Mainly the fact that it's never been done before.(edit: I found out it has been done and boy was I wrong) I know I know.... "The V8's are more practical for the low end torque" but they chew up transmissions, eat timing chains, and you can't get them in manuals. I do think it would be fun to have a manual 4.4 X5 even if it was only rear wheel drive.

I recently got a bmw scanning software for my laptop and have been playing around with an R56 mini trying to get the FRM module programmed to vin, and I think it would be interesting seeing the amount of work to get an engine (The S54 since it's the same block as the M54) (edit: Don't listen to the dummy that wrote that last sentence about the S54) that bolts up to the transmission and work on programing OEM computers to vin.

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Running:
2001 E53 3.0i Black 5MT Style 87, Eibach Swaybars, Headers, Turner Monoball Bushings, Oil pump nut fix, E60 short shifter + ZHP shift knob, Sport Swap 204K (G-power supercharger coming soon)
2011 E70 35d Blue Style 214 3" exhaust, Dudmd stage 2, 2" HR lowering springs
2012 E70 35d Black Style 215

Sold:
2002 e39 540i 6MT Black sport, M5 LSD, M60 Intake manifold
2003 e39 525i Auto Grey Sport