A glow plug engine must be operated with the correct glow plug temperature. Large engines can operate with lower temperatures, while smaller engines radiate heat to the air more quickly and require a hotter glow plug to maintain the correct temperature for ignition. The ambient temperature also dictates the best glow plug temperature; in cold weather, hotter plugs are needed. Since glow plug engines are air-cooled, an engine that "runs hot" can sometimes benefit from a lower plug temperature, although this may cause rougher idling and difficulty in tuning. The operating speed of the engine must also be considered; if the engine is to run at consistently high RPM, such as with an airplane or a car on a mostly straight track, a lower plug temperature is more efficient. If the engine is to operate at lower RPM, combustion will not heat the engine as much, and a hotter plug is required.