# Spark Plugs



## trialsmm (Jan 6, 2011)

My '66 GTO has 389 with stock Carter AFB carb. GM HEI distributor.
I've had the car one month and I'm baselining the car now.
What plugs would you guys run, it doesn't have the correct plugs in it now.
I've ordered a carb rebuild kit.
Thanks,
Mick


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## BearGFR (Aug 25, 2008)

trialsmm said:


> My '66 GTO has 389 with stock Carter AFB carb. GM HEI distributor.
> I've had the car one month and I'm baselining the car now.
> What plugs would you guys run, it doesn't have the correct plugs in it now.
> I've ordered a carb rebuild kit.
> ...


I'd start with a plug as close to the original factory spec as I could get, which was I believe probably AC 44S or 45S.

Keep in mind a plug has two jobs: 1) light the fire 2) keep itself reasonably clean (so it can keep doing job #1) without getting so hot that it damages itself and/or promotes detonation. Getting the right "heat range" for your engine is mostly about job #2.

Bear


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## geeteeohguy (Feb 2, 2008)

I agree!


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## likethat (Oct 5, 2007)

That is a big one. I have had good luck on new cars with +4 platinum plugs. I got almost 2 more miles a gallon on my crx from standard NKG plugs to the +4. I do run A/C plugs in some of my older cars(all GM now).
Bear nailed it. A nice light grey to light sandy color and no hot spots/ melting or carbon build up is a happy engine.


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## Chris-Austria (Dec 23, 2010)

These are alternatives to the AC Delco 44S:

NGK P/N: R5670-5 
NGK STK: 2298 
Accel: 137 
Autolite: 75 
Bosch P/N: WR9FC 
Bosch STK: 7516 
Champion P/N: J12YC 
Champion STK: 10 
Denso P/N: W16P-U 
Denso STK: 3036 
GM OEM NR: 5613889 
Mighty: M4-32 
Mopar: 2642913 
Motorcraft P/N: AS42C 
Motorcraft STK: 49 
Napa: X505 
Prestolite: 14-42 
Splitfire: SM008E 
Valley Forge: 44 

I use the Accel 137 and they work very good and are cheap. I was told not to use Platin plugs...


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## freethinker (Nov 14, 2009)

trialsmm said:


> My '66 GTO has 389 with stock Carter AFB carb. GM HEI distributor.
> I've had the car one month and I'm baselining the car now.
> What plugs would you guys run, it doesn't have the correct plugs in it now.
> I've ordered a carb rebuild kit.
> ...



the hei was designed to run wide gap spark plugs. the stock spark plugs would not be correct for that application. not saying they wouldnt work if you just open up the gap but when gm introduced the hei they redesigned the spark plug.


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## trialsmm (Jan 6, 2011)

What would be a suggested gap then?


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## freethinker (Nov 14, 2009)

trialsmm said:


> What would be a suggested gap then?


045 is the factory gap rec.


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## Chris-Austria (Dec 23, 2010)

As I could read on the MSD page the gap depends on the compression:

Up to 10.5:1: 0.050" - 0.060"
10.5:1 - 13.0:1: 0.040" - 0.050"
Above 13.0:1: 0.035" - 0.045"

If you know the compression, I would use a value from the list.


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