In the automotive world Hemi
is a slang or short term for engines using hemispherical combustion chambers
, the most famous being Chrysler's 426 Hemi
. Hemispherical heads are popular because they flow gases in and out of the head very easily, and valve
size can be quite large for a given size of cylinder bore
. However, the valve stem
s do not line up as they do in wedge head designs, making rocker arm
design more difficult and complex. It is also harder to raise the compression ratio
much beyond 11 to one because the valves push in at a large angle compared to piston motion. Domed pistons must be notched to allow for the valves, and the notches must be quite large and carefully machined.
The most famous hemi engines were built by Chrysler starting with the 331 Hemi engine that powered the Chrysler 300B, the Cunningham C4R race cars that won the 12 hours of Sebring, and some Allard J2X road racing cars. Chrysler steadily enlarged the hemi engines throughout the 50s until they got to the 392 Hemi, which was employed by Top Fuel Dragster and Funny Car racers into the 1970s. Eventually, every Chrysler brand built their own hemis of varying displacement. High cost led to the replacement of the hemis in Chrysler's street cars, until the 426 Hemi reappeared in 1965 to dominate in drag racing and NASCAR. Pollution controls and the energy crisis led Chrysler to end Hemi production in 1973. In 2002 the Hemi returned in a 5.7 ltr (345 cubic inch) version for the Ram truck, but has since found its way into larger Chrysler produced cars, trucks and SUVs.
However, hemi engines are not the personal property of Chrysler. Toyota corollas from the late 70s and early 80s were powered by the 2TC, and 3TC engines, both of which used hemispherical combustion chambers. Other engines employ hemispherical heads as well. Racing engines continue to use hemispherical heads, particularly those using dual overhead cam (DOHC) designs. That sort of cam design eliminates the rocker arm disadvantages hampering pushrod and SOHC designs.
Visually, hemi head engines are fairly easy to identify. The spark plugs are located in the center of the combustion chamber which means they are located in the center of the valve cover. Wedge head designs have the plugs enter the head outside the valve covers, usually between the exhaust ports.