The Chrysler Hemi engine, known by the trademark Hemi or HEMI, is a series of high-performance American overhead valve V8 engines built by Chrysler with hemispherical combustion chambers. Three generations have been produced: the FirePower series (with displacements from from 1951 until 1958; a race and street engine from 1964 through 1971; and family of advanced Hemis (displacing between since 2003.
Although Chrysler is most identified with "Hemi" as a marketing term, many other auto manufacturers have incorporated Hemispherical cylinder head designs.
During the 1970s and 1980s, Chrysler also applied the term Hemi to their Australian-made Hemi-6 Engine, and a 4-cylinder Mitsubishi 2.6 L engine installed in various North American market vehicles.
Design
The main advantage of a hemispherical head engine over other head designs is power. On the other hand, hemi head engines tend to have complex valve trains, are expensive to build, and are larger and heavier than conventional designs.
A hemispherical combustion chamber is efficient, with an excellent surface-to-volume ratio, and room for two large valves. However, it allows no more than two valves per cylinder because of their large size, and they are necessarily heavier than those in a multi-valve engine. The intake and exhaust valves lie on opposite sides of the chamber and necessitate a "cross-flow" head design. Because the combustion chamber is a partial hemisphere, a flat-topped piston would yield a low compression ratio unless a very long stroke is used. The piston crown is domed to protrude into the head at top dead center to attain the desired compression ratio. The result is a combustion chamber in the shape of the space between where the domed piston stops and the dome shape in the head receiving it.
The hemi-head design places the spark plug at or near the chamber's center to promote a strong flame front. However, if the hemi-head hemisphere is of equal diameter to the piston, there is minimal squish for proper turbulence to mix fuel and air thoroughly. Thus, hemi-heads, because of their lack of squish, are more sensitive to fuel octane rating; a given compression ratio will require a higher octane rating to avoid detonation in a hemi engine than in some conventional engine designs, such as the wedge and bathtub.
The hemi head always has intake and exhaust valve stems that point in different directions, requiring a large, wide cylinder head and complex rocker arm geometry in both cam-in-block and single overhead cam engines (dual overhead cam engines may not have rocker arms). This adds to the overall width of the engine, limiting the vehicles in which it can be installed.
Significant challenges in the commercialization of engine designs using hemispherical chambers revolved around the valve actuation, specifically how to make it effective, efficient, and reliable at an acceptable cost. This complexity was referenced early in Chrysler's development of their 1950s hemi engine: the head was referred to in company advertising as the Double Rocker Shaft head.
First generation: FirePower 1951–1958
Chrysler developed its first experimental hemi engine for the Republic P-47 Thunderbolt fighter aircraft. The XIV-2220 was an inverted overhead-valve (OHV) 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.
In addition to the aircraft engine, Chrysler and Continental Motors Company worked together to develop the air-cooled AV-1790-5B V12 Hemi engine used in the M47 Patton tank.
Chrysler applied their military experience with the hemispherical combustion chamber to its first automobile engine, an OHV V8 engine released under the name FirePower, not "Hemi," in 1950 for the 1951 model year. The first version of the FirePower engine had a displacement of and produced . Eventually, three of the four Chrysler divisions had their own version of the FirePower engine, with different displacements and designations, and having almost no parts in common. This lack of commonality was due in part to the three engine versions using different bore spacings (the center-to-center distance between adjacent cylinders). 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 did not have a version, but retained the Dodge poly-head engines. There was no Plymouth Hemi engine until the 1964 426.
Briggs Cunningham used the Chrysler version in some of his race cars for international motorsports. A Chrysler-powered Cunningham C-5R won its class in 1953. Cunningham switched away from these designs in 1959 when Chrysler temporarily abandoned the hemispherical concept in favor of the wedge-head B engine until 1964. Carl Kiekhaefer also used the Chrysler engines in his NASCAR cars from 1955 and 1956, winning the Grand National Series championship both years.
Collectively, the 1951–1958 Hemi engines are now commonly retroactively referred to as first-generation Hemi engines, which can be identified by the rear-mounted distributor and the spark plugs in a row down the center of wide valve covers.
1951 Plymouth Hemi V6 There were plans in 1951 for a Plymouth dual overhead cam Hemi V6 displacing designed by Chrysler's Powerplant Research and Engine Design division. Known internally as A173, it was meant to be a powerful, fuel-efficient alternative to Ford's V8 and to replace Plymouth's venerable flathead six. However, the plans were scrapped because of high build costs and the then-unusual design.
Chrysler and Imperial All Chrysler FirePower engines are oversquare; i.e. their bore is larger than their stroke.
331 This first FirePower engine, used from 1951 to 1955, has a bore of and a stroke of for a piston displacement of , with a deck height of ("low deck"). The bore spacing, shared by all Chrysler FirePower engines, was , the largest of any first generation Hemi engines. Most used a two-barrel carburetor and produced , with the exception of the 1955 Chrysler C-300 equipped with dual Carter WCFB four-barrel carburetors and was rated at .
