Every modelcar engine needs a glowplug for the ignition. Depending on things like the percentage of nitro used in the fuel, the outside temperature and the humidity you might need to select the right glowplug.

Glowplugs are numbered 4, 5, 6, 7 or labeled with names like HOT, MEDIUM or COLD.

As every manufacturer uses different names or signs for their glowplugs, you can not say that a no. 6 from brand A is the same as no. 6 from brand B.

The thickness of the glow wire will make the difference between a HOT, MEDIUM or COLD.

It would be nice if all manufacturers are giving the wire thickness as a reference number.

Like a no.6 = 0.6 m/m thick a no7. = 0.7 thick etc.

The following comments are gathered from experience over the past few years.

For engines with a nitro percentage below 10% a HOT plug will do OK when the outside temp is 15/20 degrees Celsius.

For engines with a nitro percentage between 10 and 25% a MEDIUM or COLD plug will do OK when the outside temp is 15/20 degrees Celsius.

For engines with a nitro percentage above 25% (for fools only) a COLD plug will do OK when the outside temp is 15/20 degrees Celsius.

A hotter plug with a thinner glow wire is less durable with high nitro percentages then the colder plug with a thicker glow wire.

A hotter plug will give better idling and bottom power
A colder plug will give better top speed and durability.

From our experiences the following glowplugs are OK for use with an average 15 % nitro and an outside temperature 15/20 degrees Celsius.

!Tip! If your glowplug fails, always check if the spriral is O.K.. If the spiral is missing do not start the engine again, but clean it first thoroughly, so the broken spiral want do any harm to the rest of the engine. A repeat of glowplug failure is pointing to too much compression or dirt particles in the engine. If you do not use your engine for a long time (more then a week), put some after run oil in the engine. The layer of oil will prevent the metal parts from rusting. A better solution is to run the engine first on a methanol/oil mixture (no nitro!) for 30 seconds and then apply some after run oil. Nitro is the part of the fuel that is adhesive for moisture and therefore creating possible rust forming. Rust particles can cause premature glowplug and engine failure!

The following interesting article has been written by Mike Queller from Speedline USA and is copied with permission.

Without the benefits of electronic ignition, the two-cycle engine in your r/c car has to rely on the physics of the two stroke principle. Upon initial startup, the glow-plug needs a little help - in most cases, a glow battery. When the engine is running, the heat generated by the combustion process continues to keep the glow plug lit.
What actually happens to the plug during the whole cycle? It actually gets hotter at the instantaneous combustion point, and then cools as the new mixture arrives from the intake. Too cool and the engine misfires or dies. Too hot, and the mixture detonates prior to the complete
compression. This is called pre-detonation.

Knowing how to control all of this has seemed to be a black science, especially to the novice. If all things are equal - the same pipe, same fuel, same engine, and same clutch - then it is possible to balance three areas to achieve the maximum performance from each of your model
engines - heat range, nitro percentage, and pipe length.

HEAT RANGE
Since the engine's timing is dependent on the size of the element in the glow plug, it has become very important to choose the correct heat range glow plug for a given engine.

Turbo plugs are a type of plug with a cone shaped seat that fits into a specially designed head insert. They require no extra sealing washer and have been proven to increase power over the entire power-band of the engine. They are available in ranges of numbers 5 through 8. In most
brands, they are even further subdivided as "C"-hotter or "F"-colder. If you don't speak Italian or Spanish [in those languages, C stands for Caliente (hot), and F stands for Frio (cold)] then the easiest way to remember is that f is associated with freezing, therefore it stands for cold). So you have, in order from hottest to coldest, 5TC, 5TF, 6TC,..... down to 8TF.

Standard plugs fit most engines and are the only legal plug for some classes. They require a copper sealing ring, and are available in heat ranges from 5 through 9, with 9 being the coldest. While these plugs are substantially different, the same basic principles apply to both types of glow plug.

To choose which plug is correct for the weather, fuel, and pipe on a particular day, start with what you normally run, say, a 7TC turbo or a number 8 standard. Warm up the engine, take a few laps and get a head temperature. Tune the engine by sound and sight on the track until it seems to be "on the pipe" and running cleanly, hopefully not too lean. Be sure to check it periodically to keep from getting too hot a temperature. If it seems very hot, (over 250 degrees Fahrenheit on a Raytek temp gauge, or over 215 degrees Fahrenheit on an Exergen temperature gauge), then you need to change to a hotter plug. That's right, a hotter plug. The engine is running too lean in order to keep the colder plug lit. Remember you're trying to get the plug to fire earlier so you need a thinner element, which will light quicker. The general rule is - the hotter the engine runs, the hotter the plug, the colder, the engine the colder the plug.

NITRO PERCENTAGE
Now that the engine is running decently, what changes to the plugs have to be made if the nitro percentage is increased? Nitro-methane burns hotter than methanol, the other principal ingredient in fuel. Because of the additional heat which is produced when changing to a higher nitro content fuel, the easiest way for a glow plug to withstand this higher heat is to change to a glowplug that uses a thicker element, in other words, a "colder plug".

A lot of testing and experience has yielded the following formula that seems to work pretty well. For every 10% increase in nitro over what is normally used, a corresponding change in plugs of one range colder is required. So if the engine is running well with 30% using a 6TF plug, then it will probably make more power at 40% using  a 7TF plug, providing that the weather hasn't changed too much, and provided that the head clearance has been adjusted properly.

EXHAUST PIPES
The third item to consider, is the effect of pipe length on the glow-plug. After some runtime, the plug's condition will determine the proper pipe length. If the element is pushed up inside, then a hotter plug is required or the pipe should be lengthened. If the element is gone, then the opposites are true - use a colder plug or a shorter pipe.
In both cases the head clearances should be adjusted but that will probably be the subject of a future articles.

PLUG CONDITION
The term "reading a plug" is commonly heard in the pits, but how is this done? In addition to the conditions that have been previously mentioned, the plug should be visually inspected for color.

If the plug body (the part surrounding the element) is wet and the element is shiny, and in its original location, then change to a "colder plug" (assuming that your engine temperature is on the cool side).

If the plug body is dry, and the element is shiny, the plug is correct for the conditions.

If the plug body is dry, and the element is a dull gray, the engine is running too lean - change to a "hotter plug" to correct the condition.

While most of this may sound like it is a lot to remember, it is important to realize that getting the most performance out of your engine takes a little patience and thought. But with a little practice and maybe a little "cheat sheet" it can become second nature to 'know your glow-plug'.

Good luck and good racing

More interesting links about glowplugs can be found here:
http://www.rcnitro.com/rn/articles/glowplugs.asp
http://www.excelsiorweb.com/rcair/glowplug.asp