When you start the car there is no difference from the stock sound. On the freeway with the windows up, it's throaty and smooth sounding, almost musical but just slightly louder than stock. It took me 2 hours to install. The system uses the same factory hardware that is on the stock units. Remus says that the hp increase is 3 to 5%. It's hard for me to tell any increase since I have only had my X5 for two weeks.