a variable valve timing system in which the cam shaft is rotated with respect to the crankshaft position by moving a shaft disposed interior to the cam shaft longitudinally has been developed . the variable valve timing system provides for varying the timing of the opening and closing of an engine &# 39 ; s poppet valves to enhance the engine &# 39 ; s efficiency and output . the timing system can be used with both intake and exhaust valves in both diesel and gasoline engines . the timing system is inexpensive to produce , and performs reliably over a lengthy period of time .