How are hot and cold plugs different in ignition systems?

Car ignitions use spark plugs to bridge electricity across a gap, kind of like a lightning bolt. For the electricity to get across the gap and make a powerful spark, it has to be at a very high voltage. That means the voltage at a spark plug is normally somewhere in the range of 40,000 to 100,000 volts. In order for the electricity to travel to the electrode and jump the gap toward the engine block, the spark plug needs an insulated passageway. The spark plug has to be able to handle the extreme heat inside the engine's cylinder, too. Plus, it can't let fuel additives build up on itself and cause problems.

That's why a spark plug has a ceramic insert: To protect the spark plug from deposits and to ensure that the spark only happens at the tip of the electrode and nowhere else on the plug. Since ceramic doesn't conduct heat very well, spark plug insulation gets very hot when in use, which is what burns deposits off of the electrode it protects. A hot plug is a spark plug whose ceramic insert has a smaller area of contact with the metal part of the plug than a cold plug has. Hot plugs allow less heat transfer from the ceramic and thus burn off more deposits. Meanwhile, cold plugs don't get as hot.

Carmakers determine which type of plug should be used in each car. When cars have high-performance engines, they generate more heat. Therefore, they're better off with colder plugs. If the spark plug were to get too hot, it would ignite the fuel before the spark even fired. That's why it's very important to use the specific type of plug that your car manual mandates.