The History of the Hemispherical "HEMI" Engine.
A Chrysler Hemi engine is one of three different internal combustion engine families from Chrysler that are Hemi engines; in other words, they utilize a hemispherical combustion chamber. A hemispherical (inverted bowl-shape) combustion chamber allows the valves of a two valve-per-cylinder engine to be angled rather than side-by-side. This design also allows the placement of the sparkplug nearer to the center, which is mandatory, given that there is minimal quench and swirl to burn fuel gasses thoroughly and quickly. A hemispherical combustion chamber often requires a dome-shaped pistoncrown to maintain the desired compression ratio.
The advantages of the hemi cylinder head come at the disadvantage of requiring intake and exhaust valve stems that point in different directions, requiring a much more complex rocker arm geometry in both cam in block and overhead cam engines. This also increases the space taken up by the cylinder head--hemi engines are not space-efficient. Also, the hemi design lacks the quench area that exists with wedge combustion chamber designs, making the hemi more sensitive to fuel octane. (A given compression ratio will require higher octane in a hemi engine than a wedged combustion chamber engine.)
The three generations of Chrysler Hemi engines for automobiles included the first (the Chrysler FirePower engine) in the 1950s, the second from the mid 1960s through the mid 1970s, and finally in the early 2000s through today.