The normal temperature of gasoline engine exhaust is approximately . This is also approximately the melting point of most aluminum alloys and it is only the constant influx of ambient air that prevents the piston from deforming and failing. Forced induction increases the operating temperatures while "under boost", and if the excess heat is added faster than the engine can shed it, the elevated cylinder temperatures will cause the air and fuel mix to auto-ignite on the compression stroke before the spark event. This is one type of engine knocking that causes a sudden shockwave and pressure spike, which can result in failure of the piston due to shock-induced surface fatigue, which eats away the surface of the piston.