What Is HEMI? How Does It Work?

What Is HEMI How Does It Work

You may have heard the phrase going around of using HEMI engines in advertisements such as TV commercials, which were very popular in the 1950’s to 70's. There was a very popular advertisement of the Chrysler HEMI engines used.

To go any further, we need to understand what HEMI is? HEMI is a type of combustion engine, and as it is cylindrical in shape, it was given the name HEMI short for "hemisphere." It was first used for passenger cars.

Where did HEMI come from?

These are one of the earliest automotive engines and came very quickly as the concept of internal combustion engines was introduced. They were used by the Belgian car makers at first and were a part of the automotive tools used in the car racing tournaments such as the Grand Prix, where it was used in the Fiat 130 HP in 1907. It was then again used in 1914 and 1917 and later became the part of the Toyota’s V8 engine, which was the first ever V8 engine introduced by Toyota.

HEMI engine

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The first proper HEMI engine was used in 1948 by Harry Westlake, who developed a HEMI 6-cylinder engine for Jaguar. Later the 180 horsepower HEMI engine was introduced. Now, the 180 horsepower may seem less popular nowadays, but it was a very big deal in the 1950’s. After this HEMI engine was being developed very quickly and in 1965, an engine with 425 horsepower was introduced.

What kind of Technology is Used?

The reason why the shape of the hemisphere was used were due to the increase in the efficiency of the combustion engine and also the prevention of heat from the chambers. Also, the head allows the presence of 2 chambers, as traditionally only one was used. The technology used in this is the crossflow head design where on one side the intake valve is situated, and the exhaust valve is situated on the opposite side.

The cylindrical head is domed at the top to get a suitable compression ratio at the piston crown. Getting a suitable compression ratio is very important, as it is one of the most important components of the engine and the functionality of the car depends on it.

How does it Work?

HEMI engine

Photo Credit: http://www.allpar.com/mopar/hemi/hemi.html

The most important thing about the HEMI engine is its combustion engine, which is the main reason for its high efficiency. The chambers are the reason that the HEMI engines can produce so much power. The main power lies in the head of the cylinder where the spark plug is located.

The main problem the flat head design had was the reduced thermal efficiency. This problem is eliminated completely by the hemispherical design. The presence of a camshaft, which is situated directly behind the valves, puts pressure on the valves, which makes the need for pushrods and rocker arms unnecessary.

The chambers located in a HEMI engine are made up of very strong metals, and when the engine is fired up, the space between the two chambers fills with high power of gasoline combustion and gives you its topmost power.

The design of the HEMI engine is such that when the fuel enters the engine, it reaches the combustion chamber with the spark plug related at the top. When the combustion process continues, the valves allow the proper intake and exhaust of the air flow.

The piston located directly beside the valve helps to let the particles move efficiently. By this way the thermal efficiency and the volumetric efficiency are improved by using a HEMI engine, and you can get more power compared to the flathead design.

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What is the Difference between a HEMI Engine and a Standard V8 Engine?

Firstly, the hemispherical shape of the HEMI engine gives a better thermal efficiency and also helps in the airflow intake due to the large storage size, whereas a standard V8 engine failed to complete this requirement due to the less airflow intake resulting in low efficiency.

Secondly, the HEMI engine features chambers to be located side by side, which allows easy access to the gases for intake and exhaust, whereas the standard V8 engine used side by side chambers, which limited this possibility.

Thirdly, the good location of the spark plugs, which are located inside the chamber, gives an advantage to the combustion process and helps in the Maxim ignition as possible. There are certain benefits and drawbacks of having a HEMI engine. They are:

HEMI engine

Photo Credit: http://gearheads.org/every-single-chrysler-hemi-engine-ever-made/


  • Surface area: it is a well-established fact that spherical objects tend to have lower surface areas. Having hemispherical heads reduces the imbalance in the temperature of the fuel. The heads help in fast loss of heat, which is good for the engine. Overheating of an engine is a very common problem, which can cause permanent damage to the engine if not treated in time. Having HEMI engine eliminates the problem and helps in stabilizing the temperature.
  • More room for valves: The main issue, which the preceding engine makers faced, was the space problem to fit the valves, which resulted in a low efficiency of the engine. This problem was solved by the HEMI engine makers, as they improved on the size of the valves and also allowed more room between them, which made the airflow system to be efficient.
  • Better angle: The HEMI engines have an extra angle, as the total diameter of the valve is increased, which exceeds the bore size due to the configuration of the overhead valve? The earlier models had chambers placed side by side, which restricted the airflow. This limitation was eliminated by the use of HEMI chambers, as they provided enough room.


  • Heavy valves: we can’t dispute the better positioning of the valves and the spark plugs in a HEMI engine, but the valves do serve a drawback of being very heavy. Due to the increased weight of the valves, it requires a higher amount of power to operate and that in turn increases the cost of the engine.
  • Oversized combustion chamber: the reason that HEMI engines are used in powerful vehicles and muscle cars is due to the massive size of the combustion chambers, which requires large engine space; as a result, they are only compatible with racer cars. Streetcars do not have the capacity to accommodate such engines, and therefore the use of these gets is limited to the only certain type of cars.

HEMI engines were no doubt one of the best thing Technologies gave us as it provided the basic foundation of the automotive, which we use today. The original HEMI engines are now hard to find, as the developers have modified them to a more efficient and compact design. Still, the legacy of the HEMI engine continues.