The ultimate hp/torque capacity of the transmission as specified by the transmission manufacturer likely has to do with the strength of the internal components, things like shafts and bearings.

When discussing things like trailer towing and duty cycle, we are getting into the system limitations, not just the limitations of the transmission itself. Just by way of examples, the system includes a cooling circuit, and software that could manage torque limits by softening shifts. If another manufacturer used the same transmission and didn't manage the engine controls to reduce power during shift events, the transmission would see very different load profiles. Conversely, if the vehicle manufacturer didn't need 600 nm of capacity in a specific application, they wouldn't put the cooling system and other components into the truck to accomodate that limit, even though the transmission itself may be capable of that torque limit.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue