Originally Posted by SWP
Your M5 will run just fine on 91 octane (Premium in CA).
Modern engine all have anti-knock systems that will adapt the engine to lower than recommended octane levels.
BMW recommends 92 octane for the M5, but 91 octane is readily available in CA because they add that gasoline derivative for environmental purposes (I can't remember the name).
The one octane difference will not affect performance. It may reduce your fuel consumption a bit, but nothing to worry about.
I don't think so or BMW wouldn't have put in the Euro English manual:
"> The engine of your BMW M5 is rated to run on: Super Plus/premium plus fuel, octane number 98 RON. Fill up with fuel of this grade whenever possible, so that hte niminal performance and fuel consumption values are achieved."
And: "> The minum permissible grade is: Premium-grade unleaded fuel, octane number 95 RON."
Don't have my US manual handy but 98 RON is ~~ 93 AKI or (RON+MON)/2 as we use in the US and ~~ 91 AKI for minum permissible grade.
It's true the car will run on 91 due to anti knock sensors but what do you think the EMC does when it sense knock ? What it does will definitely impact performance.
Further, the nominal value for octaine is based on atmospheric conditions that are nearly ideal (altitude: sea-level, humidity: moderate (don't remember the value), and tempature: moderate (don't remember)) so the higer the altitude, the lower the humidity, and the higher the tempature all increase the octane required.
So, where did you get your info about octane rating required for performance of an M5 ?