Its a resonance chamber I think. Its like a sub box or the ART decat pipes. So it cuts rasps and high frequencies and leaves you with a deep and meaty sound.
Mitz and Neil you missed post #11
He designed the 350z, it looks like it does because of him.
Ajay Panchal a quick link http://auto.howstuffworks.com/nissan-350z-concept-cars10.htm