Old thread, But gas prices being so high,, I'll stick to 89. My '06 4.0L has no issues with it.

 

Unfortunately alcohol free gas is usually the higher octane choice.
so, i figure running any alcohol at all will eventually cause carbon buildup, poor acceleration and loss of power.
it's not the fact you put 93 in and it runs better. That isn't proof higher octane is needed for your particular motor, it's that you could probable fix the poor performance on lower octane with the water (excuse me, alcohol) completely removed from the math and using a lower octain alcohol free gas.
the higher octain simply allows you to run higher compression.
if you are using 87 alcohol blend and your engine knocks and pings and struggles, octain is not the real reason. It's carbon buildup and a dirty engine suffers using its recommended octane requirement.
at this point i don't see any reason to use alcohol in a stock motor other than you will most likely pay as much for that as you do high octain straight gasoline.
I tried several tanks of 110 octain in my bike but the only difference i noticed was a slightly different exhaust note but I did not sense a power increase.
there's more energy in low octain fuel and a waste of money to use anything above what it's rated for. It's just that the alcohol causes problems throughout the entire fuel system and because of that. I think that's where it can become confusing about running higher octain. You're simply trying to overcome the problems a poorly maintained fuel system causes buy using a fuel that won't ping under those adverse conditions. That does not mean that motor needs higher octane than it's rated for.
on the other hand, pure alcohal in a motor designed for racing will run cool and great all day long.

Wanted to add that the situation is about to get worse with the recent relaxing of regulations limiting alcohol percentage.