[CanAutM3]

If you search, there are quite a few threads on the topic.

I doubt BMW "underrates" their engines. My guess is that the high numbers we see on dynos have to do with different things.

The Sport Auto test on a rather accurate Maha dyno showed 24 more ps than the official rating (465ps vs 431ps).

I think that some of it has to with the standards/methodology for official power ratings. The engine might be behaving differently while in steady state and while accelerating. For example, when the engine runs in steady state for the official rating, all the accessories need to be running, but when doing a WOT acceleration run on a dyno, the ECU dissengages the accessories. That can be worth a few ponies. I think that there is also more to it (engine management, turbo characteristics, etc...), I just have not put my finger on it yet.

Further, BMW has done efforts to reduce rotating mass (crank shaft, drive shaft, wheels), when testing a car on a chassis dyno while accelerating, inertia has an impact, so the so-called "drivetrain losses" can be less than we are used to seeing.