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In developing the features of a spark-ignition combustion chamber, optical combustion diagnostic technology was employed to understand the factors contributing to knock propensity of the combustion system. This optical combustion visualization equipment utilizes optical prisms located around the periphery of the spark plug shell to measure light intensity of combustion. By synchronizing light intensity and cylinder pressure measurements to crankshaft position, local tracking of combustion progression is accomplished. This method was used to track knock occurrences and to evaluate their controlling parameters within this combustion system. Cylinder head and piston top geometries were developed as a result of this combustion visualization. Recommendations derived from these studies were successfully used to achieve objectives of this engine development program which included improvements in torque and power output, fuel economy, and combustion stability.