Very interested in how this goes. I know the ATC700 is thought to be much stronger- and my 35d would need it. I've looked into swapping to the 3.15 of the 50i, to bring my final drive down from 3.64, to increase fuel efficiency, run at lower RPM, and generally run quieter/ more efficiently on the long haul. If you were to swap the 8 speed, I'd be very curious to see a what it takes!

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'10 X5 35d- 279k mi, deleted and DUDMD stage 2, daily and tow vehicle
'01 330i- Drift car: SLR Super angle, BC coils- keep it simple
'07 335i 6mt - Sold @ 171k miles- stage 2 daily commuter for 2 years