Hemi comes from the word hemisphere, and refers to a design of internal-combustion engines where the cylinder head is a hemispherical cavity in the engine block. This term is most commonly used in reference to engines used in some Chrysler vehicles of the 1960s and 1970s.
The hemispherical cylinder head increases the engine's efficiency, but drawbacks such as increased cost to produce has meant that the hemi has been a rare design.
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