I hate cut and dry technical articles and I’m usually not smart enough to write them anyway but in Nitro RC Racing, selecting the right glow plug for your engine and then being able to look at it and recognize what it’s telling you is a must. While it can be, at best, difficult, knowing all you can about that subject is of prime importance if you want to tune your engine to get the most out of it.

So, I have “borrowed” a lot of information from other sites that were created by a lot smarter people than I am and attempted to compile as much data as I can in an effort to make it easier for you to understand glow plugs and enable you to get all the performance you can from your nitro engines. This article is not restricted to Oval RC racing only; the information listed here applies to all forms of remote control engines. So let’s get down to basics.

So how do they work? To start the motor we must first heat up the element (the small coil of wire inside the plug) by connecting it to a battery. Most plugs use 1.5 volts to pre-heat the coil.  Once the motor is running the battery can be disconnected and the plug will continue to glow with an orange heat to provide ignition and keep the engine running.

How does it do this without a battery? It all hinges on the fuel we use (or part of the fuel) and what the element is made from. The working part of the fuel is methanol, a type of alcohol. The element is made from several metals alloyed together to make it strong enough to handle the heat and vibration caused by engine operation. The specific metal in this alloy that we are interested in is platinum. When platinum comes in contact with alcohol there is a catalytic reaction between the two which heats the platinum causing the alcohol to ignite.

But if the element is glowing the entire time the motor is running, how does the fuel know when to start burning (in other words, what determines the ignition point)?

Well this goes back to the catalytic reaction again. This reaction depends on two things, one is the temperature of the element (the hotter it is the easier it will react) and the pressure of the fuel/air mixture inside the cylinder (the higher the pressure the easier it will react).

So is this where "hot" and "cold" plugs come into play?  Exactly!  Glow plug temperature is controlled by using different heat range plugs. Just like automobile spark plugs, glow plugs come in different heat ranges from hot to cold with maybe half a dozen steps in between. If in doubt, use the plug specified by the engine manufacturer. Using a hotter plug than normal will advance the ignition point and a colder plug will retard the ignition. Read that again, underline it, draw a circle around it and stick it in the back of your gray matter for future reference. (That will be on the test later! LOL)

Glow plug tuning is almost a black art to most of us. What heat range do you chose, what does it look like if the engine is running too lean or too rich or has it over heated? Up to this point in my nitro life, if I stuck a plug on the igniter and it lit up, then that was a good plug! But the more I study on this subject, the more information I am finding and the easier it is to understand them. I am finding that you can “read” your glow plugs much like you can the spark plugs in a race motor and while I am no expert on this subject, I have managed to pick up a tip or two while researching this article. I hope you do too.

What you have to remember is you are looking at the combustion area of the engine and the more of it that you can look at in one place, the better picture you will get of what is going on inside you engine. Here are a few tips that I have picked up while searching the internet looking for information on this subject.

What can your glow plug tell you? Reading your glow plug can tell you a lot about what is going on inside your engine. The “pro” NASCAR engine tuners rely a lot on what the plugs tell them and so can we once we learn what to look for. But just as a real engine tuner has the proper instruments to use, you will need a good tool to look at the plug with.

 

This device is called an Otoscope and it’s the tool that Doctors use to look deep into your ears. The prices of these things can get pretty steep if let it but if you consider what we will be using it for, it just does not make sense to buy a high dollar one. You can do as I did and do a search on Ebay for Otoscope and you will find them as low as $25.00. That’s all I paid for the one I have and it’s a 5x magnifier just like the high dollar ones.

The black things that attach to it that are designed to go into the ear are called Speculas and they come just like they are pictured above. I cut one down quite a bit so that the opening is about ½ inch in diameter instead of those little things. This gave me a broader field of view and makes it easier for me to orientate myself to what I’m looking at. This thing is GREAT for reading glow plugs. I also tech the engines for the RC-Oval .09 Nitro Series and it is tailor made for looking at ports and such. Once you get one you will find a ton of uses for it.

