i just wanted to share this with you folks. here are two pictures of my spark plugs after running 21,000 miles on only 93 gasoline. (all but the first tank, which was filled by the dealer with 87)

 

Could you maybe leave a caption with the pictures? I have no idea what they mean.

 

that is what spark plug should look like when your engine is running the way it should.

 

when did Mazda start offering the 7cyl. option.
OK,OK, just kidding.

 

Ok but does anyone have plugs from a car run only on 87 octane with the same milage?? So we can compare them and see if there is any difference

 

Actually, the photos indicate that the heat range of the plug is a bit on the cold side. This is indicated by the clean center area (good ignition and burn) but the plug is transfering heat too quickly to the water jacket (black outer ring next to the threads). The plug is firing just fine but I'd say you have the same problem that I do. You put your foot in it a lot, and that shows up as a rich mixture indication on your plugs. The centers will burn clean under normal driving but the outside won't if the plug is too cold. This is a combination that occurs due to our too rich WOT calibration (either from too much fuel or lack of advance) then the car leaning out during normal cruise. The heat range of the plug is cool enough that the outside ring won't burn clean since the heat is transfered too quickly to the cylinder head.

A perfect looking plug would be clean on all the surfaces that are exposed to combustion, or have light tan deposits. I've confirmed that our cars run way rich at WOT from the times I've been on the dyno. The last time I removed my plugs (check gap and take a compression test) they looked much the same. The last time I was on the dyno was when I tested my header and the mixture was much better until about 5250 where it started to go way rich again. That was an improvement since the time before that it went rich at 3750. By the way, I mostly run mine on 87-89 octane and the plugs look the same.

 

traveler,

thanks for your input. i thought they were fine, but i totally follow what you are saying. and yes, i do get on it quite a bit. (and i still get 30 mpg average!) was your header the only reason your mixture leaned out, or did you do something else? (if it was only the header, i might have to get one )