If you remember the ´50s and ´60s, then the word “Hemi” can bring back some fond memories.
The Chrysler Hemi engines, known by the trademark Hemi, are a series of I6 and V8 engines built by Chrysler with hemispherical combustion chambers. Three different types of Hemi engines have been built by Chrysler for automobiles: the first (known as the Chrysler Firepower engine) from 1951 to 1958,[1] the second from 1964 to 1971, and the third beginning in 2003. Although Chrysler is most identified with the use of “Hemi” as a marketing term, many other auto manufacturers have incorporated similar designs.
Chrysler, I feel, was way ahead of their time when they produced the Hemi. It produced a large amount of horsepower which was installed in your everyday family car.
What exactly is a Hemi engine, and why has it kept its flare all these years? The Hemi is a hemispherical combustion chamber, a type of combustion chamber in a reciprocating internal combustion engine with a domed cylinder head. The hemispherical shape provides a number of advantages over a reverse-flow cylinder head but comes up short in others, particularly in carbureted engines. An engine featuring this type of hemispherical chamber is known as a hemi engine.
A hemispherical head (“hemi-head”) gives an efficient combustion chamber with minimal heat loss to the head, and allows for two large valves. However, a hemi-head usually allows no more than two valves per cylinder due to the difficulty in arranging the valve gear for four valves at diverging angles, and these large valves are necessarily heavier than those in a multi-valve engine of similar valve area, as well as generally requiring more valve lift. The intake and exhaust valves lie on opposite sides of the chamber and necessitate a “cross-flow” head design. Since the combustion chamber is virtually a hemisphere, a flat-topped piston would yield too low a compression ratio unless a very long stroke is used, so to attain desired compression ratio the piston crown is domed to protrude into the head at top dead centre, resulting in a combustion chamber in the shape of the thick peel of half an orange.
In 1951, you could go down to your local Dodge dealer and for the price of around $1,800 dollars you could purchase a beautifully built, American made car with a Hemi engine. It put out a whopping 180 horsepower.
The love for the Hemi is still alive and well with today’s classic automotive enthusiast. If you have a Hemi in your classic, you can bet car fans will flock to your vehicle, not only give it a look over, but I can bet you, conversation will be plentiful. You may even be asked to start up your vehicle just to hear that sweet sound of a hemi.
There were some disadvantages to owning a Hemi engine. Significant challenges in the commercialization of engines utilizing hemispherical chambers revolved around the design of the valve actuation, and how to make it effective, efficient and reliable at an acceptable cost. This complexity was referenced early in Chrysler’s development of their 1950s hemi engine: the head was referred to in company advertising as the Double Rocker head.[2]Domed pistons are commonly used to maintain a higher mechanical compression ratio, which tend to increase the flame propagation distance, being also detrimental to efficient combustion, unless the number of spark plugs per cylinder is increased.
Other drawbacks of the hemispherical chamber include increased production cost and high relative weight (25% heavier than a comparable wedge head according to Chrysler’s engineers). These have pushed the hemi head out of favor in the modern era.
So the next time you are visiting a classic car show, see if you can identify one with a classic Hemi engine. Happy Motoring.
[1], [2] Reference: Fifties Web.