I see that this is an E89 board thread, and I'm curious as to whether there's a difference in BMW's fuel recommendations between the M54 and N52/N54 engines. I say this because my E85 2.5i/3.0i driver's handbook states the following:
"The engine of your BMW is designed to run on Super Plus/premium plus petrol - octane number 98 RON. Run the engine on this petrol by preference, in order to achieve rated performance and fuel consumption. You can also run the engine on premium grade unleaded petrol - octane number 95 RON. The minimum permissible grade is regular-grade unleaded petrol - octane number 91 RON."
I'm by no means an expert on BMW ECUs, but my general take on higher octane fuels is this.
The ECU for a normally aspirated engine equipped with knock sensors can handle fuels with lower octane levels by retarding the ignition timing - so if the engine starts to pink/knock, the sensors pick this up and the ECU retards the ignition. Many petrol cars stipulate 95 octane fuel, in which case it's possible that the ECU map limits ignition advance to that appropriate that octane level, and thus won't gain any advantage from 98/99 octane fuel over 95. That would explain why many people report no perceptible performance improvement with 98/99 octane fuel.
However, from the above handbook reference I deduce that the E85/M54 ECU will indeed advance the ignition timing to take full advantage of 98/99 octane fuel. It's also conceivable that it can take further advantage of higher octane fuel by using the VANOS system to make adjustments to the valve timing.
Again, I'm no expert - just my 2p's worth!
R.E92 wrote: Thu Nov 12, 2020 5:37 pm
It's nice to have another option aside from just VPower and Momentum but the price makes it useful only as a last resort.
There's no benefit in using higher octane fuel unless tuned though. Both the N20 and N54 engines in their standard state will operate at maximum output on any standard petrol. There's no method by which the engine can produce more power with the increase octane, it can only reduce output when the fuel quality is bad.