In the link below, there’s good pictures of the 4.4L and fantastic commentary from readers regarding the engineering decisions. The article mentions that variable geometry is very expensive, and BMW is using very simple turbochargers, benefitting from short exhaust and intake length (thanks to air-water intercoolers) that minimize compressible volume and thus turbo lag. There’s no explanation for twin-versus-single turbos, though.
http://www.autoblog.com/2008/01/21/detroit-2008-bmws-twin-turbo-4-4l-v8/

I wonder if they’ll move to water intercooling on the N55, and how the performance segmentation will work (eg, NA -> T -> TT).

i can’t find a pic of the exhaust manifold for the N55, but i’m sure it’s similar to the one from the X5/6 M engine above (seriously, anyone know the engine code for this thing?). obviously, the N55′s would have 3 cylinders feed each of the inlets, instead of 2 on the X5/6 M engine.

not exactly ‘trick’ per se – just a necessary design to make the twin scroll turbos work.

@tk: Honestly, I thought I’d read of at least one BMW that used it, so assumed they were moving to that.

What’s this trick exhaust manifold you speak of?

Another possibility is that each scroll is shaped so as to provide different benefits.

but i do know that they don’t have variable vane geometry like the 997 porsche turbo’s turbos. low-rpm boost is accomplished through the trick exhaust manifold and twin-scroll turbos.

I understood that the benefit of these turbos was not so much that they’re twin-scroll, but that they have variable geometry (do they?) and can force faster spinup with less exhaust pressure.