To help shed some light on the technology of glow plugs and for some practical information concerning their use, I consulted a few of the most knowledgeable people in the industry. The roster of experts who lent their knowledge to this piece includes Howard McCoy of McCoy Racing, Jerry Conley of Wildcat Fuels and Alberto Picco of Picco Mfg. They all have extensive knowledge of glow plugs.

WHAT IS A GLOW PLUG?

A glow plug is the ignition system for your nitro-powered RC vehicle. 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 IS A TURBO PLUG?

Glow plugs are available in two configurations: standard and turbo. Most engines come with a standard plug. They feature a straight, threaded housing that threads through the cylinder head into the combustion chamber. Turbo plugs feature a different type of housing; the end that goes into the combustion chamber is tapered. The tapered end mates with a head that is specially designed for use with turbo plugs. The head is also tapered to accept this type of plug. The supposed advantages are less compression leakage around the glow plug and less disruption of the combustion chamber. The hole in the cylinder head that exposes the glow plug to the air/fuel mixture in the cylinder is much smaller, and there are fewer rough edges to create unwanted hot spots.

The turbo plug on the left uses its tapered housing to seal it to the cylinder head. The standard plug on the right uses a copper gasket.

There are specific racing rules for using turbo plugs. Currently, only .21ci (3.5cc) engines are permitted to use turbo plugs in competition. You can certainly use turbo plugs if you are not racing; that's a matter of preference. The prevailing wisdom, though, is to stick with standard plugs whenever possible. They are more widely available than the hard-to-find turbo plugs, and they cost less.

WHICH TYPE OF GLOW PLUG SHOULD I USE?

OK; you've blown the glow plug that was included with your engine, so it's time to get a new one. Which one do you buy? You could try to find the same plug, if information about its brand and type was included with your engine. More likely, you will have to choose from the brands and types of plugs that are available at your local hobby shop.

What makes the subject of choosing a glow plug a little confusing is the variety of types that are available. Each manufacturer offers a range of plugs, from as few as three or four up to 10 or more. A plug is usually identified by a code that indicates its effective operating temperature; not the operating temperature of the engine or the outside air, but the relative temperature of the glow plug's coil. Each manufacturer has its own unique temperature-rating system, and general application recommendations are sometimes included to try to steer consumers toward the correct plugs for their needs. The process can be confusing, however, because a universal rating system does not exist for glow plugs. For example, an O.S. R5 plug is not the same as a McCoy MC-9, although both are considered "cold" plugs. A glow-plug manufacturer's guidelines will suffice for average enthusiasts who simply want their cars to run; racers and performance buffs, however, won't get the most out of their engines without a little experimentation. So what should you look for in a replacement plug?

Some general rules about plugs are determined by the size of the engine and the type of fuel used. Smaller engines usually require hotter plugs, while larger engines favor cooler plugs. Engines that run fuel containing a high percentage of nitro favor the cooler plugs as well, while those that run on less nitro prefer hotter plugs (nitro fuels for car engines typically include 10 to 40 percent nitromethane). For example, a small, .12ci (2.1cc) engine that burns high-nitro fuel would favor a warm plug in a mid-range temperature (small engine = hotter plug; high-nitro fuel = colder plug). The same plug might also be suitable for a .21ci (3.5cc) engine running low-nitro fuel (large engine = colder plug; low-nitro fuel = hotter plug).

The size of your engine and the type of fuel are easy enough to determine, so these guidelines should get you pointed in the right direction. For racing buffs who want to get maximum ponies, however, another element that's not easily determined—yet should be taken into consideration—is compression ratio. The compression ratio of an engine will also be a factor in choosing the proper plug. High-compression engines favor colder plugs, while those with lower compression favor hotter plugs. Engine manufacturers rarely disclose an engine's compression ratio, so it may be difficult to use this information to select plugs unless you know how to calculate compression or can measure it with a compression gauge.

This information is best used when adding or removing head shims. More or thicker shims lower the compression; fewer or thinner shims raise it. Remember: when adjusting head clearances with shims, a plug change may be necessary (this should be left to experienced engine tuners).

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.

The plug on the left, an O.S. F, is rated "hot" for use in 4-stroke applications. The thin element wire and the large number of coils create more resistance, and this results in higher operating temperatures. The Enya no. 5 plug on the right is one step up from the coldest plug. The thicker element wire and lower coil count reduce the plug's temperature.

The best way to keep track of your glow plugs is to store them in their original packaging; the plug type is printed right on it. A visual inspection won't help you much, since some plugs don't have any temperature information printed on them; that's why the original packaging comes in handy.

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.

These glow plugs have very different electrode designs, but despite the visible differences, electrode design has no bearing on plug performance.

BEST-KEPT TUNING SECRETS OF THE PRO'S

One of the best-kept secrets of the most experienced nitro racers and engine tuners is that you can net some serious horsepower gains by experimenting with various glow plugs. As mentioned earlier, a glow plug is the ignition system for a nitro engine. Anyone who has ever worked with spark-ignition systems knows that ignition timing can have a profound effect on engine performance. "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. Generally, it's best to try to advance the timing or flash point of the fuel—in the case of nitro engines, as much as possible without going too far. If the mixture is ignited too early, then performance is lost and pre-ignition and detonation may occur. Picco's engineers don't use plugs to tune the engine; they simply find the proper plug for the application and stick with it. They haven't seen much difference in performance from changing to a slightly hotter or colder plug. They do, however, admit that getting too far away from the ideal plug temperature will have a negative effect on power production.

THE FINAL ANALYSIS

We hope you now know more about glow plugs and what to do with them. Most people don't think about glow plugs until they don't work. It's precisely then that this information should be most useful. Just keep in mind that there isn't a magic glow plug that will suddenly make your engine scream as it never has before; glow plugs are one of many elements that factor into overall performance.

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