser Aspen and Dodge Durango HEV: 399 hp (298 kW), 390 lb·ft (529 N·m)
The Hemi is also available in a 6.1 L (6059 cc, 370 cu in) version. The engine's bore is 4.1 in (104 mm), and many other changes were made to allow it to produce 425 horsepower (317 kW) at 6200 rpm and 420 lb·ft (569 N·m) at 4800 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 300C 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)
- 2008–present Dodge Challenger SRT-8
Chrysler displayed a larger 6.4 L (392 cu in) Hemi in 2005 with output claimed to reach 505 lb·ft (685 N·m). It is based on a iron HEMI 6.1 L engine block with aluminium alloy pistons.
A crate engine version was sold under the name '392 Hemi Crate Engine'.
As collector items
Dodge and 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 and mint condition Hemi-powered Dodges and Plymouths command bids of over US$100,000. It has also been speculated that modern Hemi powered vehicles like the limited edition Super Bee SRT8 Chargers, the Rumble Bee, GTX, Night Runner, and Daytona Ram trucks will also have collector value.