What is Spark plug heat range?

Spark plugs are designed so that they transfer heat from the combustion chamber to the head where they are mounted. If they stay too cool you will get high emissions, poor fuel economy, and generally poor performance. If they get too hot they could ignite the fuel mixture without a spark, but more of a concern is that they could overheat the piston and possibly melt a hole in it. Plugs of similar design are manufactured with different amounts of insulation around the center electrode to control heat range. Engine manufactures will sometimes recommend different heat range plugs for different applications. Engines under heavy load and/or operated in high temperature environments will use a "cooler" plug then those that are not constantly stressed. You should use the manufacture recommended plug heat range unless you are doing something the manufacturer never anticipated.