sparkplugs - explorer gt40p - Vintage Mustang Forums
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post #1 of 11 (permalink) Old 07-22-2019, 10:05 AM Thread Starter
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sparkplugs - explorer gt40p

What sparkplugs are good for the explorer gt40p engine swap? I plan on using the Duraspark II ignition.
Thanks.

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post #2 of 11 (permalink) Old 07-22-2019, 10:44 AM
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Originally Posted by gt66a View Post
What sparkplugs are good for the explorer gt40p engine swap? I plan on using the Duraspark II ignition.
Thanks.

I used the regular plugs for the Explorer. FYI I did a thread on my GT40P swap. https://forums.vintage-mustang.com/v...g-my-66-a.html
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post #3 of 11 (permalink) Old 07-22-2019, 11:16 AM
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There was a thread on here very recently where about how you don't want to use "modern" iridium/platinum style plugs in vintage engines with vintage engines. With a Duraspark and an improved coil (MSD, Accel, etc) you might get away with platinums. But for my 5.8 with GT40P heads I'm going with plain copper Autolites, #764 and about an 0.050" gap. I like Duraspark II's but might have to use my MSD box on this one. (So Zray doesn't come and repo it because I'm letting it get all dusty.)
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post #4 of 11 (permalink) Old 07-22-2019, 12:05 PM
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There was a thread on here very recently where about how you don't want to use "modern" iridium/platinum style plugs in vintage engines with vintage engines. With a Duraspark and an improved coil (MSD, Accel, etc) you might get away with platinums. But for my 5.8 with GT40P heads I'm going with plain copper Autolites, #764 and about an 0.050" gap. I like Duraspark II's but might have to use my MSD box on this one. (So Zray doesn't come and repo it because I'm letting it get all dusty.)
What was or is the upshot on that? I'm running Autolite plugs, I assume they're platinum, I forget and I'm using points.

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post #5 of 11 (permalink) Old 07-22-2019, 12:09 PM
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I have the gt40p heads and chose the iridium plugs.
I also chose one step colder in heat range, gapped the plugs at .038, and use a MSD box.
The colder heat range may have been a mistake but so far it’s running good. And I’ve pulled a couple plugs to check and they look good.

1970 Mustang coupe
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post #6 of 11 (permalink) Old 07-22-2019, 12:22 PM Thread Starter
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Thanks for the responses!


My first thought was to go with Autolite AP104 platinum, copper core, resistor.

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post #7 of 11 (permalink) Old 07-22-2019, 02:11 PM
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AP104 is the platinum version and OK to use IF you have some newer style higher powered ignition. A few people chimed in on the other thread saying that with an older ignition setup you tend to foul platinum plugs. The spark isn't "hot" enough to keep those style electrodes clean. I have seen this myself, iridiums are even more prone to fouling. In the short term with a fairly clean running engine it doesn't matter. Unfortunately our engines don't run terribly clean, even with fancy 1990's cylinder heads on top.

You can bead blast regular spark plugs clean after most kinds of fouling and though they won't be good as new, they'll be OK for most purposes. The same blasting tends to damage iridium and platinum plugs, at least slightly. So when they foul it's best to just discard them. I don't care to throw away spark plugs I spent extra money on after just a month or two, and I have had to. The main purpose of the fancier plugs is extended longevity, so if they give you the opposite you're just throwing money away on them.

All that said, there's nothing wrong with a little experimentation. We don't all have the same engines, live in the same climates, drive the same way, etc, etc. And also just because these type plugs may not work well for us doesn't mean they are the cat's meow in their correct application. The plugs on many newer cars can be so difficult to change it tickles everyone that you don't have to for a hundred thousand miles or so.
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post #8 of 11 (permalink) Old 07-23-2019, 12:01 AM
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As far as spark plugs are concerned, copper is the best conductor but the least durable. As you move up in alloys conductivity decreases but durability increases. Iridium's are popular because they can (and do) last 100,000+ miles. You also need to have an ignition system that will fire the plugs consistently, which a system using a standard oil-filled cylindrical ignition coil won't do.

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post #9 of 11 (permalink) Old 07-23-2019, 07:31 AM Thread Starter
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Looks like I could go with the 104 (non AP) that is the full length thread version of the 764.


I think the 764 is being (or has been) discontinued. Autozone indicates that once stock is depleted on the 764 to use the full length thread version (the 104). JEGS says to call before you order the 764.
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File Type: jpg Autolite104.JPG (51.1 KB, 3 views)

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Last edited by gt66a; 07-23-2019 at 07:36 AM.
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post #10 of 11 (permalink) Old 07-23-2019, 10:42 AM
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That sounds good. I was going by my Autolite application chart which has no date on it that I can find and so may indeed be outdated.
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post #11 of 11 (permalink) Old 07-23-2019, 11:33 AM Thread Starter
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Thanks for the help everyone!

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