I called local diesel shop, and they say that the MBRP turbo back 4 inch exhaust is good for 18-23 hp and 40-48 ft pounds of torque increase. That is minus the cat and muffler. Not a huge increase in power, but a must if one is to add a superchip or edge system, to keep egt in reasonable limits. As per other posts, the downpipe and cat replacement pipe are 3.5 inches. I see that some of you have the above exhaust. Do you think that their power increase claims are correct?