Ethanol was added to winter blends to make it more volatile and burn cleaner (has an oxygen in the molecule) in the colder months. Not to jump on the environmental rant wagon, but corn-based fuel blends were supposed to help our environment by burning cleaner. I personally feel we are destroying US croplands by subsidizing farmers to grow square miles of corn, ruining the soil with chemical fertilizers and pesticides. And I agree with PontiacGP - I had to rebuild the carb on my vintage Briggs & Stratton on my roto-tiller until I figured out that the ethanol grabs water, then evaporates and leaves junk behind in the tank and fuel bowl. I found the one station in 200 miles that sells alcohol-free gas (premium only, 90 octane up here, about $3.50 a gallon) and I get a 5-gallon can twice a year for all my small equipment. If you drive regularly, then the 10% blend probably won't hurt, but don't leave modern fuel in a classic car that sits for extended periods unless you like rebuilding carbs and replacing tanks and fuel lines.
Back to the original question, 87 octane is fine for a stock GM engine. When you raise compression and go to higher power levels, than you need to bump up towards premium. Modern engines with knock sensors back off the timing automatically, but old-schoolers like us have to make the adjustments my ear/hand. Octane is a big molecule and suppresses detonation, enabling the fuel to ignite more consistently. Therefore, you can advance the timing closer to the point where the piston just starts to move back down, getting the most 'bang for the buck'. Backing off the timing means that you ignite the fuel later, so the detonation does not hit the piston before it's ready to go down, but this lowers the efficiency/power/mileage from your fuel. The butt dyno should tell you if 89 octane makes a difference on a stock G. Gas is so cheap right now, the 10-cent difference is less noticeable.
Drive and enjoy punishing the petroleum industry for a change.
