The History of the Iconic HEMI: From Experimental Aircraft Engine to the 426 Elephant

HEM 426 V8 16 photos
Photo: Chrysler / Stellantis North America
1951 New Yorker1951 Chrysler FirePower Engine1955 Chrysler C-3001956 Chrysler 300B1955 Chrysler C-300 Racecar1957 Chrysler 300C Hardtop1958 Chrysler 300D HardtopThe 426 HEMI1966 Dodge Charger 426 HEMI1967 Dodge Charger 426 HEMI Engine1967 Plymouth Belvedere GTX 426 HEMI Convertible1970 Dodge Challenger RT 426 HEMI1968 Dodge Charger RT 426 HEMI1971 Plymouth Barracuda 426 HEMI ConvertibleOriginal Chrysler Hemi Head Drawing
Thanks to the original Mopar muscle cars from the ‘60s and ‘70s, as well as the Hellcat-powered beasts of today, the HEMI moniker has become almost synonymous with the American V8 engine all across the world.
The legendary powerplant got its name from an innovative feature found inside the cylinder head: the hemispherical combustion chamber. This dome-shaped design allows the placement of the spark plug in the center of the chamber, which reduces the flame travel and burn distance of the air/fuel mixture, meaning that the combustion process is greatly improved.

It’s unclear who exactly invented the hemispherical chamber. Most historians point to Allie Ray Welch from Chelsea, Wisconsin who designed one in 1901 and used it for a prototype two-cylinder boat engine. What we know for sure is that carmakers such as Pipe, Fiat, Peugeot, Alfa Romeo, Daimler, or Riley were using it in the early years of the 20th century.

By 1947, brothers Yura and Zora Arkus-Duntov (the man that revolutionized the Corvette) who had established the Ardun Mechanical Corporation five years earlier, introduced an aftermarket aluminum cylinder head with hemispherical combustion chambers for the Ford flathead V8, marking the first use of a hemispherical head on an existing American V8.

Original Chrysler Hemi Head Drawing
Photo: Chrysler / Stellantis North America
Two years later, British manufacturers Jaguar started employing the design on the legendary XK inline-six. Moreover, Lancia was already using hemispherical chambers on its V4s ever since the 1920s, and it carried them over to the world’s first series-production V6 which debuted in the 1950 Aurelia.

So, contrary to popular belief, Chrysler wasn’t the first carmaker to create such an engine, but it would go on to develop the most popular hemi-headed powerplants of all time.

The corporation began experimenting with the design during the Second World War, attempting to reduce the loss of volumetric and thermal efficiency caused by carbon deposits in their aircraft engines.

It all began with an experimental 2,500-hp, 2,220-ci. (36.4-liter), inverted V16 which was tested in 1945 on a modified Republic P-47 Thunderbolt, the largest and heaviest single-seater used by the U.S. Air Force at the time. Trial flights showed encouraging results, but the war ended several months later, and the engine never made it into production. However, research and development of hemispherical heads continued, and would be applied to a mass-produced automotive engine in 1950.

The Firepower Era

1951 Chrysler FirePower Engine
Photo: Chrysler / Stellantis North America
The first Chrysler-built, hemi-headed V8 was not called HEMI, but came with an arguably cooler name: FirePower. Released with the 1951 model year New Yorker, Imperial, and Saratoga, it was a marvel of engineering for the era.

The oversquare engine was built around a short, and rigid cast-iron block. Inside, a sturdy, forged crankshaft that rode on five main bearings actuated a set of pistons designed to "slip" between the crank’s counterweights at the bottom of the stroke. This delivered a smooth idle and created less friction. Furthermore, engineers designed the motor to run at lower piston speeds, so the long-term reliability of the internals was greatly improved. If you’re interested in more details about the FirePower, I recommend watching the authentic 1951 Chrysler Master Tech video that you can find below, courtesy of MyMopar’s YouTube channel.

Unlike today, when all sister brands use the same engines in multiple models, in the first half of the 1950s, three of the four of the corporation’s divisions developed their own HEMI versions with different displacements and almost no common components. In addition to Chrysler’s FirePower, DeSoto debuted the FireDome in 1952, while Dodge built the Red Ram starting with 1953. Plymouth was the only brand that didn’t use a HEMI. A DOHC V6 prototype with hemi heads was designed in 1951, but it was eventually scrapped due to high production costs.

