The Chrysler Hemi engines, marketed under the moniker Hemi, are a line of American I6 and V8 gasoline engines produced by Chrysler. They include overhead valves and hemispherical combustion chambers. Chrysler produced three distinct Hemi engines for automobiles: the first from 1951 to 1958, the second from 1964 to 1971, and the third starting in 2003. Although Chrysler is most closely associated with the marketing term “Hemi,” numerous other automakers have implemented similar concepts. The Indianapolis Foundry cast and built the engine block and cylinder heads.
A hemispherical cylinder head results in an efficient combustion chamber with a high surface-to-volume ratio, minimum heat loss to the head, and the ability to use two big valves. However, a hemi-head engine is limited to two valves per cylinder, and these huge valves are inherently heavier than those found in a multi-valve engine. Because the intake and exhaust valves are located on opposing sides of the chamber, a “cross-flow” head design is required. Due to the fact that the combustion chamber is a partial hemisphere, a flat-topped piston would result in an insufficient compression ratio unless a very long stroke is utilized; thus, the piston crown is domed to protrude into the head at top dead center. As a result, a combustion chamber the shape of the gap between the domed piston’s stop and the dome-shaped head that receives it is formed.
The hemi-head configuration positions the spark plug in or near the chamber’s center, promoting a strong flame front. However, if the hemi-head hemisphere is the same diameter as the piston, there is minimal squish to create sufficient turbulence to thoroughly mix the fuel and air. As a result of their lack of squish, hemi-heads are more sensitive to fuel octane rating; a given compression ratio requires a higher octane rating to avoid pre-detonation in a hemi engine than in some conventional engine designs such as the wedge and bathtub.
In both cam-in-block and single overhead cam engines, the hemi head’s intake and exhaust valve stems always point in opposite directions, necessitating a huge, broad cylinder head and complex rocker arm geometry This increases the engine’s overall width, limiting the cars in which it may be fitted.
Significant issues in commercializing hemispherical chamber engine designs centered around valve actuation, specifically how to make it effective, efficient, and dependable at an affordable price. The complexity was acknowledged early in Chrysler’s development of the hemi engine in the 1950s: the head was dubbed the Double Rocker Shaft head in company advertising.
Chrysler adapted their military experience with the hemispherical combustion chamber to their first overhead-valve V8 engine, which was introduced in 1950 for the 1951 model year under the moniker FirePower, not “Hemi.” The initial version of the FirePower engine had 331 cu in (5.4 L) of displacement and produced 180 horsepower (134.2 kW). Eventually, each of Chrysler’s four divisions developed its own version of the FirePower engine, with distinct displacements and names and nearly no shared components. This lack of commonality was exacerbated by the fact that each of the three engine variants used a distinct bore pitch (the center-to-center distance between adjacent cylinders). Chrysler and Imperial designated their own versions as the FirePower. Desoto dubbed their structure the Fire Dome. Dodge produced a smaller version dubbed the Red Ram. Only Plymouth lacked a variant, opting instead for Dodge poly-head engines. Plymouth did not have a hemi engine until the 1964 426.
In 1964, the hemispherical head design was reintroduced. These were the first engines to bear the Hemi moniker, which Chrysler had trademarked. This generation of Chrysler Hemi engines has a displacement of 426 cubic inches (7.0 L). At the time, the 426 Hemi was dubbed the “elephant engine” because to its enormous output, heavy weight, and large physical proportions. With a deck height of 10.72 in (272.3 mm) and bore spacing of 4.80 in (121.9 mm), it was the largest engine in racing at the time.
In the 1960s, the 426 Hemi was a NASCAR engine that was raced in a Plymouth Belvedere in 1964. Initially, it was not available to the general purchasing public. The 426 Hemi was not permitted to race in NASCAR’s 1965 season due to its non-availability in production vehicles sold to the general public and due to Ford’s objections about its power. However, several special production versions of the Dodge Dart, Plymouth Fury, and subsequently, the Dodge Coronet, were made available to the general public with aluminum fenders and bumpers for drag racing.
Chrysler debuted the “Street” Hemi engine for their intermediate range of automobiles in 1966 and sold the required quantity of Hemi engines to the public in order to obtain NASCAR approval for its usage in stock car racing in 1966. The “Street Hemi” was similar to the race Hemi, but featured an inline 2X4-bbl induction system (with automatic choke), reduced compression (10.25:1 from 12.5:1), and a lower-lift camshaft, as well as iron exhaust manifolds rather than lightweight steel long tube headers.
Numerous variances existed between the Hemi and Wedge-head big blocks, including cross-bolted main bearing covers and a unique head bolt arrangement. Although all manufacturers were familiar with multi-valve engines and hemispherical combustion chambers, increasing the number of valves per cylinder and constructing the complicated valve train required to improve production vehicle breathing at high revolutions per minute By canting the angle of the two valves per cylinder required by NASCAR, significantly larger valves might be employed. All Chrysler RBs and the Chrysler 426 Hemi have over square bores and strokes. The 426 Hemi and 426 Wedge had bore x stroke dimensions of 4+14 in 3+34 in
From 1965 to 1971, the 426 Hemi was produced in “street Hemi” configuration for consumer autos. Hemi-powered Dodge and Plymouth automobiles made between 1965 and 1971 have become collector’s goods.
The street Hemi was rated at 425 horsepower at 5000 rpm SAE gross and 490 lb-ft of torque at 4000 rpm when coupled with a pair of Carter AFB four-barrel carburetors.
In totally stock configuration, it produced 433.5 horsepower and 472 lb. ft. of torque during dynamometer testing.  Chrysler’s sales literature listed the gross 425 hp and net 350 hp horsepower ratings for the 1971 model year.
