What Is a Hemi Engine?

What Is a Hemi Engine?

Last Updated May 31, 2024 | Alison Smith

Dome Pistons for Hemi Engines

Most people have heard of the famed Hemi engine, popular in Ram Trucks and Dodge performance vehicles, such as the Challenger SRT Demon. So, what is a Hemi engine, exactly? A Hemi is an engine with a hemispherical combustion chamber, consisting of dome-shaped cylinders and piston tops. Hemispherical refers to a half-sphere, so the piston tops have to be rounded in order to fit properly within the cylinders. With a smaller surface area than a typical flat-head engine design, less heat is able to escape. Because there is less heat loss, peak pressure is higher in a Hemi engine. Higher pressure means more energy, which in return will give you more power. Valves are also less restricted in a Hemi motor design, so they can be larger in order to allow for more airflow, whether that’s bringing cool air in or sending exhaust gases out. The spark plugs also provide better ignition combustion due to their position within the chamber. All of these things help differentiate the Hemi motor from flat-top engines and contribute to the Hemi’s power.

Newsletter Sign-Up

Hemi Engines: A Brief History

With origins dating back to the early 1900’s, Hemi engines have been around since the beginning of automotive history. However, it was Chrysler that began to revolutionize the Hemi engine in the 1950’s. With the release of Chrysler’s first 331 cubic-inch overhead-valve V8 engine using a hemispherical combustion chamber, the Hemi motor was born. Dubbed the “FirePower” engine rather than a Hemi, the V8 produced a whopping 180 horsepower and 312 lb.-ft. of torque. Although this engine wasn’t referred to as a Hemi by name, it is largely considered Chrysler’s first-generation Hemi. During this period, Chrysler’s Hemi engine started becoming synonymous with power.

426 Hemi Engine Dodge Chrysler

Fast forward to 1964, when Chrysler debuted the 426 Hemi engine, the first to sport the official “HEMI” name. First produced for racing only, street 426 Hemi engines came out a few years later in 1966. Designed for drag racing in the 1964 NASCAR Daytona 500, the 426-cubic inch Hemi put out 425 horsepower. Advertisements for the Dodge 426 Hemi read:

“It’s that wailing stocker that’s about a car length ahead at the end of the quarter. It’s that bright red streak that’s taking home the lap records on the big ovals. And ever since the day the unmistakable sound it makes was first heard on the tracks, competitors screamed ‘unfair.’”

Subsequently winning first, second, and third place in the race, this engine solidified the HEMI name that Chrysler eventually trademarked. Because of the engine’s massive success, it was banned for the 1965 NASCAR season, until it became more readily available for streetcars. It wasn’t just a Hemi anymore, it was a HEMI. The name had meaning.

Challenger SRT Demon Supercharged Hemi Engine

Hemi engines remain in many modern vehicles, including the 2019 Ram 1500 and the Dodge Challenger SRT Demon, which sports a monster supercharged 6.2L HEMI V8 engine that puts out 840 horsepower.

Hemi Engine Advantages

  • More Power: Because of the unique design, Hemi engines offer more power than more traditional engines. What’s better than more power?!
  • Reliability: Hemi engines are known for being durable and long-lasting. So even though they might initially cost more, the longevity they offer makes up for the higher price point.
  • Fun: There’s no better way to put it — Hemi engines are just more enjoyable to drive! Fast acceleration and a menacing sound add to the fun.

5.7L Hemi Engine Ram Trucks

Hemi Engine Disadvantages

  • More expensive: A Hemi will be more expensive than other comparable engines because of how they are constructed.
  • Heavier: The hemispherical design and structure of a Hemi engine add weight, which makes it less efficient. However, the power gains offset most of this.
  • Limited to two valves per cylinder: Because the cylinder head design is a half-sphere, having more than two valves per cylinder isn’t practical due to the angles.
  • Fuel efficiency: Gas mileage is typically not as great with a Hemi and may require premium gas, depending on the version. This adds to the costs, which are already higher than other engines.

Sources: History of the Hemi V-8 Engine, MotorTrend | Dodge Truck HEMI Specs, It Still Runs | How HEMI Engines Work, How Stuff Works | The Original Chrysler Hemi Engine, AllPar | Image Credit: Dodge

Back To Top

This article was researched, written, edited, and reviewed following the steps outlined in our editorial process. Learn more about CJ's editorial standards and guidelines.