Why does 1 plug look like this? (a.k.a. help me read my plugs)
 

I changed the plugs in my fc last night (87 GX model) and some of what I see on them makes sense but I am completely puzzled by one thing. The plugs we removed were all NGK plugs and were the correct plugs. First the picture:

Attachment 758551
Full size link (2028x778) https://i.imgur.com/1Ll6H.jpg

From left to right the plugs are:
L1 - T1 - L2 - T2

The brown / black seems to indicate it is running rich (which I would expect), and the center electrode being burned away more on the leading than the trailing also makes sense to me, but why is L1's electrode burned away so much more than L2, and why does L2 have a weird silver looking ring near the center?

A friend who was over (we changed the brakes on his wrx before doing some work on my rx-7) says that is from detonation, but it doesn't quite look like that to me, and I don't know why I would have a detonation problem on a N/A rx-7?

A bit of history about the car:
I don't know the history of the car before I bought it, but since I picked it up I have driven approximate 200km on city streets, 900-1000km on the highway and have put 9 auto-x events on the car, and when I am competing against the clock I have no aversion to winding it right up to 8k and hitting the limiter a couple times if it makes a faster run, so the engine gets well exercised. It does DUMP blue smoke when I hit the limiter, but I suspect that is some artifact of the OMP and the rev limiter and not a mechanical problem (I can wind it up to 7800-7900RPM and it will not smoke, but if I touch the limiter once it starts smoking hard).

Do these plugs say anything to you guys?

Reading plugs is a relatively arcane art- increasingly irrelevant as computers/sensors provide the info that used to be teased from the hot end of a plug.
Did you perform a proper "plug chop"....run up to high RPM and push in the clutch and kill the ignition?

except for the left one,they look half decent.
try Platinum NGK's.

leading plugs take a bit of abuse in these engines, they generally start to wear quite rapidly after about 10-15k miles. platinums last longer but only slightly, the platinum coating still doesn't last like they do in piston engines, once the platinum is worn off they wear the same as the non platinums do, so maybe 15-20k for platinums between changes.

i have seen some people run them until the center electrode is completely worn to a nub in the porcelain... i feel sorry for those neglected engines.

but i will say that the rear chamber does seem to be ingesting a small amount of oil from the engine compared to the front which looks completely normal, could be the beginning symptoms of a failing rear rotor outer oil seal o-ring.

plus look pretty good actually, worn out but good. i wouldn't try to read further than that.

basically if you're trying to get info from the color/condition, you're getting the AVERAGE of the time the engine is running, so those show the AVERAGE of 1200KM's. the people who really try to get plug color, try to ONLY run the engine in the rpm range that they are interested/trying to tune.

also rotaries won't show detonation, because the "pistons" aren't aluminum....

It is because the plugs are run in wasted spark mode. Basically the metal is jumping from one electrode to the other.

Cool, so the silver ring is more prevalent on the rear plug because there is more of the electrode left to jump across. I want to make one small correction, these plugs were in the engine when I bought the car so I don't know the mileage or history before I bought the car, I can only account for what has happened during the time I owned it.

How is it obvious the rear rotor is burning some oil? I see the electrode has some residue on it but the rest of the plug looks the same from rotor 1 to 2 to me. This sounds related to the smoke I am seeing when hitting the rev limiter.

Thanks for all the input :)

If you were to flip the front and rear leading wires, it would make the front leading have the silver ring, it's just a byproduct of the electrical flow.

I have the same silver ring around some of mine too. Mine plugs go bad about every 2 weeks so I change/clean. I think this is caused by the water thermo sensor.