The loop in the back is called a "J pipe"….it is there to counter the effects of drone. Exhaust waves go into it, its blocked off at the other end, those waves bounce back towards exhaust pipe and create a harmonic calming, it takes the place of a big bulky resonator, that does the same thing but it also quiets the exhaust, the J pipe lets the sound go past however. I'm told without this the drone is terrible in the auto's.
dave