I have a 08 G35x and im looking to get maybe a catback exhaust. i was talking to a friend about it and his dad owns a muffler shop. he said it would be cheaper to buy the mufflers and exhaust tips and have his dads shop run the pipe straight as possible (they also to amateur racing) without the shop looking at it (yet) where can i buy performance mufflers (and tips) for my car and do you all reccomend this or not and why?

Does anyone know what size pipe i should get or will that just depend on the muffler?

i was looking at a catback exhaust made by tanabe but not sure!