Spark plugs

[Dark Knight]

Hey everybody. I want to change the plugs in my '67 but would like some opinions first. Right now, I'm running copper ACDelco R43TS plugs. My engine is set at 12* BTDC. The compression ratio is somewhere between 9-9.5:1. The engine is a 350 originally out of an '84 Chevy van. It's all stock with the exception of headers, Thumpr cam, flat-top pistons w/2 valve reliefs, and aftermarket heads. It runs great, but seems to miss sometimes and usually diesels when I shut it off. I read that I might need to go down 1-2 heat ranges. Anybody know for sure? What would be the best type of plug to use? ACDelco coppers again? Recommended gap? Thanks.

[webfoot]

Usually the aftermarket head manufacturers will give a recommended spark plug to use on their website or in the box. That plug is for a factory head and may not be correct for your application. Plugs are not going to cause it to diesel, this can be a timing or idle issue. What are the specs on that cam?

Be prepared to hear a lot of negative comments on that cam. They are made to sound cool with not much thought given to performance. 9.5:1 with headers will sound good with any performance oriented cam.

[oldblue1968chevy]

I have the same cam..

Mine diesels at shutoff if I dont run high grade of gasoline (87octane) runs great on 93, you might bump the timing back off a hair.

But first, what gas do you run?

[GASoline71]

Check with the head manufacturer on what the recommended plug is. Then get the AC Delco equivalent.

What ignition system are you using?

That Thumpr cam is gonna give you fits tryin' to time it correctly.

Gary

[oldblue1968chevy]

Tell us what heads ya got

[Dark Knight]

Running 89 octane. It doesn't diesel near as bad with 93 octane, but it really sucks having to pay for that stuff. haha. These are the heads I have: http://www.summitracing.com/parts/SUM-152123/
I love them. No complaints about the cam either. Before anyone hops on here and decides to post anything negative, let me welcome you for a ride in Old Blue. Those thoughts will disappear. PLENTY of torque to back up the sound. If anyone still wants to knock it, they're more than welcome to swap it for me. I will agree though, it's a royal PITA to get it timed. Oh yeah, running GM HEI by the way.

On a side note, I just realized the "suggested parts" link on Summit's website. Plugs came up as NGK 6630 V-Power plugs.

[oldblue1968chevy]

Im running the other type of head like that (summit makes it, smaller valves but bigger cc chamber) same cam too.

Ive tried running 89 I think it might do OK but i pump the 93 instead and dont think about it, I know mines diesels like crazy if it gets a DROP of 87

[oldblue1968chevy]

If thats the plug summit recommends then its probably right (id double check liek your trying).

If your running HEI id up the gap to .45 (if its not like that out the box)

[screaminchevypickup]

I would run a a/c r45ts plug with a 40 gap just my 2 cents Kyle
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[Dark Knight]

Well, even though I'm an AC Delco guy, I think I'm gonna try the NGK's. The more I think about it, the more it makes sense. It's the only plug they recommend, plus I'm sure they did their research on which plug works the best in this particular head. The NGK plug has a heat range of 4, while the AC (which I'm currently running) has a heat range of 3, theoretically making the NGK one step colder. I don't know if their scales are the same, but I guess I'll find out.

[406 Q-ship]

Going up in number is hotter, not colder. Did you check the AC equivilant to the recommended NGK? With aluminum heads the thread reach is usually longer than the iron head that GM used, R43TS is a short reach plug. The dieseling could be the idle has to set high for the Thumpr cam, get a idle kicker solenoid and wire it into the ignition circuit. Use the idle kicker to set the idle speed and the idle speed set screw on the carb to throttle closed.

On a side note: HEI's typically have a ton of timing in the advance and need to be limited down when running 12° initial. I better your total timing is really high (like 48°), HEI's come in very slow too. Remember the HEI was used on emission engines and have crappy spark curves. Bottom line get the HEI curved if you have not all ready.

[oldblue1968chevy]

Pop your rotor off and tell us what numbers you have stamped onto the flyweights and center weights.

Im running the infamous 375/41 combo, limiting my vac adv to 10*, I run 93 and no dieseling etc, also changed the adv curve with lighter springs