Hotter spark plugs - what happens

[Chris Patrick]

A friend wants to put 46N or R46N spark plugs in his Corvette 427. He says it is running too cold and fouling. I think that is too much difference from the 44N that is in there.

What happens if he runs ones that much difference. I couldn't give him a good reason.

[Duke W.]

Re: Hotter spark plugs - what happens

A "hot" spark plug will maintain a hotter insulator, which will burn off fouling deposits on an engine that operates at low average power output, but too hot a plug can overheat and act as a pre-ignition source on an engine that operates at high average power. In extreme cases of plug overheating, like using a hot street plug for road racing, the insulator can disintegrate.

That's why street plugs are "hot" and (road) racing plugs are "cold".

Choice of spark plug has nothing to do with rated engine power. It has everything to do with design detail, specifically the heat transfer resistance from the spark plug to the cooling jacket and the average power demand on the engine.

The second number in the AC spark plug system is the heat range, which ranges from cold (1 or 2) to hot (6). Typical OE plugs are colder than necessary for normal street and highway driving and tend to foul, so for SBs I recommend heat range "5" rather than the OE "4", and I also recommend hotter than OE plugs for big blocks.

The best way to approach this is to install one heat range higher than OE and observe if they show more fouling resistance. If they still foul, install the next higher heat range, but watch for detonation that might be from preignition due to plug overheating.

For road racing at least one heat range colder than OE should be used, but in normal street driving or even drag racing, you can't maintain sustained high output long enough - a few seconds at a time at most - to justify a cold plug, and cold plugs will have a tendency to foul because the insulator temperature runs too cold to burn off deposits.

A couple of other tips: Aluminum heads will usually tolerate one heat range hotter than equivalent CI heads because the aluminum heads offer less thermal resistance to the cooling jacket, so the plug will run a little cooler under identical operating conditions.

A high energy ignition sytem (like a TI or HEI relative to a point ignition) will tolerate a colder plug because the additional energy will allow a spark to form in a gap even though a significant amount of energy may be shunted due to plug fouling.

Bottom line: Your friend is on the right track.

Duke

[Richard G.]

Re: Hotter spark plugs - what happens

Duke

What do you think I should run in my 69 435hp car. I don't run it to hard and the plugs don't last long, I have to run hard to keep the plugs clear. Not a good suggestion for a 17k mile all original car. I curious to hear your suggestion. Unfortunately, I don't drive it much because of this.

Thanks

Rich

[Duke W.]

Re: Hotter spark plugs - what happens

IIRC most BB plug were OE heat range 3, which is rather cold, but since few get run hard today, either an an AC heat range 4 or 5 or equivalent in another brand would probably be appropriate.

Duke

[Wayne K.]

Re: Hotter spark plugs - what happens

Rich,

What Duke said and also consider an extended tip plug. They don't seem to foul as easy.

Wayne

[Richard G.]

Re: Hotter spark plugs - what happens

Could you give me a couple of examples of spark plug numbers?

[Wayne K.]

Re: Hotter spark plugs - what happens

Rich,

If your not going for correct (as in judging) then I would try a 45 heat range plug for the way you drive and also an extended tip. In a AC that would be a R45XLS or cross reference to your favorate brand. I personally like Autolite or NGK plugs. Any plug number that ends in an S is an extended tip and in my experience they don't foul as easily. I'm not sure but I think the factory heat ranges for your car were 43 for highway and 44 for city.

Wayne

[Verne Frantz]

Re: Hotter spark plugs - what happens

Rich,
I agree with Duke, but I'm also in favor of running the commercial plug for street driving. I've had great service with them. The positive electrode usually has flutes (similar to a spline) on it. The many tiny sharp points are where the spark emits from. I believe it helps keep the plug clean, and the "tiny dancers" act akin to Multiple Spark Discharge.
I use an AC C44N, where the original factory plug was a 43N. The commercial plugs are usually very inexpensive at swap meets.

Verne

[Richard G.]

Re: Hotter spark plugs - what happens

Thanks guys, I've been using the R44XLS but I'll try the 45 and see how it goes.