 If the tip of the spark plug is too hot, it can cause pre-ignition or sometimes detonation/knocking, and damage may occur.If it is too cold, electrically conductive deposits may form on the insulator, causing a loss of spark energy or the actual shorting-out of the spark current.A spark plug is said to be "hot" if it is a better heat insulator, keeping more heat in the tip of the spark plug.