Originally Posted by Richbot
Mass is a linear relationship with kinetic energy. Velocity is a squared relationship. The E90 doesn't need more brake because it's fatter, it primarily needs more brake because it's 10-20mph faster at the end of most straights than any M3 before it. So no, the M3's brakes have not increased in size sufficient to compensate for the increase in performance, let alone weight.
From an earlier post of mine:
E46 M3 at 3500lbs with driver braking from 135 mph for a 60 mph corner needs to dissipate approximately 550kcal of kinetic energy. E92 M3 braking from 145mph for the same corner needs to dissipate approximately 705kcal of kinetic energy. That's a ~7% increase in weight and speed and ~45% increase in heat load on the brakes. The E92 brakes are bigger but they aren't 45% bigger, and at bigger tracks the speed gap is even more significant. Faster cars are harder on equipment, k=mv^2. BMW had a choice to provide everything from a setup for a 150mph car with a sensitive driver to manage on track to one that can throw down qualifying laps for a whole race distance on a 100 degree day running 6" from another car's bumper. One is what we have from the factory, the other would probably need to be water cooled to fit under 18" wheels.
I fully agree on the impact of speed.
However I am not sure I follow your math. When I do the calculation using 3500lb and 135mph for the the E46 and 3750 and 145mph for the E92 with both slowing to 60mph I come up with 3.0Mj and 2.3 Mj repectively of dissipated energy. Which is a 28% difference, not 45%. Further, from your own calculation 705kCal/550kCal is also 28%
Further, my E46 weighed closer to 3600lb (bare bone 6MT) with me and half fuel and my E92 weighs 3750lb with me and half fuel. With this I only come up with a 24% difference. When I compare the swept area of the front rotors, they are about 20% bigger on the E92. This is not that far off...