A multi-optical fiber technique is presented, which enables one to detect the flame propagation during non-knocking and knocking conditions in real production engines. The measurement technique is appropriate to detect knock onset locations and to describe the propagation of knocking reaction fronts. With this knowledge, the combustion chamber shape can be optimized, leading to a better knock resistance and higher combustion efficiencies.Results of flame propagation under non-knocking and knocking engine operating conditions are presented. In addition, correlations between knock onset locations and areas in which knock damage occurs are shown for different engines. Presented are the effects of combustion chamber modifications on the combustion efficiency, based on the analysis of the optical fiber measurements.