Valve springs with high fatigue strength corresponding to the incresement of working stresses are required for the higher generating power and the better fuel economy of automobile engines. For this purpose, high strength oil tempered wires are being used. In order to achieve the high strength for the valve spring, modification of manufacturing processes is being applied. In this case, the cause and the effect for the improvement of the fatigue strength has not yet been explained explicitly. Therefore, in this report, the comparison of fatigue life between valve springs made by conventional processes with oil tempered wires and the ones made by a new manufacturing processes was made. As a result of the fatigue test, the fatigue life of the latter reached to maximum 7 times longer than that of the former. It was cleared that the improvement of the fatigue life was materialized by the difference of compressive residual stresses at the depth of 0.2mm below the inner side surface of the valve springs.
