i have heard many opinions on the plusses and negatives of a 4 inch system with an elbow,or as a three to four step up,or just leave the three inch downpipe alone and straight pipe it to muffler..im sure that the intended power level dictates which,and ill be honest in the fact that im looking for 300 to 325rwhp,lots of low speed response,and occasional passes down the dragstrip with my ats locked early..right now i have a banks 62402 fuel plate,3000gsk,k&n in a stock airbox,cat,and a banks 4inch to 3.5 muffler.no timing,but i am going to do it soon..boost at 28 or so now,and i have a part coming from heath to make wastegate more easily adjustable!!!!lots of highway miles will be seen,and i really like good response and fuel milage,and am saving for 215 injectors and better gauges,as i feel my banks are probably junk after 200,000.any help or ideas will be greatly appreciated,and thanks to all that have responded to my posts!!!!!!

 

I'd go straight 4" all the way from the turbo back.

 

I WENT 4 INCHES ALSO AND LOVE IT. THE TRUCK EVEN STARTS FASTER NOW.