Originally Posted by paul386
The one thing I do know for certain. Octane rating does not change the energy content in the fuel.
Our engines have knock sensors, so the higher octane fuels will allow for more advanced timing and therefore (in most cases) more efficient power development.
Most BMW engines are rated on the higher octane fuels, for output and fuel economy. Certainly my N55 535i engine is, as is stated in the UK BMW brochure.
Older brochures used to say something like the following (for the UK market). "The 6-cylinder and 8-cylinder petrol engines are designed for RON 98 fuel. They may be run on fuel down to RON 91 with a slight decrease in engine power and an increase in fuel consumption".