[Mike V. Florida]

Just to go along with what the others have said, there is a specific amount of fuel to be burned within a specific amount of time. The full force of the burn is used to push the piston down, allowing the hot gases to be "cooled" off by the cylinder walls. If the timimg is off and the burn starts late, "the fire is still burning when the exhaust valve opens and now there is a live flame blowing out past the valve, adding a lot of excess heat to the valve, port, and exhaust manifold." as well as the less time the heat is exposed to the cylinder walls.

Now early timing can also produce excessive heat but you usually get other symptoms before that happens like hard starting, starting pinging.