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Glow Plug Q&A: idle bars, temp ratings & more!

Glow Plug Q&A: idle bars, temp ratings & more!

The idle bar is there to keep the glow plug from getting extinguished when the engine is throttled up. When the engine is idling, it has a tendency to pool up some fuel in the crankcase so when the engine is throttled up, that puddle is forced through the cylinder transfer ports. These ports direct the flow right at the glow plug. With an unshielded glow plug, that fuel hits the wire element and instantly smothers it. The idle bar in front of that wire element helps prevent the flow of fuel from hitting the wire and thereby keeps the glow plug lit. If you have a problem with the engine choking out when you go to wide open throttle, you may want to try a glow plug with an idle bar.

When hooked up to a battery, the glow plug’s coiled-wire element glows an orange-white, which creates a temperature in excess of 1,500 degrees F. Once the engine is flipped over, either by hand or starter, the compressed fuel/air mixture will ignite. If the mixture is right, the engine will become self-sustaining so that when the battery is disconnected, the engine continues to run. Simply put, what happens inside the combustion chamber is that the coiled-wires are heated up from the compression stroke and continue to glow for the next compression stroke, igniting the fuel/air mixture, which in turn, heats up the wire for the next cycle.

We have different plug temperature ratings so they can be used to change the performance of the engine depending on the flying conditions. Because our engines have a fixed compression stroke and operating setup, the perfect ignition point will change with different running conditions. Some of these can include compression ratio, nitro or oil contents in the fuel, weather conditions and propeller load. By using plugs with different temperature ratings, we can adjust the ignition point so it’s not too early or late. Once you find that sweet spot, your engine will produce the best engine performance.

Updated: March 14, 2012 — 11:17 AM
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  1. good information.. thanks for the artical..the colder the plug with a good performance….the better right? Keeps from pre-igniting?
    Too hot can make the motor lean? plugs are exspensive now days so you say the most universal plug is a med. heat range for two stroke motors?
    Terry Bolin Neosho, Mo.

  2. Generally speaking, a colder plug will give a little more power at the top end and will need a leaner mix to deliver the best power at the expense of the lowest idle and best transition. A hotter plug is the opposite, requiring a richer mix up top to keep from igniting too early and giving up some rpm there in order to deliver a better idle and more reliable transition. Most of us care more about having a low reliable idle and a quick transition than we do about that extra 300-500 rpm that can be gained by going to a colder plug, so the medium range plugs like the OS #8 are pretty much standard.

  3. In addition to the compression preasures,a catilitic reaction occurs,durring WWII it was found that when methanol contacted Platinium the meth.cought fire,try it for yourself.take a glow plug in a pair of pliers and apply a small drop of fuel to it,watch er’ smoke-cool!

  4. Mr. Hamlin is right. Glow plugs are marvelous inventions. Just proves God meant for us all to fly R/C!

  5. After my OS 95 AX would not stay running in the air and trying different plug temps, fuels, needle settings,tank positions,A fellow modeler suggested using an idle bar glow plug.I did and it has not quit( while their was still gas) since .

    1. Where can you buy glow plugs with idle bars?

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