[x33rs]

XR4, that's as hot as they get.

That's equivalent or close to the R45S AC plug which is an extended reach plug (electrode and ground strap extend into the chamber slightly more) which is better for flame travel.

I don't believe NGK makes the shallow plug that would be equivalent to an AC R45 (no S in the part number) We always called those oil burning plugs because the electrode and ground strap were very shallow and not out in the path of the chamber.

I ran the XR4's in my Z for about 2,000 miles as an experiment. Worked fine up here at 5,000 ft. But had #2 and #3 showing a tad rich, which runs off the left front corner of the carb. I wanted to go to a warmer plug but NGK doesn't make one hotter. So I tend to believe the XR4's may be closer to the R44S heat ranges because those plugs worked similar. I already had the carb stagger jetted for best power so I decided to switch to Autolite 86's, which are as hot as they go and equivalent to R45S. Been clean ever since.