What is a HEMI ?

Folks owning a Chrysler car (or truck) will often broach the subject of HEMIs as a matter of pride or nostalgia, leaving us wondering what HEMI means. In simplest terms, HEMI, is an engine name that was trademarked by Chrysler in the 1950s to represent an engine with hemispherical combustion chambers. The result, as proved by more than one NASCAR race, was a series of engines with superior horsepower, fuel efficiency and torque.

Since then, HEMI has become a synonym for powerful engines that are typically used in trucks, muscle cars and increasingly, premium consumer cars. Before we go into just why the HEMI is so efficient though, we need to look at its evolution and ask ourselves, in little more detail, “What is a HEMI” ?

History of HEMI

Though Chrysler managed to secure exclusive rights to use the term HEMI, engines with hemispherical combustion chambers were in use long before the 1950s. That said, most of the passenger cars in the 1940s and 1950s were still using the older flat bed engine since it was cheaper to produce. Chrysler managed to introduce the HEMI engine to passenger cars in the form of a new line of V8 engines debuting in 1951. This engine clocked a remarkable (for that period) 180hp.

However, it was in the 1960s, however, that the HEMI became a countrywide craze once it occupied all three pole positions in the NASCAR race. By this time, the HEMI was achieving an admirable 425 hp without giving into many of the problems of flat bed engines.

As HEMI and Chrysler’s fortunes rose, other companies also began to utilize the design. Porsche, General Motors, Aston Martin and Ford are only a few names in the long list of companies that flocked to the HEMI camp, albeit under a range of names (since HEMI itself was trademarked by Chrysler).

So What is HEMI all about ?

As mentioned above, HEMI consists of hemispherical combustion chambers. What these basically mean is that the top section of the combustion chambers (or combustion heads as they are called) are hemispherical in shape, as opposed to the flat design that was used in earlier cars. This allows the spark plug to be on top of the combustion chamber in a manner that coordinates well with the valves. Speaking of valves, these are usually placed on either side of the chamber. This “across” design of the valves is different from the flat head design since the latter used valves that were placed side by side.

As one would expect, the HEMI head created a new combustion chamber design, which from the outside resembled a somewhat conical pit. The overall engine design also had to be modified, but the number of combustion chambers was not impacted by the new design. What was impacted was the number of valves. The HEMI head was designed such that it could accept two valves. Placing more valves created problems of angling them properly to each other, and for this reason, multi-valve options were severely limited.

Pros of HEMI


The unique design of HEMI created a number of benefits as well as challenges. The pros were


Via: auto.howstuffworks.com

  • Surface area concerns – It is well known that spherical objects have lower surface areas. For this reason, the hemispherical heads have lower surface area, and this reduces the chances of there being imbalances in the temperature of fuel in the chamber due to loss of heat from the chamber. Uniformity and adequacy of heat in the chamber make combustion more efficient.
  • Power-compression ratio – Since the combustion process is more efficient, the pressure generated in the chamber is higher. This dispenses with the need for higher compression to achieve necessary pressure in the chamber. Such efficiency is directly translated into better engine power.
  • Spark plug positioning – The HEMi design places the spark plugs in a position where they are able to achieve maximum ignition in the shortest possible time.
  • Valve positioning – The two valves found in the hemi head are located across the chamber, instead of being located side by side. This removes the limitations flat heads had on the size of the valves. Since larger valves allow for greater flow of intake and exhaust gases and general flow of air through the engine, this positioning is crucial to allowing faster combustion of fuel.

Cons of HEMI

  • Heavier valves – The design of HEMI was such that the valves became heavier, requiring greater power to operate. This increased the cost of the engine and acted as a counterweight to the overall efficiency of the engine.
  • Number of valves – While flat head engines during the 1940s and 1950s also used two valves, the hemi design actually limited the number of valves to two. While other multi-valve engine options emerged, design improvements could not overcome this limitation. This limitation was not immediately evident on the racing circuit because the NASCAR would make do with two large valves. However, street cars could benefit from a four-valve design in certain circumstances.
  • Combustion chamber size – If you’ve ever wondered why HEMI engines are usually associated with trucks and muscle cars, it is because of the size of the engine required to accommodate the HEMI. Street cars often require compact engines which HEMI can’t provide, and for this reason, alternatives eventually had to be evolved.

Evolution of HEMI

Despite the shortcomings, the sheer efficiency of the engine and the horsepower heights it could reach allowed companies like Chrysler to constantly improve the design. Chrysler itself has had three generations of HEMI engines, the Hellcat being an example of the latest generation.

While adding muscle, improvements have also improved the fuel efficiency and emission efficiency of the engine. Case in point is the HEMI Magnum, which doubled the number of spark plugs in order to fix the poor emission record of the HEMI.

In a rapidly evolving market like that of the automotive, it is impossible for an engine variant to hold sway for a long period. Seen in this light, the HEMI is a remarkable exception because, having been introduced in the 1950s, it remains in production today.

Of course, necessity has forced most manufacturers to move away from the original HEMI design to more complex, compact and efficient ones. Hence, while getting an original HEMI in your car is now a remote possibility, it is quite likely that you could get a car or truck that offers a modified HEMI engine that has built on the rich legacy of this remarkable engine variant.

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