Another thought, guys setting up Cummins go to the larger size turbos because when fuel rate is set up way over stock along with other modifications, the exhaust volume spins the smaller turbos so fast the compressor side can throw blades through the housing. There are photos of this happening in the Dodge Turbo Diesel Registry magazine. Our whistlers are running in the 25 pound boost range with fuel rate maxed out. I think the larger turbos only get around 15 pounds. Turbos definitely develop exhaust back pressure. That is why when building up my Cummins for my one ton stiffer exhaust valve springs were installed. It takes pressure against the exhaust side turbo wheel to spin the pressure side of the turbo, especially at high boost levels. When building high boost rates, the exhaust valves can be blown open by the pressure in the exhaust manifold; that is why the heavier springs to keep them closed. Improperly tightened pyro sensors in the exhaust manifold can be blown out of the holder. I worked on a Dodge that did it. No power or pressure is free. If we could find free power, we would have the perpetual motion machine!