Factory exhaust is the same size regardless of 318 or 360
To add insult to injury - the 3.9 V6 also uses the same exhaust as the 5.2's and the 5.9's, at least as far as aftermarket replacements goes. 2" into the cat/conv, 2 1/2" intermediate pipes, muffler, and tailpipe.
Getting ready to do an exhaust on my 96 Ram 1500 - 5.9 auto. Going to try not to put a cat on it. Here's my plan. While on the Rams the cat is part of the y-pipe, on the Dakota's and Durango's the cat is separate. Since the shorty headers on my truck also fit a Dakota, and both can be equipped with A518 based automatics, the Dakota y-pipe should bolt up to my shortys. While a Walker replacement y-pipe has "wrinkle" bends, the Eastern Exhaust P/N EE93103 has 16 ga 2" mandrel bent downpipes into a 2 1/2" outlet with an O2 ****. I have 3 bolt flanges to weld there for easy y-pipe removal. I have a 2 1/2"-3" adapter to run a 3" intermediate pipe (I have a downstream O2 **** to weld in, and a couple bends on hand if needed for crossmember clearance) into a stainless steel 3" Jones 24" Max-Flo muffler. For the tailpipe - gas Rams use a 2 1/2" pipe - V10's and diesels run 3". I ordered a (diesel) Walker 54672 pipe, which is 16ga, mandrel bent, has factory hangers, and is 3" pipe over the axle and 3 1/2" for the last bends. Both the y-pipe and the tail-pipes were reasonably priced - about $60 each to the door. All pipes are aluminized, and I will coat all welds with heat paint. If I get a cat code, I will have plenty of room for a universal cat, and if the straight thru Jones "Magnaflow" knock-off muffler drones or is too loud, I will put a resonator in front of the muffler. It should flow plenty for a stock or lightly modded 5.9. I'll post results when finished.
