This isn't completely necessary, but I'll say it anyways. Octane measurments are actually a measurement of heptane and octane. A true OCTANE is 100, because, well, then it is 100% octane. 87, for instance, is a mix of heptane(cheaper) and octane, to make it the equivalent of 87% octane, with the rest being heptane or some equivalent. RON(research octane number) + MON(motor octane number), divided by 2 is how we get our numbers in the states. It's not that's it's not combustable, it's that it resists detonation. Detonation, in essence, is spontaneous combustion under pressure/heat. Obviously, we only want it to combust on que.
Now, the reason octane is used, instead of heptane, is because heptane will ignite under very small pressure, unlike octane. And,since all this can be put on paper and figured out mathematically, any given octane rated gasoline has a certain compression ratio at which it will detonate. That being said, if 87 requires a CR of 10:1, and your engine runs at 11:1, you'll see detonation as soon as you hit 10:1 on your way to 11:1. On the other hand, if 91 detonates at 12:1, then you won't finish burning it, because you never reached 12:1. And because of this, you will never see the full potential of 91. BUT, it won't harm performance, it will just dump the unburnt fuel out the exhaust(possibly bad for the cat though). So naturally, you'd want 89, because it detonates at 11:1, and you'd get the full benefit of the gas.
Since we aren't going to figure all that out, we just run the lowest that makes it run the smoothest..
How'd you like todays science lesson? hehe
I hope it at leastedhelped a little..