The turbo on the 6.4 powerstroke is a sequential turbo. Basically it has two turbos a high pressure turbo and a low pressure turbo. The high pressure turbo is also a Variable Geometry Turbo (VGT) like the 6.0 powerstroke. The low pressure turbo feeds the high pressure turbo, by doing this it allows the turbo to spool up faster and has less turbo lag. The turbos are a great system, but have not seemed to hold up all that great, the reason for that I believe is the regen process, so you need to get rid of regen and the turbos will be good. Turbo repairs can be very expensive so you will want to take care of them.