I was looking through some literature for the tier 3 J.D. engines. They started using a variable vane turbo in conjuction with an E.G.R. My understanding is the variable turbo tailors the amount of exhaust gas that is routed back to the engine. I guess my question is since the Duramax now has a E.G.R. is that the purpose of having a V.V.T. instead of for performance issues?
Also, J.D. cools their exhaust gases before sending it back through.

To the best of my knowledge the main reason for the VVT is for Emissions. It sure isn't for performance from what I can tell. http://www.dieselplace.com/forum/smileys/Cry.gif

I don't think that it's entirely tuned for emissions - but there's no doubt that emissions take precedence over performance! The big advantage of a VVT is controlling the turbo boost. With a fixed turbo, you are coupled to a boost performance directly linked to exhaust flow with the one additional control function of wastegating. You can tailor a much wider range of finer boost control with computer control over the compressor as well as wastegating. With the exception of the complexity and potential electro-mechanical issues with the VV system, once the aftermarket gets up to speed with the VVT programing it has the potential to be a really exceptional system!