Let's talk spark plugs...

3190 Views 15 Replies 5 Participants Last post by  snrusnak
I've been researching this topic a lot and have found a lot of good information on this forum, other forums, google, etc. and am trying to decide what plugs I should use in my engine. Anyone please chime in with information, recommendations, etc. as I've gathered all this information and want to know if it's correct, etc.

Heat Range:

I read that switching to a colder heat range plug is recommended if you have the following: A lot of ignition timing advance, high compression, high octane fuel, more power output over stock(I read that you should go 1 heat range colder for every 50-100hp over stock).

How does overall engine temperature effect spark plug heat range, or does it have no effect? For example, stock engine operating temperature is about 200 deg F with the stock tstat. If I switch to a 180 deg tstat does that have any effect on what heat range plugs could/should be used?


From what I understand, the larger the gap the better the performance. The problem is you need a lot of spark energy for larger gaps. Too small of a gap can cause detonation. I wonder also, why does the lower bank of plugs in the 4.7L use .050" gap and the upper bank of plugs uses .040" gap? Does this have to do with the material of the plugs (lower is iridium, upper is platinum)? If so, would using iridiums in the upper bank with .050" gap be an improvement?


Iridiums last the longest, but are the most expensive. Platinums are sort of mid grade for longevity and price, but they can cause engine damage with high performance applications apparently. Coppers are the cheapest but last the shortest. From what I understand, plug material basically has no effect on performance, only duration of the plug...

What I'm wondering is if I should/could switch my plugs to a colder heat range (1 range colder) and if this would be a benefit/no change/bad thing to do. Also, if changing the upper bank platinums for iridium or coppers would be any benefit/no change/bad thing to do (I understand with coppers they'd need changed more often). For example, would running iridiums in the upper bank allow more gap and give more power.

The reason I'm wondering is because I have the tuner which adds a lot of timing advance, I use high octane(93), and I will be somewhere around 375hp-400hp when done (more than 50hp over stock).

Thanks for any input in advance!
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Sean, recall watching something on TV (insomnia at 2 am) about flex fuel engines, Octane rating of E85 and how high compression and heat contribute to getting the most out of higher octane, which isn't much.

Now, with that said, here is my disclaimer: I COULD BE WAY THE HELL OFF BASE ON THIS.

You had mentioned the delay in the firing of the top copper plugs and the lower irridium plugs. I'm "wondering", is this to get a more complete burn on the ethanol if someone actually decides to run E85 through their engine?

Example: Top plug fires and ignites fuel/air mixture. Bottom plug fires to complete the burn.

If I remember correctly, the manual states copper on top and iridium on the bottom. I've heard the material provides less resistance, allowing for larger plug gap, resulting in larger spark. I've also "heard" platinum and iridium both last longer.

If this is true, then here is something to consider: The manufacturer was saving money by throwing copper plugs on top. If, as you mentioned earlier, the top fires first, quickly followed by the bottom, based on the angle of the cam, then the type of plug shouldn't matter. The plug is just the light bulb. Something else is hitting the light switch.

I've thought about switching to platinum when I change out the top plugs just to make them last longer. I've heard the bottoms are a real pain in the @$$ to change out.

This is just my thought bordering on theory, based on scientific wild ass guessing.
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I miss interpreted your post. When you used the word "degree", I thought you were talking about something else.

Is it possible to pick the brain of an actual automotive engineer? Maybe get online with Mopar and ask them if this would fly? I'm dying to know this and I'm GLAD you brought this discussion up because I was going to swap out the top plugs at 30,000 miles with platinum or iridium just to make them last longer.

I'll be watching this post for the outcome.
By the way, is it true about what I heard? Are the lower bank plugs a real pain in the @$$ to change out?

SEE SEAN!!! I TOLD YOU I'M NOT A DUMB @$$!!! Oh, wait, that was me telling myself that. lol
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