A car-modifier specialist told me that when you remove the electronic protection of the M5 E39 engine (and speed limiter), the engine has 450-480 HP. This seems obvious when you see the low yield of the serial engine (80 HP/litre, M3 100 HP/litre)
BMW locked it at 400 HP, being afraid to not have enough reliability with this car.
M5 E39 owners, why not 80 HP more ??