What I haven't read here is a reason for wanting a 'deleted' exhaust. To add what HayDude mentioned regarding engineering, it is hard to believe that there is no other way to have better air than with DEF, or to get better mpg. I have known some who have deleted and their number one reason is because of mpg. Nothing about power, sound or air quality. Whether true or not, they reported upwards of 20% better fuel economy = same as the pre-DEF engines. Therefore a customer has to buy more fuel, plus they have to buy DEF to use their vehicle. One owner I know bought a brand new 5500 pickup and was only getting 10 mpg. Who wants to put up with that? With delete, the new mpg was 14. And BTW, the second reason is that if you run out of DEF, the vehicle goes into limp mode, so if you are towing something you are essentially stranded along the side of the road. I'm convinced that the way things are (w/ DEF) is cheaper than better engineering.