Does anyone know why the stock spark plug changed from Champion RC9 to RC12 with the 91 model year? I have an 87 and was about to go get some RC12's because all the info I could find basically says that the I6 is happiest with the champion RC12. Just to make sure I went to the Champion website and looked up my jeep and it says RC9. Did the engine change that much in 1991? I want to replace my plugs, wires, and coil. Would it be safe to upgrade to RC12's with the correct wires and coil? Thanks.

 

There are a lot of differences as that time line is the jump from Renix to the High Output Chrysler-ized engines. I suspect it's something to do with thermal differences (different head design, the introduction of OBD, and the elimination of the silly EGR system). The RC12 Coppers should be fine, maybe even the NGK coppers that they brought in in 2000.

 

Look at the underhood label - it tells what to use!

 

Yea, underhood label...

Seriously though, she's 25 years old. There are remnants of a label in the engine compartment but it doesn't specify spark plug type. It does however specify spark plug gap.

I wasn't asking what my OEM spark plugs are, I already know that the '87 uses RC9's. My question was asking if I could use RC12's safely with the proper wires and such. I can't find anything definitive so I'm going to stick with OE equipment. I'm not looking for horsepower boosts or crazy speeds anyway. A factory I6 is more than enough for a trail rig, in my opinion. Thanks.