Originally Posted by txgrown
I have been searching for a while now and have come up empty. Does anyone know what the 135i N55 trans càn handle in terms of pôwer? Thañks.
This is probably the wrong question. As others have noted, transmissions are rated in torque, not power, as torque is the limiting factor (assuming proper cooling). Regardless of the rating on the 6MT, we know the factory delivered it in cars with up to ~370 ft/lbs (500 Nm) wheel torque in the 1M. We also know based on experience that this is conservative, and the transmission will take significantly more than 450 wtq (with reduced durability).
Your question asked about power, however, and power at a given torque is a function of RPM, and hence how the motor is built/ tuned. If the motor is built and tuned for high rpm power but limited peak torque, then 450 ft/lbs at 7000 rpm would give you 600 whp and be perfectly safe for the transmission. Of course most N54/ N55 motors are tuned exactly opposite this, with more torque than HP, but this illustrates how high power can be made with limited peak torque.
The logical conclusion of this philosophy was shown in F1 V8s before the 18,000 rpm limit, where engines were making well north of 750 hp at almost 20k rpm, but peak torque was only a little more than 200 ft/lbs, allowing tiny and light weight 7 speed gearboxes that weigh well under 100 lbs.