detonation is a form of abnormal combustion that can cause engine damage. It occurs when a combination of too much heat and pressure exceed the octane rating of the fuel. The fuel ignites spontaneously and explosively, causing a sudden rise in cylinder pressure that produces a metallic pinging or knocking sound. like a marble in a tin can. Detonation can be caused by excess spark advance, low-octane fuel, lean air/fuel mixtures, and/or overheating. Carbon buildup inside the combustion chambers and on the piston face can also increase compression sufficiently to cause detonation. Mild detonation is not harmful, but heavy detonation can damage valves, pistons, rings and rod bearings. It can also cause elevated HC emissions. Detonation can be caused by over advanced timing, a lean fuel mixture, overheated engine or faulty knock sensor.