We all heard about it, but do we really know the origins or how it works…honestly, only a few. But even if you know little or nothing about cars and engines, the word “HEMI” might still mean something to you. The word has become a synonym for big, powerful engines.
Did a little research and came up with this interesting info. 
ORIGINS
The HEMI engine for automobiles was born in 1948 – Harry Westlake and several others developed a Hemi 6-cylinder engine for Jaguar. A few years later, in 1951, Chrysler introduced a 180hp HEMI V-8 engine on several models. The Chrysler HEMI engine had a displacement of 331 cubic inches (5.4 liters), so it is known as the “331 HEMI.”
These days, 180 horsepower sounds like nothing. For example, you can get a 2003 Dodge Neon with a stock 2.0 liter, 4-cylinder engine that produces 150 horsepower. The 5.7 liter LS6 V-8 in the 2003 Chevy Corvette produces over 400 horsepower. But in 1951, 180 horsepower was unheard of. It was an amazing amount of power for the day, and it fueled the “HEMI legend.”Chrysler continued improving the HEMI design, releasing a 354-cubic-inch design in 1956, a 392 cubic-inch design in 1957, and ultimately a 426-cubic-inch (7-liter) version in 1964. The 426 engine set the HEMI legend in stone when it won first, second and third place in the 1964 Daytona 500 NASCAR race. The 426 street HEMI came out in 1965, producing 425 horsepower. The 426 block and heads are still available today from Dodge. The 426 HEMI is a popular power plant for drag racing, funny cars and muscle cars.
BASICS
The thing that allowed the 1951 Chrysler HEMI to produce so much more power than other engines of the day was the efficiency of the combustion chamber.
In a HEMI engine, the top of the combustion chamber is hemispherical, as seen in the image above. The combustion area in the head is shaped like half of a sphere. An engine like this is said to have “hemispherical heads.” In a HEMI head, the spark plug is normally located at the top of the combustion chamber, and the valves open on opposite sides of the combustion chamber. Most cars prior to the 1950s used what was known as a flat head, and many lawn mower engines still use the flathead design today because it is less expensive to manufacture. In a flathead engine, the valves are in the block, rather than in the head, and they open in a chamber beside the piston.
The head in a flathead engine is extremely simple – it is a solid metal casting with a hole drilled in it to accept the spark plug. The camshaft in the block pushes directly on the valve stems to open the valves, eliminating the need for pushrods and rocker arms. Everything is simpler in the flathead design. The problem with a flathead engine is its thermal efficiency, as discussed in the following section.
Advantages
There are many different parts of an engine’s design that control the amount of power you can extract from each combustion stroke. For example:
- You want to burn all of the gas in the cylinder. If the design leaves any of the gas unburned, that is untapped energy.
- You want the maximum cylinder pressure to occur when the crankshaft is at the right angle, so that you extract all of the energy from the pressure.
- You want to waste as little of the engine’s energy as possible sucking air and fuel into the combustion chamber and pushing exhaust out.
- You want to lose as little heat as possible to the heads and the cylinder walls. Heat is one of the things creating pressure in the cylinder, so lost heat means lower peak pressures.
The last item in the list is one of the key advantages of the HEMI head versus the flathead engine. Surface area causes heat loss. Fuel that is near the head walls may be so cool that it does not burn efficiently. With a flat head, the amount of surface area relative to volume of the combustion chamber is large. In a HEMI engine, the surface area is much smaller than in a flat head, so less heat escapes and peak pressure can be higher.
Another factor with a HEMI head is the size of the valves. Since the valves are on opposite sides of the head, there is more room for valves. The engine design that preceded the HEMI was a wedge-shaped combustion chamber with the valves in line with each other. The inline arrangement limited valve size. In a HEMI engine, valves can be large so the airflow through the engine is improved.Disadvantages
If HEMI engines have all these advantages, why aren’t all engines using hemispherical heads? It’s because there are even better configurations available today.
One thing that a hemispherical head will never have is four valves per cylinder. The valve angles would be so crazy that the head would be nearly impossible to design. Having only two valves per cylinder is not an issue in drag racing or NASCAR because racing engines are limited to two valves per cylinder in these categories. But on the street, four slightly smaller valves let an engine breathe easier than two large valves. Modern engines use a pentroof design to accommodate four valves.
Another reason most high-performance engines no longer use a HEMI design is the desire to create a smaller combustion chamber. Small chambers further reduce the heat lost during combustion, and also shorten the distance the flame front must travel during combustion. The compact pentroof design is helpful here, as well.
ps - I will add some pics and more info to finish up this explanation when I get home from work
Information taken from howstuffworks.com
