BMW has been testing both variants, the Twin-Turbo V8 and the V10 for the new and upcoming BMW F10 M5. It seems that the boys up in Munich have decided to downgrade the M5 power-plant down to a V8 (from a V10), but with the addition of Twin-Turbos.
The V8 selected would be the same V8 powering the current X6-M and the X5-M (4.4-liter V-8 Twin-Turbo).
We’ve got sources reporting that the next generation M3 will also be powered by a pair of Turbos, this time instead of the V8, BMW has also decided to downgrade it to a Straight-6.
Wow, turbo M cars eh? What are the purist going to say?
Sources: Edmunds
Loves speed, and gadgets. Not a geek. 
The BMW above is ugly….. prototypes must be beautiful too… :p
Jessen: Yeah, well its covered by camou, and fat suit too! Just so that the real thing will look more like a surprise.