The 331 engine was used in the following applications: 1951–1955 Chrysler New Yorker 1951–1954 Chrysler Imperial and 1955 Imperial 1951 Chrysler Saratoga 1952 Chrysler Saratoga Club Coupe 1952 Chrysler Imperial Parade Phaeton 1954 Dodge C Series 1955 Chrysler C-300 1956 Facel Vega FV2B 1958 Hongqi CA72 The Chrysler Air-Raid Siren. At 138 decibels, it is reputed to be the loudest siren ever made.
354 The 354, released in 1956, had a bore of 3.9375 (100mm) in and stroke of 3.625 (92.1mm) in, and the same 10.385" (264mm) low deck height, for a displacement of . The 300B engine was rated at , while the New Yorker and Imperial 354 engine configuration produced . For the 300B, an optional version was available, making it the first American V8 to be rated at one horsepower per cubic inch. (Note that before 1972, horsepower was SAE gross (equivalent to maximum output); thereafter, it was rated as SAE net (power delivered to the transmission, less accessories, emission controls, exhaust system, and other power-sinks).) The 354 was also modified. The Hemi was optimized for heavy-duty truck service. These were available with one or two four-barrel carburetors, and were offered in Dodge's heaviest-duty models as the 'Power Giant V-8' from 1957 through 1959; they were the largest of four Hemi truck engines offered by Dodge in the 1950s. The 354 was also offered in certain models with polyspheric heads rather than hemi heads. The combustion chambers on these had similarities to both hemi and wedge heads, but were closer in weight to wedge heads. Thus, both 354 poly and 354 Hemi V8 engines were variously available in 1957.
The 354 engine was used in the following applications: 1956 Chrysler New Yorker 1956 Chrysler 300B 1956 Imperial Custom & Crown 1957 Dodge D-501 1957–1959 Dodge C Series Pickup
392 The 392 raised-deck engine released in 1957 had a bore and stroke. The actual displacement is . The deck height, at , was taller than that of the previous blocks. Because its deck was taller, the heads were cast with wider intake ports so that earlier manifolds could be used with the new heads on the new taller block. For 1958, Chrysler offered the 392 in two configurations: with 9.25:1 compression and with 10:1 compression, both with a single four-barrel carburetor. A dual four-barrel version of the 392 available in the 1957–58 Chrysler 300C and 300D cars was rated at ; the 300D, and some marine and industrial engines, used a (now rare) adjustable rocker.Hot Rod Magazine online (retrieved March 9, 2018) An extremely rare option available on the 1958 300D was Bendix "Electrojector" fuel injection, with which the 392 was rated at . Due to reliability problems with the primitive onboard computer which controlled the injection system; however, 15 of the 16 300D cars built with the fuel injection option were recalled and retrofitted with carburetors.
The 392 engine was used in the following applications: 1957–1958 Chrysler New Yorker 1957–1958 Imperial Custom, Crown, and LeBaron 1957 Chrysler 300C 1958 Chrysler 300D 1958 Facel Vega Excellence (EX)
In the late 1950s and early 1960s, drag racers found the 392 to be a formidable engine and continued to run them competitively into the 1970s. Usual color of the block was silver.
DeSoto
DeSoto's Hemi engines were called FireDome and served as the naming convention for the DeSoto Firedome sedan.
276 In 1952, DeSoto introduced its version of the FirePower (FireDome), with a bore of and stroke of , for a displacement of . The bore spacing, shared by all DeSoto FirePower engines, was . Power output was . It was a hot seller, with 50,000 vehicles using the engine until it was replaced in 1954.
291 In 1955, DeSoto increased the displacement of the FirePower (FireDome), by increasing the bore to for a displacement of . Power output was with a 2-barrel carburetor for the Firedome models and with a 4-barrel carburetor for the Fireflite models.
330 For 1956, the displacement for the Firedome and Fireflite models was increased again to by increasing the stroke to . Power output was with a 2-barrel carburetor for the Firedome models and with a 4-barrel carburetor for the Fireflite models.
341 Displacement was increased again for the 1956 (DeSoto Adventurer only) and 1957 (Firedome and Fireflite models) to . Bore was increased to , but stroke remained at . The DeSoto Adventurer had a compression ratio of 9.5:1, using a special camshaft profile for hydraulic lifters, and produced using dual Carter WCFB four-barrel carburetors.
The 1956 Adventurer was the premiere named high-performance version—the DeSoto equivalent of the twin four-barrel Chrysler 300.
345 The largest DeSoto engine for 1957 was the DeSoto Adventurer offering with square bore and stroke dimensions of 3.80 inches. The DeSoto Adventurer used dual Carter WCFB four-barrel carburetors for a rating of , producing one horsepower per cubic inch (the first American car to do so as standard equipment) using a similar intake manifold to the 1956 341 Adventurer and a similar camshaft. The compression ratio remained at 9.5:1.
Dodge