Reading glow plugs: So, what “are” we looking for when we try to read a glow plug? Well, I’m finding that it’s not that hard to do. It will undoubtedly take a lot more practice and data accumulating before I can say that I can really read them correctly but it’s not the black art that I once thought it was. I’m not saying that you will have instant knowledge once you read this and buy an Otometer but maybe this will get your feet wet and get you started in the right direction.

As I said, you are looking inside the engine to see what the combustion area looks like so don’t just take the plug out and try to read it, take the head button off with the glow plug still in it. This will let you look at more of the area at one time and gives you a better idea of what’s going on. Sort of like me cutting my specula, it gives you a broader field of view.

 

This is what a “rich” condition looks like or what your head and plug would look like after a proper break in. Both the plug and head are wet and hardly discolored at all. If, however, this is what our engine looks like after racing it for a while, you probably want to start leaning it out. Continuing to run it this rich could cause a build up in the crankcase and cause the front bearing seal to leak. Not to mention that you are not getting the full performance potential out of your motor.

 

This is a too lean condition. Pitting of the head is starting, there is no oil on the head or plug and the plug can start to get a bluish color to it (not easily seen here). The pitting is caused by detonation. Detonation is when the fuel pre-ignites in different parts of the combustion chamber at one time. Too much compression can also cause pre-ignition. Adding another head shim could help with that.

Continued running in a lean condition can cause more overheating and the engine could start to flame out when running wide open. Once you let the engine run this lean for this long, it will take a lot of cleaning up before you can get it in good enough shape to be able to read it correctly again. Leaving the pits will make it look like it is always running too lean.

Keeping a close eye on engine temperatures can catch a fuel mixture condition before it causes any damage. That’s why you see racers using the head temps as a tuning tool. It gives them a good idea about motor condition and operation. You can’t remove the head while you are on the track tuning the engine but you can take a lot of head temps.

 

This is a good example of why you want to keep the plug in the head when you read it. The head in this case, has started to pit but the plug looks good. If you would have pulled the plug out to read it, you may have thought you were just right on tuning when in actuality; it is running just a bit too lean.

 

This is a “safe” condition. Too rich for all out racing but not so rich as to hurt the engine. This is a “basher” setup. The engine will run fine and start good and not run too hot for the basher to play all day.

 

This engine is running too lean and has gotten too hot. The coil wire will have a dusty look to it and may even be starting to break. If it does it could get in the bottom of the crank case and cause a lot of problems but in most cases it will just blow out the exhaust. The plug above has seen better days and needs to be replaced AFTER the carb is richened up.

What you have seen above are the extremes of the rich, lean, hot conditions. The trick for us, as racers, is to see the more minute changes and catch them before they get anywhere near as bad as these illustrations are. The point here is to show you what the real bad stuff looks like, you will have to learn how to properly read the more minor conditions on your own. BUT stick with it; reading glow plugs is one of the very best ways to learn what your engine is telling you and how to properly tune it.

Finding the right plug for the application. Getting the right heat range plug is a bit more difficult because the manufacturers seem to have no standards what-so-ever when it comes to grading them. One companies’ “59” may be another companies “A-3” (just an example) and this makes it a lot harder to compare them. As you will see with the chart below, it can get real confusing to make any type of comparison. The best advice that I can give is to use your own “filters” and sort it down to one company that you trust and work with what they offer. (Or cheat and ask the fast guys what they are running!)
Another thing to keep in mind is that the wire diameter of the coil determines the heat range with the thin wire being the hotter plug and the thicker wire being the colder.

