91 octane gas, could it harm my engine? Options

[6strngs]

right, so normally, since my car is almost all stock, I just run it on the basic 87 which is the lowest they sell here. well, last time I went to the pump, I decided to fill up with 89, just to see what kind of difference it made. I feel like it added just a little bit to the accelleration, and I loved it! not only that, but I usually fill up when I've got a quarter tank left, which usually occurs after about 250 miles, but I've gone about 230 so far and it's just barely past the halfway mark... this could be a result of alot more freeway driving and alot less city driving though... anyway, I was thinking that next time I fill up in the next couple of days that I wanted to try 91.. but my friend said that it would cause my eninge to "ping". I asked if that was bad, he said no and that my knock sensor would pick it up and just flow more fuel in to remedy it, so I asked if it would give me worse gas milage that way and he said no it would give me better... can someone please explain this in possibly greater detail and tell me if there'll be any downsides to filling up with 91 or if it'll harm my engine or anything? thanks in advance!

[celicaST]

first off your friend has it backwards. high compression ratio engines (which the 5sfe or 7afe are not) require high octane gas to avoid knocking/detonation. so the problem occurs when you are running a LOWER, not higher, octane rated fuel than the engine requires. on a side note, if the knock sensor does sense engine knocking, it doesnt send more fuel as far as i know, it retards the ignition timing. running 91 octane will not hurt your car in anyway... on the other hand its not going to help it either, stick with 87 and save your money.

This post has been edited by celicaST: Jun 14, 2006 - 5:23 PM