What does the glow plug do? I know it starts the engine but after you take it off does it do anything or does the compresion in the engine ignite the fuel?

You're not actually unscrewing the glow plug are you?

When you put the glow starter on the glow plug it heats the plug. Once the engine starts the glowplug constantly stays lit after you remove the glow starter.

I had this article in my archives. I don't remember where I got it, but it does a nice job explaining the function of glow plugs.


WHAT IS A GLOW PLUG?
A glow plug is the ignition system for your nitro-powered engine. Instead of a spark-ignition system such as those found in automobile engines, the remarkably simply glow plug is what we use to ignite the fuel in our engines. It doesn't have a single moving part or adjustment; its only functioning component is a simple, stationary coil of wire. All glow plugs are not created equal. The housing, wire element, type of plating and hole size determine the relative temperature range of a glow plug.

HOW DOES IT WORK?
Starting an engine is one of the functions of a glow plug. When a glow igniter is attached to a glow plug, it causes the glow plug's coil to "glow" white-hot. This ignites the air/fuel mixture in the cylinder when the engine is started. Once the engine is running, the heat generated during compression and combustion keeps the glow plug's element hot enough to continue igniting the fuel mixture on its own without the help of an external power source. In the simplest terms possible, that's how a glow plug works.

WHAT HAPPENS IF I USE THE WRONG PLUG?
If you have used a plug that is too hot or too cold for your application, this will be revealed in one of two ways. If the plug is too hot, the engine may suffer from detonation, pre-ignition and high running temperatures. Detonation occurs when the fuel mixture explodes quickly rather than burns. You don't want this to happen because it can damage the engine. The telltale signs of detonation are a "miss" in the exhaust tone at high speeds and a pitting of the cylinder head around the glow plug and the top of the piston. Severe detonation can cause the coil element of the glow plug to come loose, and this can severely damage the engine. The primary cause of detonation, however, is excessive compression. Simply using a hot plug will not usually cause detonation, so don't be afraid to experiment. If you're using high-nitro fuel and have increased the compression by reducing head clearance, however, a hotter plug may just push the engine too far and cause damage. At the very least, an excessively hot plug will cause pre-ignition, in which the fuel mixture begins burning well before the piston reaches the top of the cylinder. Using a plug that is too cold will result in a loss of acceleration and top speed and will cause poor engine idle. If the plug is much colder than it should be, you might notice an excessive raw-fuel discharge from the exhaust pipe, but don't confuse this with an excessively rich fuel mixture.

WHAT MAKES A GLOW PLUG HOTTER OR COLDER?
Many elements influence a glow plug's temperature range, but primary is the thickness, length and composition of the wire used to form the coil. It will be impossible to find out anything about the composition of the wire because most manufacturers keep it a secret, but the wire can certainly be measured. Other factors that affect a glow plug's temperature include the size of the hole in which the wire is installed, the type of plating used on the glow-plug housing and the material the glow plug's housing is made of.

IS ONE TYPE OF PLUG MORE DURABLE THAN ANOTHER?
If the fuel mixture is properly adjusted, most plugs should last equally long. When the fuel mixture is on the lean side, the hotter plugs tend to be more susceptible to damage as a result of their typically thinner elements. The quality and consistency of the material within the glow plug's element can also affect durability, and this varies among manufacturers but can be sorted out with experience. So yes; within the various brands, there are slight advantages to colder plugs, but these advantages are not enough to justify choosing them if the result is diminished performance.

WHAT IS IGNITION TIMING?
A glow plug is the ignition system for a nitro engine. "Ignition timing" is when the fuel mixture is ignited in relation to the position of the piston and crankshaft during the compression stroke. When the piston is at top dead center (TDC) of the cylinder, the crankshaft's journal, to which the connecting rod is attached, is pointing straight up. This puts the piston at the highest point of its travel in the cylinder; therefore, the crankshaft is at "zero" degrees. The crankshaft must rotate a full 360 degrees to make a full cycle, so the amount of crankshaft rotation in degrees is used to measure the events that take place inside the engine; for example, ports opening and closing and ignition. Although you can't measure or definitively set when ignition takes place inside a nitro engine, it helps to be able to visualize what's happening when you experiment with different plugs. Let's say, for example, that the fuel mixture is ignited precisely at the moment the piston reaches the very top of the cylinder. This would effectively mean the ignition timing is taking place at zero degrees of crankshaft rotation. Installing a hotter plug in the same engine makes the fuel ignite sooner because less compression is needed to heat the plug's element to the point that the fuel will ignite. Let's say that now, ignition occurs 10 degrees before the piston reaches TDC. In ignition-engine-speak, that would mean that the timing is set to 10 degrees advanced, or 10 degrees BTDC (before top dead center). What does all this mean? Simply knowing that plug temperature will affect when combustion takes place will, hopefully, help you understand why choosing the proper plug will improve performance.

No. I'm not unscrewing them. I'm not stupid just ignorant.

Thanks for the info Insane.

Scott

Actually, I have seen operators of racing boats with high-compression glow engines slightly loosen the glow plug while starting the engine with an electric starter, then tighten the plug down and proceed with their race.

A good explanation above but it left out one important aspect. It is a chemical reaction between the platinum and the alchole which maintains the heat after the initial electical connection is broken. That is why a coating over the element will cause a plug to fail.

The only DUMB question is the one you DON't ask......

There's a reason for loosening the plug on racing boats.....40-60% nitro in the fuel......otherwise they't never turn 'em over.....