Below is a comparison chart compiled by the Raleigh Durham RC Club http://www.rd-rc.org/

OS Glow Plug Information

# 8 Hot Recommended for most current O.S. (and other) 2-stroke engines
Type F Mildly Hot Special long-reach plug recommended exclusively for O.S (and other) 4-stroke engines
Type RE Hot Special long-reach plug designed exclusively for O.S. Wankel rotary engine
A5 Cold Recommended for most current O.S. (and many other) 2-stroke engines particularly for 1/10th & 1/8th scale off-road car engines
A3 Hot Dependable O.S. quality makes A3 the most durable and longest-lasting glow plug available at an economical price
R5 Very Cold Recommended for high-nitro fuel and high r.p.m. engines, particularly 1/8th track racing car engines

ENYA Glow Plug Information

# 3 Hot All Enya engines such as TV & four cycle engines
# 4 Mildly hot All Enya engines, especially those used with 10%or greater nitromethane fuel
# 5 Medium All Enya engines, especially the .40CX, .45CX and high nitro methane fuel
# 6 Cold High compression engines and high niro methane fuel used in racing.

Fox Glow Plug Information

All 1.5 Volt Plugs are Dry Cell or Ni-Cad All 2 Volt Plugs are Lead Acid Battery

Standard Short Hot 1.5 Volt, Standard Short Hot 2 Volt
Standard Long Hot 1.5 Volt, Standard Long Hot 2 Volt
Gold STD Long Plug Hot 1.5 Volt, RC Short Mildly Hot 2 Volt
Gold RC Long Hot 1.5 Volt, RC Long Mildly Hot 2 Volt
RC Short Mildly Hot 1.5 Volt
RC Long Mildly Hot 1.5 Volt
Miracle Plug Hot 1.5 Volt
Pro 8 Short Cold 1.5 Volt
Pro 8 Long Cold 1.5 Volt

McCoy Glow Plugs with OS Equivalent

MC-8 Medium Hot #8
MC-9 Cold A5, R5
MC-50 Hot IDLE BAR - LONG
MC-55 Medium Hot A3, #8
MC-59 Hot

STD ROSSI GLOW PLUGS BI-TURBO GLOW PLUGS (without idle bar) (conical w/o washer)

Rossi Glow Plugs (cold for pattern type work / high nitro fuels, hot for sport / low nitro flying)

R1 Extra hot 0.8 to 2cc RB4 Hot
R2 Hot from 2 to 3.5cc RB5 Medium
R3 Medium from 3.5 to 6cc RB6 Cold
R4 Cold from 6 to 10cc RB7 Extra cold
R5 X-cold for nitro fuel & R/C RB8 Super cold
R6 Cold nitro 10 to 13cc
R7 Cold for nitro 13 to 15cc
R8 Cold for nitro 15 to 30cc GLOW HEAD FOR R15
G1 Hot

R/C GLOW PLUGS

G2 Medium (with idle bar)
G3 Cold nitro 15 to 30%
RC Hot for 2.5 to 6cc
G4 X-cold nitro 30 to 50%
RC Cold for 6 to 15cc
G5 Cold nitro 50% or more

Glow Plug Usage Tips

Your glow plug temperature range is too cold when:

    The engine power is weak or has weakened from previous levels.
    The engine slows down considerably or stops after removing the glow plug battery, despite correct adjustment of the needle valve. For example (Enya), if a # 4 plug gives you these problems in your engine, switch to a # 3 plug instead.
   
Your glow plug temperature range is too hot when:

    The engine suffers from pre ignition and loss of power.
    The overall engine running is rough
    The glow plug filament is broken or collapses frequently.

HOT GLOW PLUGS (for low nitro and FAI fuels)

Enya: # 3
Fox: Miracle, Standard, and R/C Long (2V)
Fireball: Hot (1.2-3.0V), and S-20 R/C Long
Fire Power: F 6 (warm), and F 7 (hot)
K&B: 1 L
McCoy: MC 55 R/C Long, MC 59, and MC 14 (very hot)
O.S. Engines: # 0, # 1, # 5
Rossi: R 1 (extra hot), and R 2
Sonic Tronics: Glowdevil # 300
Thunderbolt: R/C Long

MEDIUM GLOW PLUGS (for 10%-15% nitro fuels)

