Spark plug heat ranges determine how efficient the plug is at removing heat from the combustion chamber. The length of the insulator nose, or part from the firing tip to the shell, decides the heat range. Spark plugs must maintain an operating temperature between 500 and 850 degrees Celsius (about 950-1550 degrees Farenheit). Different heat ranges are selected for engines based upon these needs. Too low a temp and the electrode won?t be hot enough to burn off carbon and combustion chamber deposits. The plug could foul or cause misfire. Too high a temp and the electrode may melt causing pre-ignition (fuel explodes too early and reaches peak pressure before the piston is at TDC) or detonation (occurs between ignition and TDC when the gases explode before the spark plug flame reaches them). A ?hot? spark plug has a longer insulator nose and the firing end heats up more quickly, but dissipates heat more slowly. A ?cold? spark plug has a shorter insulator nose. Since the heat has less distance to travel, the plug can transfer the heat more quickly. Colder plugs are necessary on highly modified cars, cars under load, or cars run at high rpm for long periods of time or long distance high speed driving. Hotter plugs are good for stop and go cars as they will burn off the oil and carbon deposits easier.