Engines Exposed: The Hemi and its 6 Decades of Dominance


SRT Charger Hellcat
Source: Fiat-Chrysler

Unless you have been living under a rock for the past few years (which I don’t advise — see 127 Hours for supporting evidence), you have heard about the Hellcat versions of the Charger and the Challenger. We have covered them extensively on this site, including talking about how efficient they are. Now, you might not expect that a 707-horsepower muscle car from Detroit would be particularly efficient, but the engineers at Dodge, which is now part of Fiat Chrysler Automobiles, have leveraged one of the keystones of the muscle car era to make this magic happen: the Hemi.

There are times in life when you have a question that you don’t want to ask because it seems like you are the only one that doesn’t know the answer and you don’t want to appear uninformed. When I started studying cars, “What is a Hemi?” was one of those questions. Allow us to save you the trouble of asking as we explain what it is and why it matters.

Source: iStock
Cylinder head with a dual cam-shaft layout | Source: iStock

Let’s start with the basics. Hemi, though it is now a registered trademark, is simply short for hemisphere, or hemispherical. This is due to the fact that the combustion chamber (the space in the cylinder head where the combustion will occur — unless it’s a Heron head, but that’s a topic for another day) is in the shape of a hemisphere. Seems like a lot of hoopla for what is essentially a round combustion chamber. Why does it matter?

Our article that discussed how camshafts help your engine to breathe alluded to the fact that the efficiency of cylinder flow is improved if the intake and exhaust valves are arranged on opposite sides of the cylinder head, in what is known as a cross-flow arrangement. This cross-flow style of cylinder head has become extremely popular due to increased efficiency. Contrary to what you might expect, the increased efficiency is not due to the alignment of the air passages through the cylinder head: It’s due to the ability to have multiple or larger valves to utilize inertia of the airflow to draw the intake charge into the cylinder, and to control heating of the intake plumbing. The Hemi helped designers to exploit all of these advantages.

The hemispherical combustion chamber has been around since the early 1900s but found limited production acceptance until the middle part of the century. It was widely tested during WWII, and it was at that time that engineers realized that conventional wisdom surrounding the hemi design, namely that it was rough and could only run on high-octane fuel, was flawed. After a lot of design work and testing, the first Hemi V8, named Fire Power, was introduced in 1950 for 1951 models of the Chrysler Saratoga, New Yorker, and Imperial. This engine, when mated with the Saratoga Club Coupe, blitzed the 0 to 60 miles per hour run with a time of 12.5 seconds.

Source: Fiat Chrysler Automobiles
A 1951 Chrysler Saratoga Club Coupe | Source: Fiat Chrysler Automobiles

The leadership at Chrysler responded to enthusiasm from the consumers and attempted to optimize the performance of the Hemi engine. James Zeder, vice president for engineering, stated in 1952:

The basic Fire Power cylinder gives performance comparable with Indianapolis engines, which have been developed for power without regard to any other purpose … we remain unalterably convinced that, in the battle of the combustion chambers, the spherical segment chamber has demonstrated unquestionable supremacy.

Going back to basic geometry shows us some of the reason why the hemisphere is a logical choice for a combustion chamber. Spheres have the smallest ratio of surface area to volume, which means that they allow for a sizable combustion chamber in a small space. More importantly, the spherical shape allows for a splayed valve angle (meaning the two valves are in the shape of a V), which means it is possible for the two valves to be wider than the bore of the cylinder. This helps to increase airflow to the engine and improve power.

Source: Fiat-Chrysler

While the Hemi name is most commonly associated with Chrysler, many manufacturers have produced engines with hemispherical combustion chambers. Chrysler wasn’t even the first to do so, nor did it claim to be. However, it was the one that helped the Hemi to become a household name. The Fire Power Hemi V8 went on to achieve a cult-like reputation in the automotive world, which was continued by the 426 Hemi that was introduced in 1964. During the 1964 Daytona 500, Hemi-powered Mopar cars swept the results and took the first four spots.

Over the past 60 years, Hemi engines have set new records in the world of racing and powered many of the most iconic cars. In fact, the first car ever to come with 300 horsepower, the Chrysler C-300, which was introduced in January 1955, was powered by a Hemi V8. This proves that the Hellcats, currently two of the most powerful cars on the market, are not unusual. For the Hemi and FCA, it is merely a return to its roots.

Like classics? It’s always Throwback Thursday somewhere.