A Hemi Engine gets its name from having a hemispherical combustion chamber which is shaped like a dome or half of a
sphere at the top of the cylinder.
This spherical shape is contained in the engine head as the cylinder itself in the engine block retains the regular
The area in the engine head that forms the top of the combustion chamber along with the intake and exhaust valves
and spark plug is usually flat or wedge shaped.
The spherical shaped chambers in the head of a hemi engine.
The top of the piston may also be similar to the dome shape and may extend up into the engine head when the piston
is at top dead center.
The usual configuration for the hemispherical chamber is to have the spark plug at the top/center of the dome with
the valves on both sides (left & right) of the plug.
Hemi engines have no over head camshafts but use pushrods and rocker arms to operate
The hemispherical chamber is very efficient. There are no corners or hidden pockets so the entire air/fuel mixture
The spark plug being in the center of the sphere along with the shape of the chamber allows the explosion to quickly
travel throughout the chamber.
The shape of the chamber also allows it to have a reduction in the amount of heat loss as the surface area is very
Hemi engines are high revving and a little more fuel efficient than an engine of similar specifications with a
regular head. Forced induction is also easily facilitated.
Popularity of the engine began in 1948 when a six cylinder hemi was developed for Jaguar and in 1951 Crysler developed
a V8 hemi for some of their models.
Hemi engines are still used on high horsepower, performance engines and can be seen in vehicles from Crysler/Dodge/Jeep
SRT performance vehicles.
The downside to a hemi engine is that the head can only accommodate one valve per intake and exhaust while many other
high performance engines have two valves each.
Although the hemi engine is not used by most automotive marques presently, it is still a powerful and unique engine with
a unique sound and feel to it.