1955 Chrysler C\-300
Photo: Chrysler / Stellantis North America
These engines were continually developed and in January 1955, Chrysler introduced the C-300, triggering a horsepower war between all major U.S. carmakers. Advertised as “the most powerful production car built in America”, the two-door hardtop developed for NASCAR homologation purposes was powered by the first, true performance-oriented iteration of the HEMI. It had the same displacement as the initial FirePower, but could spit out a whopping 300 hp thanks to upgrades like dual Carter WCFB four-barrel carburetors, a race-spec camshaft, solid valve lifters, or a performance exhaust system.

For 1956, this amazing engine was bored and stroked to 354ci (5.8-liter), making it even more powerful. In the new 300B, it could produce either 340 or 355 hp, depending on the configuration.

The biggest, most powerful version of the first-generation HEMI was the 392-ci. (6.4-liter). It was used on multiple Chrysler models, but its most rabid variants were found under the hood of the 1957 300C (375 hp) and 1958 300D (380 hp). There was also an extremely rare option available on the 300D where the engine was fitted with a Bendix fuel injection system which boosted the output to 390 hp.

Produced until 1958 when they were replaced by the smaller, more efficient wedge-head V8s, the initial Chrysler HEMI V8s became the weapon of choice for many hot-rodders and racecar enthusiasts. These engines helped Carl Kiekhaefer’s team win the NASCAR Grand National Series in 1955 (Tim Flock) and 1956 (Buck Baker). Moreover, they powered mean machines that broke virtually all the drag racing records conceivable during the 1950s.

The First Comeback

1968 Dodge Charger RT 426 HEMI
Photo: Chrysler / Stellantis North America
The hemispherical head design made a comeback in 1964 with the introduction of a completely redesigned engine, the humongous 426-ci (7.0-liter) also known as the Elephant. A race engine developed for NASCAR; it was the first unit to officially bear the now-trademarked HEMI moniker. Rumored to be capable of no less than 750 hp, it was used in the Plymouth Belvedere racing cars that finished first, second, and third at the Daytona 500 that year.

The engine was so dominant that NASCAR was forced to alter the homologation regulations, effectively banning the HEMI for the 1965 season. To return to the competition, Chrysler had to make the engine available in mass-produced models, so the street HEMI was born.

1967 Dodge Charger 426 HEMI Engine
Photo: Chrysler / Stellantis North America
Based on the race unit, the production version had a lower compression ratio, as well as far more plebeian internals or manifolds, and ditched the dry-sump lubrication system for a regular wet sump. Initially available for the 1966 model year Dodge Coronet, Plymouth Belvedere, Dodge Charger, and Plymouth Satellite, it was advertised by the manufacturers with both the gross 425 hp and net 350 hp ratings, but actual dynamometer tests showed an output of 433.5 hp and 472 lb-ft (640 Nm) of torque.

Considered by many the most epic HEMI engine of all time, the 426 was built until 1971. Throughout its six-year lifespan, it was only optionally available on some of the best Dodge and Plymouth models of the era, like the Charger, Super Bee, Challenger, GTX, or Barracuda. Few were sold with HEMIs and even fewer survived in stock form to this day. One of the rarest of the bunch is the 1971 HEMI ’Cuda convertible. Only eleven of them were produced, one of which was auctioned off in 2014 for $3.5 million. Another example was listed by Mecum this year and although the highest bid reached 4.8 million, it wasn't high enough to meet the seller's $6.5 million reserve.

1971 Plymouth Barracuda 426 HEMI Convertible
Photo: Mecum
The only vehicle that was set to offer a 426 as standard was the Monteverdi Hai 450, a Swiss-built supercar that unfortunately never made it out of the prototype stage.

David Freiburger from MotorTrend made an excellent video of 20 moments that made the 426 great, and you can watch it below.

While the first generation of HEMI engines proved the superiority of the hemispherical combustion chamber design, the 426 took it to a whole new level of performance and popularity. Although it was discontinued far too soon, the HEMI would make a second comeback in the 21st century.

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About the author: Vlad Radu
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Vlad's first car was custom coach built: an exotic he made out of wood, cardboard and a borrowed steering wheel at the age of five. Combining his previous experience in writing and car dealership years, his articles focus in depth on special cars of past and present times.
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