Pictured above is a combustion chamber. The combustion chamber is in the bottom of the heads. The circular pieces are the bottoms of the valves. This combustion chamber has 2 intake valves (the larger ones) and 2 exhaust valves (the smaller ones). Exhaust valves can be smaller than intake valves, because it is easier to push air out of an engine than it is to draw air in.
The combustion chamber is a depression in the bottom of the heads where the valves are located. Its geometry is very complicated, so it is not factored in to hipermath, yet. There is nothing to do, for now, except become familiar with what combustion chambers are. HiPerMath combustion chamber sizes range from 0 - 100 and are measured in cubic centimeters (cm3).