Glow plug heat range selection.

[2revo1maxx]

Ok I've been running nitro for almost ten years now. All along I've used medium to hot plugs. I've never ran in freezing weather til this year and so far only for 10 minutes. I read the sticky from Rolex and it says cold plugs for cold weather and hot for hot. I believe it, I know he has way more knowledge than me, but can someone tell me why this is. I've always thought it was the other way around.

 

[olds97_lss]

Huh, I had never heard that before.

From what I know about plugs, the relationship of hot/cold is directly related to the compression of the engine and nitro percentage of the fuel. The variable that the plug fills is the combustion timing. Hot makes the combustion happen earlier whereas cold makes it happen later.

High nitro ignites easier than low nitro.
High compression causes the fuel/air mix to ignite easier.

Easier combustion usually requires a cooler plug to cause the fuel to ignite at the right time. Cooler plugs allow the combustion to happen just as the piston reaches TDC, then as it's rolling over the top of the stroke, the fuel has time to burn before the exhaust port is open.

If you ran too hot of a plug for your compression/nitro content, the combustion would happen to early in the stroke as the piston is coming up. I believe this is called detonation. When this happens, the explosion causes the piston to want to go back down when it should be coming up. Inertia keeps it going up, but it's fighting itself.

If you run too cool of a plug for your compression/nitro content, the combustion would happen as the piston is already on it's way back down, which means your loosing power directly out the exhaust.

Sometimes it's required that you actually add head shims when you increase nitro content beyond a certain point or regardless of the plug you use, you get detonation. Extra shims lowers the compression ratio mechanically.

I can try to find the info I read that stated most of the above, but from what I remember, that is the hot/cold plug purpose. A hot/cold plug doesn't have much to do with the ambient temps your running the engine in.

Here's a few articles I've read on the issue:
http://www.hooked-on-rc-airplanes.com/rc-glow-plugs.html
http://www.rchobbies.org/cars_eng-tuning.htm

 

[2revo1maxx]

Ok thank you. I'm probably just going to stick with medium plug/30%....med hot/20% like I have been. As for the detonation thing, it's pre-ignition when the fuel/air charge is set off too early by a plug, detonation is when compression sets it off too early. Detonation is very common when people boost a normally aspirated engine and don't compensate by using race fuel. The compression ratio is already high unlike a motor that is built for boost.

 

[Rolex]

That info was a cut and paste from a magazine article. I still agree with cold plug in cold weather since cold air is far denser than warm air and will compress more. That's why you would want the ignition delayed. Running against extra compression and igniting before TDC would rob you of a lot of power.
Granted, you still have the option of adding a shim or two.

 

[olds97_lss]

I still agree with cold plug in cold weather since cold air is far denser than warm air and will compress more. That's why you would want the ignition delayed. Running against extra compression and igniting before TDC would rob you of a lot of power.
Granted, you still have the option of adding a shim or two.

That's a good point. Didn't think of the extra "air" when cold causing the same (or similar) issue as higher nitro content. Just never read it before was all.

I had read many posts/threads/articles that suggest running a hot plug in cold weather is better due to the ambient temps... that never sat right with me though.

 

[PaulC]

Here's my take, based on some research, knowledge about engines and common sense.

Cold air has more oxygen. Nitro like all (most) engines engines burn oxygen and fuel. The more oxygen the richer the mixture and the more power. The more power and higher rpms the more work on the glowplug. Hot glue plugs are thin wire, same current at start, so they glow hotter and stay glowing hotter in a running engine. Thin wires break easier.

Put together, a cold thick plug will last longer in a cold climate as the engine uses a richer mixture and produces more power = glow plug stress. A hot plug will wear rapidly under those conditions.

Or, inversely, a hot thin plug will burn out on a cold day, but on a hot day with less oxygen the mixture will need to be leaned and the engine will run with less power, so the hotter plug won't burn out as quickly.

Humidity and altitude play a huge part too.

Hotter is generally better, with the afore mentioned stuff on pre-ignition/detonation considered, for a more defined ignition and less chance of cutting out. Will it last the day though?

 

[olds97_lss]

Honestly, I run McCoy MC8's (medium heat) all the time, winter and summer in my big blocks running 25% nitro fuel. They seem to run best with the engines and fuel I run regardless what the outside temps are.

In my jato, I run OS LC3's which are hot with the same fuel but in a TRX2.5 and it runs fine as well in both winter/summer. Just useless to try and run a jato in the snow, so I don't.

 

[2revo1maxx]

I gotta agree with what rolex said. The only engines I'll run under 40 degrees are my 3.3's so I'll just stick with medium. I am getting some cold plugs though I want to see if I can notice a difference. Thanx everyone!

 

[revo_rob_1984]

.

In my jato, I run OS LC3's which are hot with the same fuel but in a TRX2.5 and it runs fine as well in both winter/summer. Just useless to try and run a jato in the snow, so I don't.

it would be entertaining to see especially if you still have the anaconda's on it

 

[Scrogg]

Don't bother with a cold plug in a Traxxas engine....it will not tune or idle.
30% and medium plugs, you can't go wrong. A lot of engines come with excessive head shims for breakin purposes only.. STS is one brand. Picco, you can pull one or two. My new JL is supposedly safe to remove 1-2 shims AFTER a full breakin of course. This is still running 30% nitro. It works, a big difference in my Picco when i pull two thin shims.
No sign of detonation either.

 

[2revo1maxx]

Awesome info, thank you. I have a picco 21max and sirio s27. Should I run medium on both? How about 20% in the 21 and 30% in the 27?