Currently have the 3.9L and wanting dual exhaust that will interchange for when I put a 318 in in the future.

Question is will 2.25" dual be enough or will I need 2.5" dual?

Mods will be the normal HCI, Throttle body, Headers...Was looking at Hearthrob's Dual Deep Tone 2.25".

 

Plan on running cats? Short or long tube headers?

 

Yes on cat. short tube headers for now.

 

I'd go 2" for straight flow muffler 2.25 for chambered. X pipe. 2.5" will give you a little deeper sound but lose some low end tq

 

even with modified 318 in the future? ok I shall go with the 2.25" Hearthrob Deep Tone Dual

 

Unless you plan on going all out, I wouldn't go more than 2-2.25 depending on muffler setup. What did you mean by HCI btw? Heads Cam Intake? If that's what you meant change recommendation to 2.25-2.5 depending on muffler.