Hotter or colder plugs, what are they?

[BruceH]

We have so much discussion on forced induction and spark plug heat ranges that I'd like to get a clear, concise answer about the properties of both.

My understanding is that a hotter plug will retain heat longer in the tip and that this property will promote early combustion or detonation when used with a combination of fuel and cylinder pressure. Basically it makes combustion easier.

On the flip side a colder plug doesn't retain as much heat in the tip. This reduces the chance of premature ignition. Especially important with gasoline and high cylinder pressures.

My question is do I have it correct? I'm looking for more than just opinions, want the solid correct answer.

Thanks,
Bruce

 

[hamish]

It has more to do with the transfer of heat to the cylinder head material.
Colder plug transfers more heat to the head .

 

[WJBertrand]

Mostly correct, but you want to avoid early combustion at all costs, so if a hot plug is causing pre-ignition, it's too hot and engine damage will ensue with continued use!

Mostly the retained heat in the tip is about keeping the tip clean. You need enough retained heat to burn off deposits and no more. Taking the same engine, if you drive around slowly on surface streets, making a lot of short trips and rarely venture onto the highway or "put your foot into it", you may need a hotter plug to avoid excessive deposits and misfiring.

Conversely, if you mostly take your car to the track and run it hard, fast and long, the retained heat in the standard plug may be more than you need to keep the plug clean and you risk moving into pre-ignition. In that use condition there's enough heat in the engine to keep the plugs clean and you can use a colder plug to avoid problems.

The standard plugs fitted to most engines are the engineers' best compromise to cover the range of normal expected operating conditions.

Because fouled plugs and misfires can send unburned mixture down the exhaust system, causing emissions problems and damaging catalysts, most manufacturers push their heat ranges toward the high end and then use knock sensing and precise fuel metering technology to prevent damage. Precise fuel delivery also helps avoid fouling. New precious metal sparkplug technology allows for plugs with broader heat ranges as well.

Back in the old days (of which I'm familiar) of points, condensers, distributors, carburetors, and no knock sensing technology (other than your ears) plug heat ranges were more important considerations. Owner's manuals often identified/recommended several different heat range plugs based on your driving style.

As long as your keeping the tip clean and avoiding pre-ignition there's no performance advantage to changing the heat range and you could introduce new problems.

 

[psfracer]

It has more to do with the transfer of heat to the cylinder head material.
Colder plug transfers more heat to the head .

Yep--which is very important in a motor like mine that has 16:1 compression. You can tell buy just looking at them where the cold plug has a very shallow insulator nose, where as the hot plug has a very deep insulator nose.

Running too hot of a plug in a nitrous motor can be bad news, they won't be able to dissipate the heat fast enough like a cold plug, and you will melt the electrodes off lol.

 

[05stroker]

So, even if on E85 one should proboly use the cooler plug with high C/R and high psi? For example, I have 20 psi and 12:1 C/R. I was going to test it on the HT1 plugs, This thread makes me not want to do that!

 

[Pentalab]

VMP sez once you are at 10.5 psi, then you should go one heat range colder. That's for 91-94 octane gas, and the 9.8 cr on the 3v 05-10 cars. I believe Brisk.com has a good write up on heat ranges of plugs. They depict what happens if they are too cold. Conversely you don't want them too hot either. Your E85 throws a new twist into the mix. You might want to consult with some of the plug maker's..and run your parameter's past them. 20 psi + 12:1 CR is a bunch.... but your E85 may well cool things down.

 

[stkjock]

So, even if on E85 one should proboly use the cooler plug with high C/R and high psi? For example, I have 20 psi and 12:1 C/R. I was going to test it on the HT1 plugs, This thread makes me not want to do that!

Not as cold as you would with gas

20#, 9.5:1 CR on mine and I run a TR6 plug which IIRC is one step colder

 

[05stroker]

Not as cold as you would with gas

20#, 9.5:1 CR on mine and I run a TR6 plug which IIRC is one step colder

I think I will tune on my HTOs and then switch to HT1s to test.

 

[Dark_horse]

Jeremy went back to the HTOs for e85 and I will be doing the same... not sure of his compression but I'm at 10.5:1..

 

[stkjock]

in general from what I've read, E85 does a bit better with a hotter plug than the same set up on gas would necessitate.

 

[Sky Render]

in general from what I've read, E85 does a bit better with a hotter plug than the same set up on gas would necessitate.

I would ASSume that's because ethanol burns cooler?

 

[stkjock]

I would ASSume that's because ethanol burns cooler?

I can't recall if that was one of the reasons mentioned in some of the reading, however, it's logical.

colder plugs are used to avoid preigniton in a hot combustion chamber and with E85 you chamber is cooler given the same boost/timing.

 

[Eel Mit]

I would ASSume that's because ethanol burns cooler?

That's a big part of it, but it is also effectively 104-105 octane. Higher octane numbers mean slower flame travel and less propensity for detonation. Its a double win, you can keep more timing in the tune.

 

[FAST3V]

On my previous setup..I ran 725whp....STOCK plugs gapped at 30, on E85....no problems ever noticed.

 

[Unreal]

I've always used the rule of every ~150-200 hp go one step colder.

 

[psfracer]

I think E85 is a game changer vs what is normally called for.

 

[05stroker]

Since I love working on my car so much, after I am done I will do a back to back dyno test with the HT0s and the HT1s. I have both in the garage so I might as well try them. It is an easy swap on the dyno.

 

[MustASH]

Thread revival, I'm about to change my plugs, and I need some advice. I'm mostly tracking my car, running 93 octane tune and cams.

I'm very lost on all my reads over the internet.

Part # would be great ! Thanks !

 

[07 Boss]

Thread revival, I'm about to change my plugs, and I need some advice. I'm mostly tracking my car, running 93 octane tune and cams.

I'm very lost on all my reads over the internet.

Part # would be great ! Thanks !

Plugs have to be at a certain temp for them to work their best. Some engines, depending on mods, will run higher cylinder temps and need a "colder" plug. All this means is that they are designed to shed heat faster to keep it in that optimum temp range. Too "cold" of a plug may never get up to that optimum level in a regular or mildly modded motor.

 

[Pentalab]

Ok, what if folks are just cruising around town, or on the local hwy in rush hr traffic etc. 1.5 krpm in OD @ 50 mph.. isn't exactly going to heat things up too much.
In winter, it might be a possible issue if the car is used as a DD, and driven close to freezing..or below.