Understanding the Hemi Engine

Small hemi racecar

When people think Hemi, they usually think big and powerful engines – images of muscle cars or full throttle drag racing come to mind. For over 50 years, many iconic auto makers have incorporated the Hemi design, from Fiat and Alpha Romeo to BMW and Chrysler. If you know cars or have some auto mechanic training, than you might be aware that Chrysler trademarked the term Hemi back in the 1960s, making it a household name through extensive advertising campaigns.

Now, people are familiar with the trademarked name but not necessarily what it stands for, or the important role Hemi played in the evolution of engine design.

Hemi Is Short For…

Hemispherical. As in the shape of the engine’s combustion chambers:

With the hemispherical combustion design, the intake and exhaust valves are on opposite sides of the chamber. Why is this significant? Because it sends the combustion mixture flowing directly across the chamber.

Hemi Advantages

An auto mechanic knows that for optimum efficiency, you want to get as much power as possible out of your engine. For this to happen, the engine should leave no gas unburned, take full advantage of cylinder pressure, and waste as little energy as possible sucking air and fuel in – and pushing exhaust out.

The Hemi design really scores on that last point. Hemi surface area is smaller than that of a flathead engine, so less heat escapes during combustion – and hotter fuel burns better. And because they’re positioned on opposite sides of the chamber, the Hemi valves can be bigger than on previous models, which put them side by side. Bigger valves means improved air flow through the engine.

Hemi Disadvantages

At this point, you might be asking yourself, if Hemis are so great why don’t all auto makers put them in their cars? Most experts agree that a major drawback of the Hemi design is that it cannot incorporate four valves per cylinder. And that’s OK when it comes to racing cars, which are limited to two valves anyway – but modern cars use an alternative design with four slightly smaller valves that let the engine breathe more easily.

Also, if you’ve taken auto mechanic courses, you know that high performance auto makers prefer smaller combustion engines for their vehicles. They help prevent heat loss with smaller chambers.

Lastly, the Hemi design tends to be more expensive to produce than other models. And it’s heavier than a comparable wedge head. Companies still using the Hemi concept have adapted and manipulated the design to boost cost efficiency, extract more power, and include capabilities like reduced emissions.

Check out this video for a closer look at the Hemi engine:

What’s your favourite Hemi-engine car?

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