When I was in High School, I cracked the snout on a 904 behind a 318 duster. I was dogging it, liked to downshift manually when coming to a stop light so the engine would gurgle back pressure like a stick. There were a few times that I barked the tires by downshifting at too high a speed (and kissed the steering wheel)... and then came the cracked snout! Since then I've run 904's hard without any problems (and letting them downshift themselves).