Ah ha... Thanks to R32 for pointing out the link the the mk5 GTI Tips and Tricks
Here is the simplified reason why higher octane is better for FSI engines
Regards FSI owners mainly.
The higher the octane rating of the fuel the longer the FSI engine will run in FSI mode.
You see, when the FSI is running in 'Fuel Stratified Injection' mode it creates a lot of NoX (NoX - Very bad as far as emisions go) due to it being such a lean burn, so it has a NoX Cat which abrorbs the NoX, once its full the Nox probe (fancy lambda probe) senses this and the engine switches back to normal running and can safely clear the NoX out with the other gases through the main Cat so it can switch back into in FSI mode again.
The problem with octane levels is, and this is just an example depending on driving styles.
Out of 100 miles average :-
95RON - 90 miles normal, 10 miles FSI (lots of NoX made in FSI mode)
97+RON - 60 miles normal, 40 miles FSI (some NoX made in FSI mode)
Now regardless of what anyone says this is how the engine is built and as its more efficient in FSI mode VW always recommend the highest octane rating for this reason, they did introduce a 1.4FSI for the uk market that is made to run on 95RON fuel for longer but you want to see the size of the NoX Cat, more like a tanker. lol
And after all that, you will get better miles to the gallon running 97+RON regardless of it being an FSI, its they way they are made these days, the ecu recognises when the fuel mixture combusts and will adjust it accordingly, the cheaper the fuel the more retarded the timing, the more retarded the timing the less power, the less power the more you 'need' to put the foot down