Enya: # 4 (medium hot), and # 5 (medium cold)
Fireball: Standard (1.2-2.0V)
Fire Power: F 5 (medium), and F 6 (warm)
Fox: R/C Long (1.2-1.5V), and Gold
Hanger 9: Sport Long
McCoy: MC 50, and MC 8
O.S. Engines: # A 3, # 8, # 9, # 7 (with idle bar)
Rossi: Medium, and R-3
Sonic Tronics: Glowdevil Standard
Tower Hobbies: Tower Power Performance plug, and Reg. (w/bar)

COLD GLOW PLUGS (for high nitro; 25% +)

Enya: #6 (cold)
Fireball: Cool (1.2-1.5V)
Fire Power: F 2 (extra cold), F 3 (cold), and F 4 (cool)
Fox: R/C (1.2V), and # 8
K&B: Long & Short high performance nitro plug
O.S. Engines: R-5
Rossi: R 4 (cold), and R 5 (extra cold)

FOUR-STROKE GLOW PLUGS (hot)

Fox: Miracle plug (often used in 2C's W/low nitro)
McCoy: MC 14 (very hot, often used in inverted 4C's)
O.S. Engines: Type F
Sonic Tronics: Glowdevil ST 301/302

IDLE BARS

 

Idle bar glow plugs came about because some engines were having trouble transitioning from idle to high speed. When the throttle was opened from idle, the incoming air and raw fuel would strike the glow plug's heated coil, cooling it to the point where it would no longer support the combustion process, so the engine would die. To help prevent this, the idle bar was added to the glow plug to serve as a physical shield, helping to keep the coil from cooling off too quickly.

A glow plug with an idle bar will not increase peak RPM (it may even reduce it in some cases), but it may improve the idle with some engines, since it simply helps to keep the plug hot enough to light the fuel. If your having transition problems, you might want to try using a glow plug with an idle bar. Some modelers use idle bar plugs in the winter only, since the glow plug tends to loose heat faster in the colder environment.

Naturally, all of this assumes that you have the low speed mixture adjusted correctly to begin with.

HOT PLUGS

So what is a 'hot' plug, and how does it differ from a 'cold' plug?

Naturally, a hot plug will heat up faster and stay hotter, but that's not the whole story. When discussing this aspect of glow plugs, another very important aspect must be considered, the amount methanol in the fuel. The more methanol we're using (i.e., less oil and less nitro), the hotter the plug we should use. Conversely, the more nitro and/or oil we use, the less methanol we're using, so we use a cool(er) plug. An extreme example would be when using a very high nitro content fuel in a very high RPM engine (a typical ducted fan engine, for example). Here we'd use a very cold plug. For most sport pilots using fuel with just 5-15% nitro, however, a hotter plug would probably do well.

Probably? Yes, trial and error is often the best (and sometimes 'only') way to determine the right glow plug for your application. Most 4C engines need either high nitro or hot plugs to run at their best, since they have combustion strokes only half as often as 2C engines.

RULES OF THUMB TO LIVE BY

    Use a hot plug with low nitro (less than 24%), and a cold plug with high nitro (more than 25%).
    If you remove the glow starter from you idling engine, and notice an immediate drop in RPM, you may need a hotter plug or more nitro.
    If your engine has a tendency to backfire a lot, you may be using a glow plug that's too hot, or you may need fuel with less nitro.
    Most hot plugs can take up to 2.0 volts starting power without burning up, while most cold plugs prefer 1.2 to 1.5 volts starting power.
 

Keep looking. Don’t think that what you have read here will make you a master at reading plugs or finding the right one for your engine tuning methods but it is a start. Check some more places on the net and you can find more information but maybe this will wet you apatite enough to try to learn more.

The following are sites that I “borrowed” information from and you should check them out as well:

http://www.rc-truckncar-tuning.com/rc-glow-plug.html

http://www.scootworks.com/rdrc/gloplugs.html

http://www.maxxtraxxusa.com/Glow_Plugs___It_s_gettin__hot_in_herre_.cfm