as in dodge motor engins

It is not initals, they stand for Hemi sphere shaped valves that give it that distinct sound and horsepower.

hemispherical combustion chamber

Its short for a hemispherically shaped combustion chamber.

Hemispherical heads

It is not an acronym, it is a prefix - short for hemispherical, which is the shape of the combustion chambers.
Combustion chambers come in all sizes and shapes - wedge, high swirl, masked and so on. The hemi heads are more expensive to manufacture, but have proved over the years to be very efficient at producing horsepower.

Well, the Hemi is short for ''hemispherical and it means the spark plug is right square in the middle of the cylinder bore. Most engines have the plug off to one side of the cylinder, making it easier to put in bigger valves. Also, it weakens the head by putting less metal between the valve seats and the plug hole. Dodge made the term famous, but alot of other car manufacturers use the same design. It is supposed to get you the max power out of the fuel in the cylinder.

l believe hemispherical' there might be more but i'm not sure

Hemi is an abbreviation for hemispherical ( as in the cylinder head has a hemispherical dome shape for each cylinder ). They are not initials.