The importance of minimizing fuel consumption has necessitated the study of knock which drastically limits the attainment of high combustion efficiency in current s.i. engines. In the present work, four aspects of this phenomenon have been examined: 1.Knock intensity levels encountered during actual service operation of European cars. 2.Knock intensity levels that do not cause engine damage in endurance tests. 3.Factors affecting the knocking behavior of a fuel. 4.The meaning of the knock rating characteristics of a gasoline using the research and motor methods. It was found that the most severe knocking conditions were those met with by small displacement engines at a sufficiently high constant speed (4000-5000 rpm) and wide-open throttle.In these conditions, high knock intensities, much greater than the trace level, are needed to cause engine damage. Motor Octane Number is the most important parameter in controlling high-speed knock at all engine speeds, this was determined using the CFR engine under fuel stress conditions very similar to those present in commercial engines.