You must know that, all exhaust got bigger piping. So it mean louder too than oem. Some of the good catback for the B15 SpecV that are know sound good, Apexi WS2 , Stromung, Nismo.
Nissan Sentra SeR SpecV 2002 with QR25DE: Apexi WS2-OBX header-AEM CAI-TA 17-No DLR-BS chainless-Faktion SS-RevolutionAutoworks TD-ES MMI-Technafit SS brakes & clutch lines-Unorthodox crank pulley-Tanabe NF210-Nismo RSB Nissan Silvia K's 90 with CA18DET:Exedy Stage 2 clutch-GAB front strut bar-Tom's Motorsport rear strut bar-Tein HA-Megan Racing tension rod-Nismo chrome knob shifter.
I've had a Nismo on my 03 Spec V for 3 or 4 years. Adds a nice tone, you can hear it coming, but it's not annoyingly loud at all. With that and a Nismo CAI the car sounds good, runs better and still averages 28 mpg in mixed driving.
