Choosing the Right Glow Plug

from O.S. Engines - Model Aviation

Several factors influence the use of the correct glow plug for your engine.  Here are some guidelines:

Engine Type
Standard engines (engines with a 1-piece head) are most common. Standard plugs are easily available, inexpensive, and fit almost all standard engines. Standard plugs are installed with a washer, which creates a compression seal with the head.

The Turbo head. Many newer O.S. engines feature a special 2-piece turbo head. The biggest benefit of turbo plugs is superior performance. Unlike standard plugs, turbo plugs (identified by a “P” in the description) feature a tapered “seat” that matches perfectly with the head. That creates a superior compression seal and with it, maximum efficiency and power. Turbo plugs are the choice for racers who want and need top performance.

A word of caution: you should never install a turbo plug in a standard engine or vice versa. Doing so risks doing serious (and expensive) damage.

Displacement
Small displacement engines need a hotter plug than larger displacement because larger engines have more mass and retain heat better.

Nitro Content
Use of higher nitro fuels will require a colder plug than lower nitro fuels.

Temperature
The hotter the day, the colder the plugs need to be.

Additional Things to Know:

• 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.
 

• 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 in” 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, and burn up a plug [Tech Editor’s Note: More than the plug might be lost, excessive leaning can ruin an engine!] before it’s time. Do not over lean!

Final Thoughts
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.

• Buy quality plugs. You’re protecting your investment.

 
• Store plugs where it’s dry. Moisture can ruin them.
 

• Use the right glow plug. Follow the guidelines above.
 

• Follow the proper break-in procedures.
 

• Tune your engine carefully. Running too lean will make your engine “blow” plugs more often. Proper tuning helps extend plug life.
 

• Never touch the filament of a glow plug. Doing so can break the filament and ruin a plug.
 

• Don’t over tighten your plug. Tighten it until it’s just snug.
 

• 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.
 

• Use only a glow starter or 1.5V battery to heat your plug. Otherwise, your plug may burn out ahead of it’s time. Don’t be afraid to ask for help. Experienced modelers have already “been there, done that.” Their experience can save you time and money; and most are glad to help.
 

• Glow plugs get very hot, enough to glow the filament red or white hot, and removing a glow plug while power is applied can cause burning if appropriate care is not taken. [Tech Editor’s Note: Removing the plug while it’s still being “heated” strikes me as nearly impossible, since any attachment to the plug, that would be doing the heating, must be removed so that you can have access to remove the plug. The only other way to heat the plug is from the combustion in the engine, so if you are handling the engine, it’s generally a good thing to have stopped it running while you “fool” with it.] Special caution must be taken while near fuel sources.
 

• Some connectors for glow plugs can short circuit and damage batteries, or cause them to explode. Batteries may get hot during the use of a glow plug. This especially applies to homemade or nonstandard connectors.

Tech Editor’s Note: One source of material that can ruin glow plugs is detritus left in the engine during/after manufacture. Before you run any new engine, it’s a good idea to flush, at least the combustion volume, to eliminate as much “stuff” as you can. Flush it with fuel or methyl alcohol, never water. The best way to flush is to remove the head and the back plate and flood the engine with the flushing fluid. If you are uncomfortable removing the engine parts, then take the glow plug out, move the piston down so that the top of the piston is below the exhaust port and run the flushing liquid down the plug hole and out the exhaust. If you catch the flushing fluid on a white paper towel, you should be able to see anything that the flush eliminated.