The heat range numbers used by spark plug manufacturers are not universal, by that we mean, a 50 heat range in Autolite is not the same as a 50 heat range in Champion. Some manufacturers numbering systems are opposite the other, for domestic manufacturers like Autolite (and Champion), the higher the number, the hotter the plug. For other manufacturers (NGK, Denso, Bosch), the higher the number, the colder the plug.