I went to a mountain range the other day with my Seat Toledo from 2001. Going downhill, with 12% inclination, I downshifted to 2nd gear to avoid braking all the way. The car still picks up speed, maybe 60km/h, reaching 4500 RPM (redline is 6000).
I wasn't sure if this was harmful for the engine, and whether I should tap the brakes occasionally (not because of the speed being dangerous, but because the RPM were too high). Father-in-law says it's bad, forum says it's fine.
As for harm to the engine... there is none.
So, is it harmful for the car to reach high RPM when going downhill in a low gear (a.k.a, when braking with your engine)?