# 44s ac delco spark plugs for 389 '66 gto



## PONTIAC CLASS (Feb 4, 2014)

I have an original 1966 GM shop manuel that states the following.
It says to use AC-44S spark plugs for 389 GTO.
Also, I am running the original 389 in the car, a 1966 GTO.
Compression is high with this engine.
Has anyone used or is using this AC-44S spark plugs.
From what I understand this AC-44S plug is a hotter plug than the AC-45S.
The GM shop manuel says to use the AC-45S in the 6 cylinder engine and the 326 c.i.d. engine. 
Would I be accurate in concluding that the AC-44S plug provides more power and best efficiency for the 389 engine.
Thanks for all your input on this subject.


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## BearGFR (Aug 25, 2008)

Nope, not correct. All a spark plug does is get the fire started. How much power an engine makes is a function of displacement, volumetric efficiency (how well it moves air), combustion chamber design, friction losses, and overall mechanical condition.

The heat range of a spark plug has to do with how hot the ceramic insulator around the center electrode tends to be. This matters because a plug that's too "cold" will tend to foul up with crud because it doesn't burn it off, and a plug that's too "hot" will tend to promote detonation in the engine (a very bad thing) and also won't last as long. That's about it.

Bear


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## rickm (Feb 8, 2012)

PONTIAC CLASS said:


> I have an original 1966 GM shop manuel that states the following.
> It says to use AC-44S spark plugs for 389 GTO.
> Also, I am running the original 389 in the car, a 1966 GTO.
> Compression is high with this engine.
> ...


that's a non- resistor plug , which are hard to find nowdays. your gonna find the R44S ac plug for replacement. the difference in heat range between 44S n 45S is very little. back in the '70s when spark plug co,s began to consolidate their plugs champion (which I dislike) had 1 part # champion plug that they claimed was a replacement for both ac plug #s. also , back then Neanderthal mechanics would ask me for a" hotter plug" for a motor that had an oil fouling problem.


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## geeteeohguy (Feb 2, 2008)

The 44 is a cooler plug than a 45, and is the right heat range for your engine. A 45 will work, but better off finding some 44 or equivalent plugs. The resistor plugs are available, and the ones you want are R44s. 45's are hotter...which can contribute more to your already too-high-compression-with-today's-poor-fuel issue. I too dislike Champions in these cars. Have had nothing but bad luck with them over the years. NFG!!!


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## PONTIAC CLASS (Feb 4, 2014)

Thanks gentlemen. So, the 44 is indeed best for an original 389. Are all plugs
now resistor plugs? I believe the original plugs were not of the resistor variety and from an overall information standpoint for myself are the resistor plugs a manifestation of where we are in the 21st century.
Also, when the heads were done i was told they were "decked" and that they may be in the compression range of 11-1 or 11.5-1 - the original 10:75-1 compression was increased.
I run only 93 octane pure gas no ethanol.
My other question has to do with the original 093 heads on the 389, are these 093 heads all H.O. ?


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## BearGFR (Aug 25, 2008)

PONTIAC CLASS said:


> Also, when the heads were done i was told they were "decked" and that they may be in the compression range of 11-1 or 11.5-1 - the original 10:75-1 compression was increased.
> I run only 93 octane pure gas no ethanol.


If that's truly the case, there is no way that engine is going to survive for long on anything less than 100+ octane racing fuel. It's only a matter of time.

In order to correctly id the heads, the first bit of information that is required is the 4-character date code. These links will show you the possible places to look for it.

















Bear


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## geeteeohguy (Feb 2, 2008)

O93 heads were made one year, 1966. And came on GTO's and 421 engines in the big cars. They were Pontiacs 'best' performance head at that time, but were never designated "HO". At 11:1, you will need 100-110 octane fuel at all times. No if's, and's or but's.


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