Found this on a website may help
Tips:- Hot plugs promote better idling and acceleration. If your engine runs rough or accelerates sluggishly, a hotter plug will help.
Cold plugs produce more power and may improve performance if your engine runs hot. The downside is rougher idling and more difficulty in tuning.
Where you run also plays a part. If the track/course has a lot of twists and turns, a hot plug is fine. If the track/course has long straights where you'll reach maximum rpm, a colder plug is best.
Fuel-air mix not only affects how your engine performs; it can also have an impact on how long your plug lasts. If you run rich, it means that you're using more fuel than necessary for top performance. Modelers are often advised to run rich during engine break-in, because it helps cool the engine. However, running too rich can also cause an engine to "bog down" or quit entirely. In addition, it also means that the glow element is being exposed to more contaminants than necessary, which shortens plug life.
Running lean means that you're using less fuel. "Leaning down" an engine has a positive effect on performance. However, care is needed here, because over-leaning an engine can harm it, by raising operating temperatures, "burn up" a plug before its time.
Choosing the right glow plug not only improves performance, but can also extend the life of your engine and the glow plug itself. With the guidelines above and the tips below, you're well on your way to achieving both.
1. Buy quality plugs. You're protecting your investment.
2. Store plugs where it's dry. Moisture can ruin them.
3. Use the right glow plug. Follow the guidelines above.
4. Follow proper break-in procedures.
5. Tune your engine carefully. Running too lean will make your engine "blow" plugs more often. Proper tuning helps extend plug life.
6. Never touch the filament of a glow plug. Doing so can break the filament and ruin a plug.
7. Don't over tighten your plug. Tighten it until it's just snug.
8. Be sure to shim your engine correctly. A plug that's too close to the piston can cause pre-detonation, which will quickly damage a glow plug.
9. Use only a glow starter or 1.5V battery to heat your plug. Otherwise, your plug may burn out ahead of its time.