I did a supercharger sound for xrg once. I saved the transmission sound (press shift +A in a race, you will see "Gear Whine". Press the "S" that is besides it. Name it whatever you want and click Okay) and then I loaded this transmission sound as the intake sound (You will see "Air Intake". Click the "L" besides it, and click on the sound you just saved, and click Okay). Adjust the volume of the intake to your preferences, around 1.1 isn't bad. I hope I helped ! (I can't do it for XRT, I'm only a demo racer)