The combustion chamber of the YZF-R15 engine is a hemispherical 4-valve type and its shape is designed with the minimum area necessary for the two intake and two exhaust valves. At the same time it is designed with a minimum intake and exhaust valve angles to realize an optimum combustion chamber shape. As an important factor determining combustion efficiency, the surface area/volume ratio (S/V ratio) has been minimized as well. This combines with the effects of the fuel injection system to produce outstanding engine performance characteristics and contributes to more enjoyable running performance in long-distance riding in the suburban areas and running in the mid- to high-speed range.