The 4-valve combustion chamber of the kind adopted in this engine has two intake and two exhaust valves. The first merit of this design is that it allows the spark plug to be positioned in the center of the combustion chamber to provide more efficient flame spread and combustion. In other words, it enables highly efficient combustion. Also, the 4-valve design enables greater overall valve area than a 2-valve system for more efficient (per unit of area) intake and exhaust function. This is the second major merit.