How Does a Hemi Engine Work?

Chrysler's branded synonym for high power is a powerplant with a long history.

2023 Dodge Hemi ChallengeDodge


A quest by Chrysler in the 1960s to dominate car racing in the U.S. helped turn the hemispheric V8, a design variation of the internal combustion engine, into the Hemi, an enduring synonym for street-racing power. While Chrysler, Dodge, Jeep, and Ram have long offered high-output versions under the Hemi trademark, the hemispheric engine's history and applications extend well beyond just one carmaking family.

Chrysler FirePower engineChrysler

The First Hemispheric Engine Was Used in a Boat

The hemispheric engine was invented in 1901 by a Wisconsin man named Allie Ray Welch, with a two-cylinder version first used to power a boat. The concept was applied to an automobile for the first time by the long-defunct Belgian carmaker Pipe, which put a hemispheric four-cylinder engine in a vehicle in 1905.

Hemispheric engines later became a popular power option in European brands such as Alfa Romeo, Daimler, and Peugeot, especially for early race cars.

Many automakers — including Aston Martin, Ford, Jaguar, Mercedes-Benz, Porsche, Mitsubishi, and Toyota — have experimented with their own variations of the hemispheric engine. Chrysler first attempted to develop a 16-cylinder version to power the World War II-era P-47 Thunderbolt fighter plane, but the project was scrapped as the war ended. The company's first automotive application of the design came in 1950, with a hemispheric engine marketed as the FirePower.

Chrysler FIrePower engineChrysler

How Hemis Are Different From Traditional Engines

Hemispheric literally describes the shape of the engine's combustion chambers. Picture a domed cylinder head and matching piston tops with spark plugs placed on the top of the cylinder. The shape helps optimize the dynamics of the burning mixture of fuel and air that drives the engine cycle. Exhaust and intake valves are positioned on the opposite side of the chamber, which further improves air flow.

The hemispheric design, with its faster and more efficient combustion, is great for pushing bigger power. The engines' enhanced ability to draw in air helps make them ideal for high-performance applications.

On the downside, Hemi engines tend to weigh more. Early Hemi engines weighed as much as 1,000 pounds. The focus on high output can mean poor fuel economy and higher emissions. The hemispheric design also means the engine can have only two valves, while four valves are more common in modern engines, and allow for more oxygen intake.

Dodge Hemi engineDodge

NASCAR Helped Hemi Engines Become a Household Name

Chrysler, Dodge, and DeSoto vehicles of the 1950s featured hemispheric engines, but it wasn't until Chrysler began to develop hemispheric racing engines for NASCAR that the Hemi name was inexorably linked to the brand.

The Plymouth Belvedere race car's massive 426-cubic-inch (7.0-liter) Hemi was almost too powerful for racing and necessitated the creation of Street Hemi engines for Chrysler family automobiles to make the Belvedere legal for competition. That helped launch the muscle-car era, with Hemi-powered classics ranging from the Barracuda and Road Runner to the first Challenger and Charger.

2023 Ram Hemi EngineRam

Hemi Engines Are No Longer Technically Hemispheric

Stellantis resurrected the Hemi name in 2003 for the Dodge Ram, and the engine name has been a prominent selling point for many of the company's car, truck, and SUV models since then. The rapidly unfolding switchover to electrified technology is bringing an end to the Hemi era, though the design can still be found on models ranging from the final editions of the Dodge Charger to the Jeep Wrangler Rubicon 392.

Modern Chrysler Hemi engines, however, are no longer truly hemispheric in shape, as the half-circle-shaped head has been flattened to improve efficiency and power output.

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Andy Stonehouse
Andy Stonehouse literally fell into the world of auto writing while working as a ski-town journalist, and has not looked back since. A childhood spent dealing with the eccentricities of a 1976 MG Midget has made any subsequent auto experience a more safe and reliable drive. He has been blessed with nearby mountain trails and snowy roads in Colorado to do TV-adventure-styled test drives on a weekly basis.