knock conditions

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In normal spark-ignition combustion, the fuel is consumed as a flame-front starting at the spark plug and spreading out to all corners of the combustion chamber. This occurs rapidly but at a rate determined by the type of fuel, the fuel mixture, and the turbulence level in the combustion chamber. The relationship to turbulence level is why the combustion speed is roughly proportional to engine speed. When knock occurs, some portion of the fuel combusts before the flame-front reaches it, and this occurs as a sudden explosion (autoignition). The more fuel that gets auto-ignited, the more intense the knock is. The tendency to knock is proportional to the following conditions: engine load (throttle opening), engine temperature, inducted air temperature; and inversely proportional to Octane and the relative humidity of the inducted air. Air-fuel ratio also has a major effect on knock tendency, with slightly-lean being the worst-case. Combustion chamber design, including compression-ratio and valve timing are also key factors in knock tendency.
At light-loads, engines do not knock unless the spark and air-fuel ratio are way-off. At light-to-moderate loads the ignition timing can usually be set to the optimal efficiency point. At higher loads, the calibration should be close to best-timing (called MBT) without knock. Retarding the timing to avoid knock (something the knock-detection system does) reduces efficiency, power, and raises the exhaust temperature. (MBT=Minimum Spark advance for Best Torque)

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