If you thinking of going to a "true" dual exhaust DO NOT DO IT!!! Some people have that on the V6 and it cause's way to may problems. First he Check Engine light comes on allot. Second they loses ALLOT of low end torque. I know, I have a V6 and I looked into doing that. I went with a cat back duel and lost a bit of low end torque. But I gained MORE Hp at higher RPM's. Some people put a bigger single cat back on the V6. They don't loses as much low end torque BUT they don't gain as much HP in higher RPM's as I do. It's a trade off. If you go FI then a dual might be ok. I'm not sure.
As far as the Cavalier or I4 Accord, you could put a Dual on it, BUT you need small piping. Instead of getting like 2.0 or 2.5 inch you would have to get 1.75 or something like that. Not 100% sure on the size piping. You should be ok that way. I have seen Neon's with a Duel exhaust. It is just the Y pipe split. Just like a stock V6 Accord's, and almost ALL stock V6 car's. Even most V8's are that way, they just have a little bigger piping. That way they keep the noise down but still have a nice sound. Just look under a Mustang and you will see. Hope that helps.
