Indexing spark plugs? And what gap to run for 347?

I index mine. I don't know what kind of gains, if any, you can see by doing it because I also side-gap mine. I have never done "one or the other". I do know that you CAN feel a SOTP difference by doing both ...... but like I said, I don't know about indexing alone. And I don't have a dyno sheet reflecting such.

IMO, side-gapping is more productive than indexing. But, keep in mind, your plugs won't last quite as long if you do.

Zero Signal said:

Has anyone done this? Is it even worth it? I'll be working on putting my motor back together this weekend and have the chance to easily index my plugs since I'll be assembling all of it outside the car.

Also, I'll be running about 9.9:1 compression untill I can get good aluminum heads. What gap should I consider and which plugs should I pick up? I was thinking of going a couple heat ranges cooler than stock untill I break it in and feel good about the tune. I do have slightly dished pistons, I'm not sure if that makes much difference. Currently my combo is pretty mild, see my sig.

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I run AR3924 they are autolite's racing plugs. These particular ones are already factory sidegapped and 1 step colder than stock.

I run the 24's with 10.5:1 CR. I don't think you need to go any colder than that.

i wouldn't go colder than one below stock. so the 3924s in other words.

He has gt40 iron heads, none of the plugs listed will work.

I believe autolite 764's are for the gt40 iron's. IMO you should be fine with those, if not go 1 step colder.

I run autolite 3923's in my n/a 11:1 pump gas car gapped @ .045".

They had a big write up in Car Craft or Hot Rod or ? one of those mags where they dynoed with indexing and without. It made no difference on a 350-400hp motor. Maybe with a really high hp motor it might who knows? They said it wasnt worth the trouble.
On a motor like yours you will never even notice. Even when they add an ignition box over stock it really doesn't do much. The box will help with a supercharger because the added pressure will blow out the spark easier (Depending on how much boost) So imo don't bother indexing.

I've never seen dyno proof that said it was worth it. I see it as a "blueprinting" technique, it doesn't take that long and probably is worth a couple of HP.

I personally index mine to point them toward the exhaust valve simply to keep them out of the incoming flow of nitrous into the chamber. I dunno how effective it is, but I'm getting away with a .040 gap on a 150 shot with no issues. I also have an MSD Digital 6+, Blaster TFI coil, and superconductor wires, mostly because of the nitrous not for any other reason.

I run Autolite 103's normally, currently NGK TR6's since the local advance was out of the 103's at the time. They are 1 HR colder than a 764, which is techically a heat range cold anyway. The Cobra's from the factory had the motorcraft equivalent of 765's, but recommended replacements are 764's. I guess Ford decided they needed a bit colder plug.