You're overanalyzing my example.

My point was simply to illustrate that you cannot tell an engine's torque output, from its horsepower rating.

Anyway, we weren't talking about axle shafts. The weak point in most hypoid gear axles is the intersection between pinion and ring gear teeth.

As the torque applied increases, the force trying to push the gear teeth apart from each other increases. The gear teeth then become loaded closer to the tooth's tip which increases the moment on the tooth's attachment to the gear. Result is the tooth breaks off. The reason the Ford 9" can take so much torque is its pinion support. There is another smaller bearing surface behind the gear teeth and thus the pinion is supported on both ends, lending stability to the pinion. It's harder for the pinion to move away from the ring gear.

I saw a picture recently that showed the pinion sheared off right behind the inner bearing. While this looks at first glance to be the engine twisting the pinion shaft too hard. My theory is that it's a fatigue crack that was stated by the pinion moving away from the ring gear through whole revolutions of the pinion. The metal fatigued and eventually snapped with a brittle fracture. This is the same idea used to fatigue test metals.

R.