The surface-to-volume-ratio in the moving combustion chamber is so complex that a direct comparison cannot be made between a reciprocating piston engine and a Wankel engine. The flow velocity and the heat losses are quite different. Surface temperature characteristics are completely different; the film of oil in the Wankel engine acts as insulation. Engines with a higher compression ratio have a worse surface-to-volume ratio. The surface-to-volume ratio of a reciprocating piston diesel engine is much poorer than a reciprocating piston gasoline engine, but diesel engines have a higher efficiency factor. Hence, comparing power outputs is a realistic metric. A reciprocating piston engine with equal power to a Wankel will be approximately twice the displacement. When comparing the power-to-weight ratio, physical size or physical weight to a similar power output piston engine, the Wankel is superior.