Originally Posted by Richbot
Sal - Not trying to get you to give up the farm but what does the stock spark advance plot look like? Is it more like the Dinan car in this comparison (in terms of magnitude and shape) or more like the Evolve tuned car? Just trying to understand what the ECU is up to when it's compensating for reduced octane. Is it simply going to retard timing by several degrees across the board (what I would expect from experience with less sophisticated knock control systems) or does it also change the shape of the curve/map (what we see in the two different plots above)?
It's threads like this that make hanging around these forums worth it, awesome stuff.
Here is the stock full throttle ignition target for an M3:
With high octane fuel these targets will be met.
With 91 US they simply will not.
Light knock is detected and the ECU is continually pushing and retarding ignition.
You can 'force' ignition by raising the preset ignition maps by quite a few degrees.