[sear]

Timing advance and octane levels has obviously been a hot topic on this forum for a long time. Having read tons of threads on related subjects I've still got one area of open questions about how the N54 can "sense" that octane levels may be lower than desired and thus take appropriate action(s).

Clearly understood (I think) is the function of teh knock sensors and the DME. These sensors listen for audible frequencies emitted by the block when knock is beginning to occur (note: already occuring) and the DME takes action with I beleive a radical drop in timing advance. It may even take further mitigation steps beyond step 1. Not looking to clarify all deatiled actions based on knock sensors being triggered, but am looking to understand the following:

What other sensors/logic is in play by the N54 DME to take proactive measures when octane levels are not ideal? I assume that the knock sensor role is one of a reactive reaction, but assume that some proactive DME and/or Tune actions are also in play...

[Joshboody]

Your understanding is correct... just knock sensors. Maybe some adapted ign advance over time, but I never really noticed this when logging the stock tune.

[tscdennab]

It does take proactive measures via adaptation. You have "reset octane adaptation" in the BT tool. What does this mean ? There is probably a tendency to not use high timing values once it has been determined that low octane fuel is used for a long time, and to slowly try to raise timing to see if the knock sensors are ok with it. The ECU will not try to use the maximum timing values like mad regardless of the octane, and rely on the knock sensors to cut the timing each second. It would be stupid.

[Joshboody]

Yeah, you would think. I never went so far as to put 87oct in the tank of course. But when running in harsh conditions I would have multiple timing corrections per gear and the ign target didn't seem to change. I also reset octane adaptation and again timing target seemed not to change. These were my experiences using BT and mainly only reviewing WOT. If it does adapt, then it doesn't do a good job when WOT on 91oct in hot conditions. Basically I wouldn't rely on the DME to match timing to poor conditions.

You would also think there is some correction for baro, and I would be very curious to see high altitude logs of the stock tune, as in the OPs case. But I doubt there's adequate adaption.

BMW seems to very confident in the DME's ability to dynamically adjust per the immediate conditions. Just my observations.

[sear]

Interesting... thanks for the responses so far. Clearly begs the question for sure of what the Octane Adaptation is for then?

Just spectulating on your altitude question, baro adjustments may just be expected to be compensated for via IAT sensor inputs. This given that the trubos have to spool more to achieve boost targets and this has the side effect of higher IATs for equivelent boost target at sea level. Maybe there is more to it than this though

[Joshboody]

Originally Posted by sear

Interesting... thanks for the responses so far. Clearly begs the question for sure of what the Octane Adaptation is for then?

Just spectulating on your altitude question, baro adjustments may just be expected to be compensated for via IAT sensor inputs. This given that the trubos have to spool more to achieve boost targets and this has the side effect of higher IATs for equivelent boost target at sea level. Maybe there is more to it than this though

yep, you are correct IAT should account for baro. For reference, the below link includes a portion of my logs.
http://www.e90post.com/forums/showthread.php?t=491784

[JamesM3M5]

Maybe the octane adaptation takes longer to change than just a few pulls? Of course the only way to tell is to put in low octane gas and run the engine moderately hard for a while to see whether it sets any different parameters / targets. No thanks.

I think it's mainly for countries that do not have higher octane fuels readily available?? 80s Bosch DMEs for many different cars (BMW and Porsche come to mind) had a "fuel quality switch" with 8 positions to increase/decrease fuel and decrease timing as needed across the board. It was a very rudimentary way to detune an ECU for a country without high octane fuels available. South African and other models also got old M5/M6 engines with lower compression from the factory for this very reason.