Had our first test and tune last Saturday, car was a little sluggish. I had decided to move from an Autolite 85 to an NGK XR5. I’m running a 906 with about 9.5 to 1 compression. I run 90 no ethanol gas with a little Boostane, which is where some of the orange comes in on the plugs, should put me around 95 octane. . I was making some jetting and timing changes during the day with some success but after pulling some plugs I begin to wonder if the plugs were too hot. When I got home I looked my plugs with an otoscope and determined that there were black spots on the porcelain so denotation appears to be happening. My question is, is it due to timing or the plug or both. I bought some AR73 and was thinking of moving the timing back to 38 and re-start from there.

These are three different plugs at different timing. The car has always seemed to favor high timing, around 40 and there are times when 42 shows a pick up.

Anyway just looking for feedback and a recommendation.