Originally Posted by shiv@vishnu
On 91oct (and even 93oct), the DME is rarely able to achieve the desired ignition advance set point due to knock activity. Which is why adding race gas or meth picks up the power it does. When you run it, you feel the advertised power that BMW claims. I just wish they would admit to it not being able to achieve that with US pump fuels. Same goes for the newer Porsche turbos but that's another story.
But even the timing achieved when running race gas isn't necessarily "ideal". There is still room for 2-3 deg more timing between 6000-7500rpm if the DME would allow it. This alone is worth 10-15whp. There is also some gains to be had in the 4000-5000rpm range. But the timing maps were calibrated to run on pump gas so there isn't much reason for BMW to implement these higher advance targets.
Hope that helps..
Thanks for the great, and specific, info Shiv.