They have a bunch of classes, so how ever your modified or not modified defines what class your in.
Well I got an iPOD interface installed the other day. To install it they had to update the software on the car. This had quite the effect on the exhaust valve. Before it ran closed until about 4000rpm, now it's open unless it thinks your going to want power. Then it shuts for a bit to make back pressure until the turbos are spooled at which point it opens again. Anyway, what this does is it creates a much louder exhaust note most of the time and I love it! Course, I can't open my trunk now.......