Well, using a lower octane fuel (which ends up being a longer chain molecule) does have an increase of BTU's per volume of gas, which can lead to more power and better fuel economoy, to a point. The difference in energy content between 10% ethonol and preimum (which can contain up to 10% ethonol without having to have it listed, and if you ask for a gasoline breakdown sheet, this will be proved) isn't much more than about a 2% BTU differance. And running the lower octane in an engine with a high compression ratio, it will knock, as detonation is not based on the spark event but instead of the ignition temperature of the fuel. Knock sensors are used to tell the computer when spark timing has been advanced to far, and to pull back timing. Ignition timing really has no effect on detonation. So if you use low grade fuel and your engine is detonating, the computer will see this as overadvanced timing, and pull a few degrees out, this learned spark table may then lead to a decrease in fuel economy, while still not curing the violent detonation.
If it says 92, run 92, its not worth an engine for a few bucks. If you can honestly say you know all there is to know about the fuels/engine, combustion chamber characteristics, air distribution, you could probably figure what you can get away with. But if you don't know those things, just use what the book says.
On the other hand, if there is a supercharger kit for these engines that will still run on premium fuel, that is a pretty convincing argument that a stock engine will run on 87 fuel, as the difference isn't that great, but is still enough.
Now in my turbo 4cyl, I ran 12psi of unintercooled boost down an engine rated at 9.5:1, I did have minor detonation problems, as this was the edge for the gas I was running (92 octane) and chamber design. After pulling the engine back apart this fall, it was obvious another 2psi would have led to an engine failure..... Things to think about when playing with fuels......
