This may seem like a controversial statement:
"You should run your car on the lowest octane fuel it is designed for."
But if you look into what happens during combustion it becomes a bit clearer. For the best performance (be it economy or power) there needs to be a controlled burning of the fuel air mixture. This is dependant on pressure, mixture, fuel, temp., ignition and whatever else.
By raising the octane rating of the fuel it will resist detonation (where the fuel/air mix explodes rather than burns), i.e. the infamous "detonation" or "pinging". Put simply, the higher the compression ratio (or boost) then the higher the octane rating required for the fuel.
Now, if you are running 98 octane in your engine that is designed to run on 91 then all the fuel is not being burnt during combustion as the temperatures and pressures are not high enough. Therefore a reduction in performance will be the result.
If your engine has a knock sensor fitted then it will automatically adjust the timing to suit whatever fuel is going through the pipes for the required performance. That's about it as far as my technical knowledge goes.
Now someone please correct me if I'm wrong:
The knock sensor is also called a lambda sensor and measures the unburnt oxygen via the oxygen sensor, right? If the system is a closed loop system then it can adjust the timing to suit the fuel used.
The RB25DET (stock ECU) does not have this as you have to tune it for the fuel being used , right?
Any corrections appreciated.