Now-a-days Hemi engines are used as a nickname for powerful engines. But word Hemi is used for the Hemi-spherical shape of the combustion chamber.
The two most basic engines are flat-head and Hemi engines. There are more types also but these two started the era of motors. We all know that the shape which occupies least surface area is circle/sphere. That is if there are two metal pieces of the same volume – one with a cuboid shape and other with a spherical shape. The piece with spherical shape will occupy less surface area. This concept makes hemi engines work more efficiently.
It is a simple concept that in a combustion chamber, the air-fuel mixture near to the walls and head of the cylinder will be cooler than the mixture at center because of the air flow whenever the valve opens. And cooler air-fuel mixture will not burn efficiently. In flat-head engines, there will be more surface area occupied by combustion chamber due to their cuboid shapes. This will let the more air-fuel mixture to aggregate near walls of the chamber.
The Hemi-spherical shape of the combustion chamber will occupy less space and hence its shape will not let more amount of air-fuel mixture to attach to walls. Which means more amount of air-fuel mixture will be present at the center causing it to burn efficiently and thereby increasing the power of the engine.