The current-generation “HEMI” engine heads are flatter and more complicated than the Hemi V8 chambers from the 1950s through the 1970s. The combustion chambers have lost their hemispherical shape. It has a coil-on-plug ignition system and two spark plugs per cylinder to decrease flame travel, resulting in more uniform combustion and reduced emissions. The 5.7-liter Hemi-head engine, like the majority of Chrysler’s previous-model Hemi-head engines, is rated at around one horsepower per cubic inch (the current engines are SAE net, whereas the old Hemi engines were rated SAE gross). Power was increased to 357-395 horsepower) and 389-410 lb. ft. for the 2009 model year, depending on the application. Additionally, it improved fuel economy by 4%. Additionally, variable valve timing was introduced.
In some variants, a novel variable displacement technology called Multi-Displacement System is used, which allows two cylinders on each bank to be turned off under mild load to enhance fuel economy.
The 5.7 L HEMI engine was introduced in 2003 to replace the Magnum 5.9 engine in Dodge Ram pickup trucks. As of 2004, it was the Ram Heavy Duty’s only gasoline engine option. Chrysler eventually made the 5.7 L Hemi engine available in all models of the 2004 Dodge Ram and Dodge Durango, as well as the 2005 Chrysler 300C, Dodge Magnum R/T, Jeep Grand Cherokee, 2006 Dodge Charger R/T, Jeep Commander, 2007 Chrysler Aspen, 2009 Dodge Challenger R/T, and the 2022 Jeep Waggoner. Cylinder deactivation is not included in manual gearbox applications
The Ram’s 5.7 L Hemi engine produced 345 hp) and 375 lb. ft. but only 340 hp and 390 lb. ft. for the 300C and Magnum R/T, which is exactly 100 hp more than the previous 5.9 engine. It is a 90-degree V8, two-valve pushrod design, similar to the previous Magnum series engines, with a displacement of 5,654 cc), a bore of 3.917 in, and a stroke of 3.578 in.
For the 2009 model year, Chrysler made numerous changes to the 5.7 L. The first is what Chrysler refers to as Variable Camshaft Timing, or VCT. VCT utilizes an oil control valve to regulate oil flow to a special camshaft sprocket equipped with a phasing device that advances or retards camshaft timing based on the oil control valve’s functioning.
The cylinder heads have been modified to improve flow. While the intake manifold has been replaced on all applications, the alteration is model specific. To improve torque and horsepower, the Dodge Ram, non-hybrid electric vehicle Chrysler Aspens, and non-hybrid electric vehicle Dodge Durango all include an active intake manifold with a short runner valve. The valve is closed at lower engine speeds, resulting in increased low-end torque from the longer runners. At higher engine speeds, the valve opens, diverting incoming air toward the manifold’s center. Increased horsepower is a result of the shorter runners. Passenger automobiles, Jeep vehicles, and hybrid electric vehicles such as the HEV Chrysler Aspen and HEV Dodge Durango do not employ this manifold; instead, they use a passive intake manifold that does not include a short runner valve.
Additionally, the Hemi is available with a 6,059 cc displacement. The bore x stroke of the engine is 103 mm 90.9 mm, and numerous additional modifications have been made to enable it to produce 425 bhp. at 6,200 rpm and 420 lb. ft. of torque at 4,800 rpm. The engine block is modified from the 5.7, with redesigned coolant channels and piston cooling oil jets. Durability is enhanced by a forged crankshaft, lighter pistons, and stronger connecting rods. A cast aluminum intake manifold is optimized for maximum power at high revolutions per minute and does not have variable-length technology.
For 2015, Chrysler unveiled the Hellcat, a brand-new high-performance supercharged variation of the Hemi engine.
It shares the 4.09 in bore and 3.578 in stroke with the 6.4 L Hemi, giving it a total displacement of 6,166 cc The supercharger is a 2,380 cc twin-screw IHI unit with integrated charge coolers and a boost output of 11.6 psi This engine produces 707 bhp at 6,000 rpm and 650 lb. ft. of torque at 4,000 rpm. It also features a 9.5:1 compression ratio. Until the Dodge Demon was launched, this engine was Chrysler’s most powerful production engine and the most powerful production engine ever installed in a muscle vehicle. This engine is not equipped with the Multi-Displacement System developed by Chrysler.
The Demon version of the Hemi V8 boasts several enhancements above the Hellcat model. It features a 2.7-liter twin-screw supercharger, stronger reciprocating components, a redesigned camshaft, and numerous other valve train modifications. The Challenger SRT Demon now produces 808 horsepower on 91-octane regular gasoline and 840 horsepower on 100-octane unleaded race gasoline. A functioning Air-Grabber hood scoop aids in cooling, as does a novel charge cooling system that utilizes the air-conditioning coolant to lower the intake charge air temperature. High-speed launches are aided by a factory line-lock system that enables the car to conduct a burnout to warm the rear tires, the world’s first production-car transmission brake, and the Torque Reserve Launch System.
Chrysler introduced a larger and more powerful 392-cubic-inch HEMI engine in 2005, rated at 525 horsepower and 510 lb. ft. torque. It is fitted with pistons made of high-strength forged aluminum alloy. Since 2007, this engine has been available as a crate engine with the designation 392 HEMI. The 392 HEMI engine was introduced in the 2011 Dodge Challenger SRT8 with variable camshaft timing and MDS in automatic transmission vehicles. The new 392 HEMI engine, dubbed “Apache,” is based on the third-generation 5.7 L HEMI engine, codenamed “Eagle,” and shares only a few components with the 392 crate engine.