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Octane is not a "grade" of gas, that's a misnomer, the gas is all the same "quality". What a higher octane rating does is limit pre-detonation, which is much more important the higher your compression is.
Basically as the piston compresses the gas, the heat caused from this compression will cause a low octane fuel to ignite before the piston hits apogee or top dead center (is that the correct term?) which causes the piston to slam down before it's supposed to, this causes the knock or ping.
Higher octane makes the gas harder to ignite, which allows the higher compression engines to wait till TDC before the spark ignites it vs the pressure.
The ECU will retard timing if it senses knock but it can only do so much. Running 85 on a car tuned for 91 will be bad as the ECU can't alter it enough to prevent this. Undue stress on the pistons and other internals will then shorten the life of the engine.
Now the reverse is also something to take into consideration. Most family vehicles are built to run on 85-89. Buying 91 or higher is just throwing money away, there is literally no advantage to doing this.
Something else to consider, and I do this with my Hemi, fill up with the proper rated fuel, 91 in my case, and if 89 is all that's available when traveling, don't worry about filling say half full of that. It will dilute the 91 but keep you above 89 for the remainder of the tank. And assuming you fill up when no less than half empty, the next time you get 91...things start to even out.
I would much prefer this method over octane boost btw.
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