Originally Posted by bruce.augenstein@comcast.
Bottom line, I'm not trying to say anything about which car is better, just that based on what I see in this string plus my own experience, Quattro is far from "horrible", and it certainly isn't particularly power hungry.
I'm won't comment on the relative losses comparing Quattro vs. BMW except that the 100% rear bias available on the BMW does help substantially. I think the point is that regardless of the design or manufacturer AWD systems produce a significant amount of loss. If you believe the particular dyno numbers in the OP the RS4 losses are 18% vs. 12% for the M3. This difference is consistent with numbers used in simulation (numbers thanks to you, with my tweaks) of 3% for axles and 1% for a diff (12+2x3+1 = 19). No matter how you slice it 6% is a lot and for cars in this power range it is ~ 25 hp. In lb, at 9 lb/hp, this is ~ 225 lb.