AC Delco 41-902 or 41-979 Plugs? What's the difference?

These seem to be the only plugs, other than basic copper plug, for my K3500 7.4l Vortec that I can find listed.
Anyone know what the difference is? Thee auto stores can't tell me anything as the specs seem to be identical.

Is one preferred over the other or does it matter?

Thaks!!

Wild Guess: Heat range.

You're not supposed to know the difference. That's why they changed the part numbers from something meaningful to something totally random.

I wondered about heat range. The insulator on one is slightly longer and thicker. Other than that I see no difference!

"You're not supposed to know the difference." HAHA, I am sure you are on spot there! They make two plugs with different part numbers but don't tell you why they are different.

I guess I'll just pick one and hope it is fine. I'll save the 41-993's for the Tahoe since they supposedly are correct for it.

From AC-Delco:

In the early 1990s, an all-numeric, identification code was developed to ensure that the
proper spark plug would be selected for replacement in specific engine applications. The
numeric code does not in any way correspond to the heat range of the spark plug. Therefore,
selection of a spark plug with a different code number is not recommended. The heat ranges
may be drastically different between two different plugs, and engine damage or poor
performance may result. See your local installer or retailer for details.
The prefix “41” stands for the manufacturer’s product line (e.g., 41 = spark plug). Numbers
after the prefix indicate the type of spark plug and also the specific application:
· 100 – 199 = Iridium
· 600 – 699 = Resistor
· 800 – 999 = Double Platinum

Click to expand...

I found that I got slightly better mileage & a tad better performance in my BBC's by running plugs 2 steps colder than the factory recommendation.
The way it was explained to me, this creates a more even burn across the top of the piston.
The term colder plug and hotter plug refers to how fast or slow the plugs transfers the heat from the tip. A colder plug transfers heat faster while a hotter plug is slower.
The trick is to find a temperature range cool enough to prevent knock and not melt the plug tip, but still hot enough to burn off the carbon and prevent fouling.
The biggest problem in running a colder plug when it isn’t needed, is that it will foul up quickly. This happens because the plug tip doesn’t stay hot enough to burn off the carbon deposits, and so a build up occurs.
Another common problem is that colder plugs can make the car harder to start when it’s cold outside.
This may or not be of any concern when it comes to a computer controlled engine. My experience with plug temps involved an old-school high compression, points & carb, big lumpy cam'd BBC, with the timing advanced to near pre-ignition levels.
Higher cylinder temperatures at the near knock level brings the spark plug firing end temperature closer to the pre-ignition range.
Not sure weather a computer would recognize the slight performance diff between the different temperature plugs & then compensate or not.
It would be interesting to see if there was any advantage or disadvantage.
Anyways, it's always best to err on the side of a colder plug, rather than suffering knock, or even the possibility of blowing a hole in the piston by going too hot.

I think Schurkey is correct. The higher number the hotter.

How the devil do we tell what plug is ok? Both the 41-902 and the 41-979 are listed as fitting the 7.4 Vortec. Is one just a newer version or is there something else to know? Or does it even matter?!

You could go with NGK plugs but, keep in mind, the higher the number, the colder the heat range.

I would say the main difference is length if that particular spec is accurate. The heat range is supposed to be the same.

On Delco's site it shows the insulator of one is slightly longer and wider but everything else is the same.
Some of the specs on Rock don't make much sense.