Originally Posted by Davidka

Glow plugs are essensially the timing adjustment in an engine. A hotter plug causes the ignition charge to happen slightly earlier, a colder one causes it to happen slightly later. Medium and hotter plugs usually make for a better idle and a wider tuning window, colder plugs usually make for a crisper response and cleaner top end while more difficult to achieve a good idle. If you go too hot on the plug and too lean on the needle, you could have detonation.