Originally Posted by Nigelo
Main issue is the final drive ratio difference. The M3 diff comes in two OEM final drive ratios - Manual is 3.84, DCT is 3.15. Compare this to the 335d fd of 2.81. This means that only the DCT one is remotely feasible.
You'd need to work out what rpms would equate to which speeds, particularly at motorway cruise speeds as higher rpms could be awful to live with. I think divide the rpm at your cruising speed by 2.81 and times by 3.15 to find what rpm the new diff would give you for that speed.
Here's the thread regarding M3 diff into a 335i - should be exactly the same procedure.
Thanks for the info
Will try and do some reading, its certainly not a simple drop in fit!