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Old 05-04-2009, 10:28 PM   #1
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What plugs for GT40x heads?

Pretty simple:

What plugs to go with my 302 with GT40x/e-cam/gt40 intake...

What should I gap said plugs at?

Thanks!
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Old 05-04-2009, 10:31 PM   #2
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With basically that same combo, I ran Autolite 3924's gapped at .52
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Old 05-04-2009, 10:39 PM   #3
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autolite 104 or 764

3924 is incorrect.

.052
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Old 05-04-2009, 10:51 PM   #4
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Quote:
Originally Posted by rob View Post
autolite 104 or 764

3924 is incorrect.

.052
what is the problem with the 3924 plugs. I use the 24's with my novi 1000 and i have the gt40x heads
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Old 05-04-2009, 10:52 PM   #5
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i use 764 also
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Old 05-04-2009, 11:20 PM   #6
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Quote:
Originally Posted by rob View Post
autolite 104 or 764

3924 is incorrect.

.052
Explain.
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Old 05-05-2009, 12:24 AM   #7
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Quote:
Originally Posted by 95StangRC View Post
Explain.
i'll start with the gap .52 vs. 0.52, thats a big difference.

ford gt40 iron, p, y and x use a non washered/tapered (or whatever it is) and the 3924 is washered.

does that explain it better for you?
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Old 05-05-2009, 12:30 AM   #8
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3924
[IMG][/IMG]
104
[IMG][/IMG]
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Old 05-05-2009, 12:32 AM   #9
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i personally run 104's with my x heads and run 764's with my gt40 iron like the parts store calls for a 93 cobra.
3924's with tfs, afr, holley, edelbrock, etc...
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Old 05-05-2009, 07:54 AM   #10
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Quote:
Originally Posted by rob View Post
autolite 104 or 764

3924 is incorrect.

.052
The GT40 X head accepts a tappered or gasketed plug. The 3924 plug is a warmer plug(heat range), equal to a 42 series(agsf-42c, stock 5.0 plug) motorcraft plug I believe. Autolite produces Motorcrafts plugs. 3924=42series, 3923=32, 3922=22. Autolite also produces a racing plug(AR series) in the AR 3924, AR 3923, AR 3911, and AR 3910. NA application I would run a AR 3923, in a boosted application go with the AR 3911 or AR 3910(the 3911 is 3 heat ranges colder then the 3924). I run the AR 3911 in my car(Vortech 351).
After talking to a Autolite rep for awhile I was told to run as cold a plug as possible(esspecially in a blown application). If you go to cold, the motor can load up while idling, thats why I went with the 3911 over the 3910(not a hint of loading up).
The AR plugs have a thicker electrode and body(heat transfer), and the colder plug helps pull heat out of the chamber as well. You can also bring down the gap, most AR plugs are factory gapped at .025 to .035. The tighter gap helps prevent a blower or turbo from blowing out the spark. Summit sells most of the AR plugs, there under $4 a plug.

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Old 05-05-2009, 09:57 AM   #11
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I just went through this with the 95 GT I bought recently for next to nothing because the guy that built it didn't have a clue. It had 4 thread plugs in X series heads among a bunch of other stupid crap. I like the Motorcraft copper plugs personally and am running the AGSF-22C in this car. FRPP 302 short block with GT40X heads, Motorsports intake, 70mm BBK TB, 24# injectors, Mac shorties, UPR catted X into Dynomax's. TwEECer tuned.

Acutally I just double checked Ford Racing on the GT40X and it says it can use either the tapered or gasketed 8 thread plugs.
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Last edited by PGreenSVT; 05-05-2009 at 10:02 AM. Reason: updated info
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Old 05-05-2009, 10:17 AM   #12
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rob, I wasn't trying to be a smart a$$ if it came across that way. I just wanted an explanation of why it was the wrong plug. I've run washer type plugs in my X heads for over 2 years now without problems and wondered why it was wrong to do so. As for the gap, that was a typing error by me...I meant 52 thousandths, not hundredths. Sorry to the op for the clerical error.
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Old 05-05-2009, 10:42 AM   #13
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edit.....double post
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Last edited by 95StangRC; 05-05-2009 at 10:44 AM. Reason: double post
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Old 05-05-2009, 10:52 AM   #14
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ford recommends a motorcraft plug that is tapered. i checked the specs on the x heads and it does say tapered or gasketed, so i guess everybody is right.
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Old 05-06-2009, 06:02 AM   #15
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I read thru some of my notes I had from Autolite, and the 3924 Autolite plug has the same heat range as the 32series Motorcraft plug, not the 42. If anyone cares?

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