Topic: Spark plug heat range - real world experience

I'm familiar with the reasoning behind hotter vs colder range spark plugs and their intended purposes. I'm curious about the concensus among the racing community here. Since the vast majority of you are running stock engines, have you found the need to run a colder range spark plug to combat increased cylinder temps associated with long term run time and high rpm use?

We will be running a Ford modular 4.6 v8 if anyone has specific experience there.

Thanks!

Re: Spark plug heat range - real world experience

Infiniti v6 and v8, stock heat range never a issue, one colder prob wouldn't hurt

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Re: Spark plug heat range - real world experience

So a basic primer on spark plug heat range looks like:

  • Colder-range spark plugs have more porcelain insulator in contact with the steel body of the spark plug. This allows more heat to be transferred from the cylinder into the cylinder head, and then into the cooling system.

  • Hotter-range spark plugs have less porcelain in contact with the steel body keeping more heat in the cylinder.

  • When you make more power by burning more fuel you may want to remove more heat to keep detonation/pre-ignition issues at bay.

When my Skunk SHO was struggling with overheating my tuner suggested using as hot a plug as I could to keep heat out of the cooling system and in the exhaust. It didn't seem to make any difference.

  • When your engine is old and tired, burning a bit of oil, you can go to a hotter plug to keep it from fouling due to the oil contamination.

  • You can determine if you need a hotter or colder plug by looking at the porcelain under strong magnification. There will be heat discoloration on the porcelain tip that can provide clues to which way to go.

Unless you have spent way more money than you should have on making engine power, you really only need stock plugs and good fuel.

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Re: Spark plug heat range - real world experience

LOL ... on the 4.6 or 5.4 you should be worried about keeping any spark plug from blowing out of the block.

But on the topic of shifting to a cooler plug, IMO you shouldn't. ECU, knock sensor (if equipped) and closed loop sensors will compensate. If you have tackled the oil cooler to maintain an acceptable oil and water temp in race conditions and you are still seeing evidence that you need to cool the cylinder then remap the ECU to create a richer fuel mix. Don't change it just because you think it might be a good idea.

Re: Spark plug heat range - real world experience

moparfan wrote:

LOL ... on the 4.6 or 5.4 you should be worried about keeping any spark plug from blowing out of the block.

But on the topic of shifting to a cooler plug, IMO you shouldn't. ECU, knock sensor (if equipped) and closed loop sensors will compensate. If you have tackled the oil cooler to maintain an acceptable oil and water temp in race conditions and you are still seeing evidence that you need to cool the cylinder then remap the ECU to create a richer fuel mix. Don't change it just because you think it might be a good idea.

Haha, fair enough. If I remember correctly though, the 3 valve mod motors had the most issues with unintentional plug ejection. This is just a 2 valve motor in a crown Vic. It's a p71, so it's already equipped with oil coolers.

Mainly I was just curious if most guys we're sticking with stock heat range plugs or not. Thanks for the input everyone.

Re: Spark plug heat range - real world experience

If not a boosted engine and not burning excessive oil, worry about everything else but using a non-stock heat range.  You know things like passing tech, and passing tech, and replacing hoses and belts and fluids....and passing tech.  How many copies of "How not to fail tech" do you have?

I only mention this because 4-posts in with the most reliable American V8 in Lemons history and spark plug heat range is your concern.  Normally, this is a sign of misaligned priorities for a first race.

Re: Spark plug heat range - real world experience

They are 5 posts from domination!

Troy

#35 LRE
1973 Datsun 240Z

Re: Spark plug heat range - real world experience

OnkelUdo wrote:

If not a boosted engine and not burning excessive oil, worry about everything else but using a non-stock heat range.  You know things like passing tech, and passing tech, and replacing hoses and belts and fluids....and passing tech.  How many copies of "How not to fail tech" do you have?

I only mention this because 4-posts in with the most reliable American V8 in Lemons history and spark plug heat range is your concern.  Normally, this is a sign of misaligned priorities for a first race.

Dually noted, and I appreciate the input. I have read the "How not to fail tech" several times and will continue to read and re-read as we build this car. I am new to the world of racing, but not new to building vehicles, roll cages and fabrication. I tend to research everything exhaustively, and things like spark plug heat ranges are something I have no real world experience with. I was just curious if endurance racing like this warranted adjusting the heat range of the plugs. Because if so, it's a simple change. Simple adjustments for the sake of performance and/or reliability are something I will always seek out.

I do not underestimate the importance of building a safe car first though and I understand that "performance" (almost an oxymoron in this conversation) is secondary. However while building the car we will still be addressing things like this as we can to avoid doing things twice. I like to dig into the details. smile

Re: Spark plug heat range - real world experience

It's going to be engine- and vehicle-dependent. Different compression ratios, different fuel delivery systems with different emergent issues from endurance racing, different cooling system efficiencies... but it sounds like you already got the advice you need regarding a stock 4.6 mod motor. Don't fuck with it.

Just for the reference of those running very old engines, we run one heat range colder than stock on a 60s Chrysler slant six (NGK 5 vs. 4). I've read that the original plugs had to be hotter to prevent fouling when burning the lower quality fuel of the day. Ours are a beautiful light brown whenever we bother to check them. (We also have a slightly higher compression ratio than stock, so I can't directly confirm.) Might consider going even colder if we ever get around to adding forced induction.

One other point for those driving slant-sixen or other 60s tech. Some people like to run extended-tip plugs (eg. NGK ZFR5N) on older street cars that didn't originally call for them, because it supposedly improves combustion efficiency. These are not generally a good idea for racing as the extended tip can be a hot spot and contribute to pinging under heavy load. YMMV

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Re: Spark plug heat range - real world experience

I always run at least 1 heat range colder than stock. Sometimes 2.

The stock plugs are specd for no fouling on part throttle cruise for infinite periods of time and in cold climates and every sort of driving condition including grandma driving to the store once a week.

We drive WOT for 8 hours straight.

Because I drive aggressively on the street, most of my (even lightly) modded cars get 1 range colder and I've *never* in 25 years had a problem with fouling.

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Re: Spark plug heat range - real world experience

aventari wrote:

I always run at least 1 heat range colder than stock. Sometimes 2.

The stock plugs are specd for no fouling on part throttle cruise for infinite periods of time and in cold climates and every sort of driving condition including grandma driving to the store once a week.

We drive WOT for 8 hours straight.

Because I drive aggressively on the street, most of my (even lightly) modded cars get 1 range colder and I've *never* in 25 years had a problem with fouling.

This was my train of thought, I'm glad to hear I wasn't completely wasting my time looking at the potential (however minor) benefits of this simple change.

Re: Spark plug heat range - real world experience

An NGK rep replied to this question once on the CC forum. Can't find the thread, but his answer was that for what we're doing, one step colder isn't a bad idea.

Re: Spark plug heat range - real world experience

ross2004 wrote:

An NGK rep replied to this question once on the CC forum. Can't find the thread, but his answer was that for what we're doing, one step colder isn't a bad idea.

Perfect smile