Easiest way is to replace the back box with the largest (in volume ), straight through stainless box you can get in there, preferably 2 in 2 out. 2” is fine. Y piece on the entry, the increase in volume will slow the gas allowing all the high frequencies to dissipate without being too loud.Picked up a cheap z3 but want a little better sound than stock. Any ways other than spending 1/3 of the cars cost on a eisemann exhaust? (I also don’t want the car to sound like a riced Honda civic)