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Spark knock:

 
Spark knock (detonation) is caused when the fuel-air mixture in the combustion chamber burns ahead of the advancing flame front. Once the spark plug ignites the fuel-air mixture, the flame consumes the mixture, increasing the heat and pressure in the combustion chamber. As the piston compresses the burning charge even further, the unburned fuels auto-ignition temperature is reached, and detonation occurs. Engine design, fuel rating, and operating variables affect the engine spark knock performance. Different engine designs have been found to increase knock resistance. Centered spark plugs, lower compression, a swirling of the fuel-air mixture in the cylinder, and two spark plugs per cylinder have been found to decrease spark knock. Increased compression, off centered spark plugs and a slow-burn combustion chamber increase spark knock. Engine operating variables also influence spark knock. Spark timing, coolant temperature, and air-fuel ratio, outside air temperature, and humidity, are an important part of the equation.