AC Delco Spark Plug Heat Range

[John P.]

Hello, A friend of mine recently purchased a 1972 Corvette with a 454 engine. I am doing a tune up for him. The plugs that I removed from the engine are AC Delco 41-817. The plugs that are required are AC Delco R45TS or 41-803. Are the 41-817 a hotter or colder plug than the 41-803? Would the 41-817 be more in line with a R46TS? The AC Delco sight is of little to no use. Thank you so much in advance for any and all help, John

[Owen L.]

Re: AC Delco Spark Plug Heat Range

Original spec was the Delco ACR44T. I don't think there's any significant benefit to installing the fancy iridium, platinum, etc plugs – just use old-school nickel electrode tech. I use the NGK YR5 which cross references to the 44, 45 & 46 (I believe it's on the hotter end of the range; NGK uses an inverse heat range scale compared to ACD.)

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https://www.sparkplug-crossreference.com/convert/AC_DELCO/R44T

[Thomas B.]

Re: AC Delco Spark Plug Heat Range

I was looking at plug replacement options yesterday and found this. Don't know the source.

[Mark E.]

Re: AC Delco Spark Plug Heat Range

1970 454 uses R43T. 1972 may be different since it has open chamber heads; check the owners or service manual. I use one range hotter, R44T, because they don't foul as quickly. You can try a range or two hotter to see what works for your engine and driving style. The "S" suffix extended tip plugs (eg R44TS) work too; I haven't noticed a difference in performance.

[Duke W.]

Re: AC Delco Spark Plug Heat Range

As a general rule AC heat range 5 or equivalent in another brand work best for normal road driving when the engine only sees brief periods of high load. OE plugs were typically heat range 4 for small blocks and 3 for big blocks.

The lower heat range OE plugs are more likely to foul than heat range 5.

If the ignition and fuel systems are in proper working order and unleaded fuel is used a set of "two-dollar" spark plugs should last at least 15K miles and maybe as long as 30K. I don't see the point of buying an expensive "100,000 mile" spark plug for a car than may only be driven for a few hundred to a few thousand miles annually.

Duke

[Ray K.]

Re: AC Delco Spark Plug Heat Range

As referenced above, the called for spark plug is an R44T - both my P & A catalog and Delco book agree with this. However, I personally prefer the extended tip ( S ) plugs for street driving applications. In my opinion, the R45TS is what I would choose, but the R44TS should offer equal performance.

Ray

[Mike E.]

Re: AC Delco Spark Plug Heat Range

1970 454 uses R43T. 1972 may be different since it has open chamber heads; check the owners or service manual. I use one range hotter, R44T, because they don't foul as quickly. You can try a range or two hotter to see what works for your engine and driving style. The "S" suffix extended tip plugs (eg R44TS) work too; I haven't noticed a difference in performance.

[Owen L.]

Re: AC Delco Spark Plug Heat Range

Owner’s manuals can be a good start, but can lead you astray... So accept owner’s manual info with some skepticism.

To cite the '72 LS-5's R44T, I used the Specs doc that's downloadable from the GM Heritage Center. That pdf doc is invaluable in many ways and needs to be in everyone's arsenal of information.

[Mike E.]

Re: AC Delco Spark Plug Heat Range

To cite the '72 LS-5's R44T, I used the Specs doc that's downloadable from the GM Heritage Center. That pdf doc is invaluable in many ways and needs to be in everyone's arsenal of information.

That is the only GM place that has correct information on the LS6 plugs. Use that site all the time.

[John P.]

Re: AC Delco Spark Plug Heat Range

Thank you all for the information. I installed the AC R45TS, and it runs great. I still cannot find the heat range difference between the AC 41-803 and the 41-817. The manufacturers are not supplying the data they used to. Thanks again!

[Owen L.]

Re: AC Delco Spark Plug Heat Range

Thank you all for the information. I installed the AC R45TS, and it runs great. I still cannot find the heat range difference between the AC 41-803 and the 41-817. The manufacturers are not supplying the data they used to. Thanks again!

A little backward sleuthing seems to indicate they are very similar in heat range. The NGK cross-reference indicates both cross to the UR5GP. The NGK 5-heat range is pretty similar to the Delco 45.

(This NGK cross-reference is extremely detailed across brands.)