I am new to the diesel world and hoping someone can answer my question. please allow me some latitude if i sound or come off a bit ignorant to this. i purchased a used 2015 6.6L duramax silverado Z71. it had been already converted and not knowing anything about diesels or conversions I assumed I would be able to find somebody locally to help. I have not yet found that person. The conversion to my truck is, from what I have been told is extremely professional in appearance and the materials used. no emissions, no muffler and no DEF tank. The only indication of a conversion in the cab is a small black 5 position knob to the left of the steering column. I was told by a tech that it is a good quality knob but he had no idea what it does. another believes it controls only the O2 sensors. Me not knowing anything about diesel, was left to simply feeling out what it did to the performance of the truck. In all 5 positions the truck is in my opinion super fast. the smoke it puts out is minimal at best. I was also told that that my not be a good thing. I am a bit to nervous to push the truck beyond normal driving because i was told that i could blow the motor up. Gas mileage tests are what I have done. From a clockwise perspective, position 1,2 and 3 get around 18 MPG combined. Position 4 seems to get better 21 MPG on the highway and 15 MPG city. And position 4 eats fuel when in idol. I will start my 1 week running of position 5 tomorrow. Pleas your thoughts and knowledge will be greatly appreciated.