[Clovef]

The e diff is the computer using the rear breaks to try to even out the resistance to the tire lacking it. Power goes to the place of least resistance with an open dif instead of a dif locking so power is 50/50 to the rear tires. If you are putting the power down out of a corner and your inside wheel only has 20% of the traction you will overcome that 20% easily and spin the inside tire. The e dif applies the break to the rear wheel so your power has to overwhelm both the 20% traction that inner tire as well as the breaking force being applied to that one corner which they hope to get the remaining percentage to 50 so that the inside rear wheel does not end up spinning with the least resistance for the power to go to, which is not very useful. In the above example they want to be able to apply ~30% traction worth of breaking power so that 50% power can go to the outside tire which has the weight on it and traction to allow the power to be useful.

The n55 engine is newer than the n54 and as such does not have as much aftermarket support yet. The n54 seems to have a little more power with FBOthan the n55 which can get to a little under 400 at the wheels but are happier around 370whp but with over 450 lb ft of torque at the wheels and over 400lb for most of the rev range. N54 has a good number of members over 400. The twin turbos have a little more flow for up top but take a bit longer to build the power than the twin scroll single turbo on the n55. To me the difference in transmissions is grater than the difference in engines. The 7speed DCT is so very well matched to the n55. The gearing in the 7 speed is actully shorter than in the 6 speed so you are always where you need to be power wise. I love it in all the traffic I have to drive in. If you see a gap just a flick of the fingers for a downshift or two, point, and shoot. 40-80 mph is just so easy and quick that you can just pass.

I think you will love the car no matter the engine and transmission. They are all so good. BMW made some great decisions when they decided to make the 135i. Enjoy.