confused about statements of differences between 91 and 87 octanes...91 has relatively same btu's so any improvement in mileage would have to be about timing(advancing) and the engines ability to burn 91 without pinging and see no advantage because engine will burn 87 with out a problem.
Are you serious?
First of all, BTU is the absolute most ridiculous unit for measuring a fuel's ability to make powerful combustion.
An engine is not a water heater or a dryer.
This is especially true for turbo motors because exhaust gas pressure/energy contribute directly to the car's power.
Second, how are you both asking a question and making a statement?
Yes, running 91 octane allows you to run more aggressive timing which translates into more power.
Ever been in an older high performance car using a distributor with bad centrifugal weights?
The weights spin outward based on engine speed and
advance timing - when the weights are stuck the timing won't advance and the power flattens out early in the RPM band.
The horse has been beaten about the dual-map in the Sonic's ECU.
You fill up the tank the ECU uses 'the good map', an aggressive timing map, until it detects knock then bounces you down to the ****ty map.
You could, in theory, stay in the good map if conditions are great ; maybe cold weather, not going to hard on the throttle from stops/high load but I wouldn't count on it.
I don't mean to sound rough, but this issue is getting a little old especially since the benefits of aggressive timing go back to the beginning of motor vehicles.
If there's ever any question feel free to pop the hood on a vehicle and retard the timing and see how the vehicle performs.