RC:
reginos:
Porsche use turbines with variable pitch vanes in an attempt to have the best of both, but only in the 911 and the Diesel.
http://www.porsche.com/microsite/technology/default.aspx?pool=uk&ShowSingleTechterm=PTVTG&Category=&Model=&Searc...
Yes, the 997 Turbo was actually the first petrol run car worldwide with VTG turbo chargers but I also remember that tuners hated them. 
I actually wonder why Porsche doesn't use VTG chargers in the Panamera Turbo and Turbo S, so maybe this technology isn't really delivering the benefit everybody has hoped for. I am curious about the next generation Turbo, the 991 Turbo...VTG chargers or not?! 
The problem with the VTGs apart from their complexity (which affects their reliability/longevity) is the heat that they generate. The hot side produces its maximum boost from ~2500rpm to 6500rpm so 4000rpm of maximum boost = loads of heat. For the GT2RS they developed the newer Spanish made intercoolers which enabled 620hp but that 620hp soon reduces in ambient temperatures above 20C.
The only way to produce more power with VTGs is to use exotic aerospace intercoolers which are way too expensive for mass production, they could increase the size of the VTG hot side (to cool it down - spin slower) but then the boost comes in later and the max boost will be at 3XXXrpm not 2XXX so defeating the object somewhat.
The real benefit of VTGs is their response to the throttle, the way a VTG spins up is incredible and makes a K24 type turbo seem from a different era. In the age of idiocracy where many people only look at 0-200kph or simple in gear acceleration times on constant throttle the VTG engines do not appear to have a significant advantage but a side by side reveals all.
With the Panamera/Cayenne being 4.8 litre my guess it simply does not need the complexity of VTG to hit the power/torque targets and I also expect that the packaging (ie turbos not hanging low in the air stream like a 911) probably creates way too much heat which is hard to control
--
3.9 GT2 2011 make over
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