This depends on the type of engine being used. Low power engines such as lawnmowers and chainsaws don’t produce large amounts of heat, meaning that a ‘hot’ spark plug (with a low heat range) is needed to reach optimum performance temperature. High-performance engines, however, generate a large amount of heat, resulting in the requirement for a ‘cold’ spark plug (with a high heat range). The heat range number, which typically varies between 5-10, is featured within a spark plug’s code (found on the ceramic insulator). This number illustrates how ‘hot’ or ‘cold’ the particular plug in question is. The lower the number (e.g., 5 or 6), the hotter a spark plug is, whereas the higher the number (e.g., 9 or 10), the colder the spark plug is.