Not necessary always OEM.... you can’t go wrong with AC Delco for sure but the thinking that they are always superior because they spend millions on R&D is flawed.
That millions spent is for “descent” reliability at the cheapest cost to GM. Basically “good enough” which delco plugs are “good enough”.
NGK specializes in plugs and they are indeed far superior to AC delco. In a lot of cases the OEM will test several plugs already in production with an engine they just developed.
I guess in the end it’s just my opinion but it does come with a boatload of experience and data to back it up.
On the torque and heat range subject it’s actually the depth of the plug in the head/how far it protrudes into the combustion chamber. The difference is minimal and varies from plug to plug/brand to brand. For the assembly plant to stress proper torque most likely plugs with crush washers were being installed. Once contact is made with crush washer to head you can actually feel the crush or squish that last half to 3/4 turn. The diff in proper crush or seating would only be 3-4 thousandths (just an estimate). This type compared to most automotive plugs which are solid chamfered seat without washer will make little to no difference in depth or heat as long as it’s not loose. Snug, properly torqued, or tight as hell would all 3 perform in the same heat scale and not vary in combustion chamber protrusion.
The “spark plug life” issue is a interesting one for me. The older ignition systems for sure would be hard pressed to “wear out” a spark plug. Engine may foul it up but not physically degrade it in most cases. However, the newer ignition systems with multiple coils, higher voltages, and more ECU control and variables will indeed degrade or wear out the best of plugs. An extreme example would be the 2004 yamaha R1 I owned. 4cyl one liter engine pumping out 190 hp! Redline for engine RPM was 15,500!
After the first 8k miles I put on it to the mile it all the sudden didn’t want to crank. Finally did and it ran like garbage. I thought Oh crap what did I do to it. Keeping it simple I began to troubleshoot replacing the plugs since it was maintenance either way whether it fixed it or not. The NGK’s were laser iridiums, 16.45 a piece! (Which I may add were the OEM plug from factory and to my knowledge delco makes no plugs that will cross over much less meet the massive requirements of this 1 liter engine cranking out the Hp of most 4 cyl compact cars.. I digress..) installed new plugs and voila ran like new. My 09 Silverado I bought in 2010 had 17k miles. At 19K it had a slight rough idle. Yanked the delco plugs out and put NGK IZTR5B11 laser iridium’s in it. 34K miles later(twice as much life as OEM delco) replaced them due to same very slight misfires. Currently I have 86K miles on the 5.3 and had to replace the plugs today exactly 34K miles after last replacement. Now, I know what folks are thinking with their 100K mi spark plug claims. Could I have gone longer? Yes. I’m a lifelong mechanic though and very keen to the slightest hiccup in my engine compared to oblivious people that just get in and go till it won’t go then to the shop it goes. These 100K plug claims are ludicrous on these newer NNBS engines.... the wires, caps, and plugs just don’t last that long period! This isn’t fringe info, crack your maintenance log and even GM will recommend at least plug replacement well before 100k(since we are trusting all those millions in R&D better stick with your logic and follow maintenance guide)
What am I saying? I’m saying AC Delco will work but if you want the best source for lighting off your air/fuel mixture buy NGK spark plugs.
((by the way, that 1 liter that ate plugs every 8500mi. After replacing I would inspect the ones that came out and could find literally nothing cosmetically wrong. No electrode degradation or burning, nothing but a nice brown tint on the business end. That’s the first time I experienced or realized modern ignition systems were capable of internally degrading a spark plug))
