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Never posted before but since this is my expertise thought i'd pipe up.
The 87/91 Rating that you see is 87, the number you see at the gas pump and 91 is the RON rating. the 87 number comes from something like averaging RON and another number to get your average of 87. Why they put the RON rating is beyond me. Over at sportbikes.net we get a billion of these posts about that rating and people not knowing wth it really means. so there ya go.
87 octane actually burns faster than 91 or 93 or whatever, the higher the rating the slower the burning and more resistant to pinging or engine knock it is. So if you run 93 in an engine designed to run on 87 your not only wasting money but losing performance and economy.
Now before you say, "but my car's faster on 93!! So shutup you nub." It can very well run faster on 93 or whatever than 87 even if its designed for 87. The reason being is that your engine will accumulate carbon buildup on the pistons and that will raise your compression. That can cause knock which in turn your ECU will detect and start retarding your engine timing so you end up with a good loss in performance. So when you run a higher octane you eliminate your knock and so you get your timing back.
Also i saw someone say to unplug your battery to reset your ECU so that it'll relearn everything for new gas or whatever it was about. You should never have to do that unless you just put some monster turbo on the thing. The whole system constantly updates itself 100's if not 1000's of times a second once in closed loop. The whole map thing is for open loop, when the car isnt warmed up yet and it cant get an accurate reading from the O2 sensor. It's also as a reference usually, for say, when you slam the throttle, the O2 sensor's not as quick as we'd like it and it will give a good estimate to what it should do before the O2 sensor catches up. Of course the map is usually updated every so often too, like every so often times the motor is ran.
Hope that made some sense. Now your that much smarter.
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