I would like to do the exhaust on my 56 plymouth, right now it has a kind of patched together dual exhaust exiting out of the side with GM turbo mufflers on it that the Previous Owner installed. I want to put full dual exhaust out of the back with Dynomax Ultra Flow mufflers. My question is what diameter should i run? right now i measured the pipe coming out of the side it is 2-1/4. It is a stock 361 with a 2 barrel and exhaust manifolds but i plan on changing it over to a 4bbl soon. Should i go with 2-1/4 or 2-1/2?

It seems like most guys recomend 2 1/2 but not me. Go with the 2 1/4 if you are having someone bend it up for you. I worked at a muffler shop from 95 to 01 and helped with a lot of dual exhaust jobs. The pipe bender that most shops have don't bend 2 1/2 very well. The machine tends to slightly collapse the pipe at the bends hurting exhaust flow. It does that with any sized pipe but 2 1/2 is a lot worse. You can get 2 1/4 to fit much nicer...keep it closer to the floor so you don't see it hanging down. If you are wanting a good sounding, good performing exhaust system and arn't looking for maximum horsepower, go with the 2 1/4. If you are going for max power, have someone custom build you a "mandrel" bent system. I have always been told that a 2" mandrel bent system flows better than a 2 1/2" system bent with a conventional pipe bender.

2 1/4 will also be a little bit quieter than 2 1/2, if that's a concern. I don't think you'd see much more power out of a stock 361 with the bigger pipe.

2 1/4 is MORE than adequate for a 361.

You couldn't modify it enough, or spin it fast enough that the 2 1/4 wouldn't work.

Is there a hp rating or a chart that we could reference for the 2-1/4", 2-1/2", and 3" pipes,or is there just an understanding that if you have so much hp that you should run a certain size pipe?

I too am considering a larger exhaust pipe once I install my TTi's this Winter but I would like to understand some reason before I consider swapping out my 2-1/4" system.
Thanks and Bump

I too agree on the 2.25" exhaust system. If you had a cam and headers, and you were aiming for performance, then 2.50" would do some justice. A 2.25" system will also keep the noise down. Another way to figure out what exhaust size to use, is pull the existing exhaust from the manifolds and measure the inner diameter of the collector flange.

Hope it helps,
Bill

One main thing that you have to realize is that you can't physically start out with a large exhaust pipe anyway. You may only be able to start with 2 1/4 off of stock manifolds. From that point, what does it matter if you increase the size of the rest of the system.

Just make sure that the mufflers are the same size as the rest of the system all the way through. Don't kneck it down. That goes for the tailpipes too. Some people say that the muffler is the main restriction in an exhaust system and put smaller exhaust pipes on. THat's stupid. Make the entire sysytem the same size all the way to the back.

When larger systems come into play, is when you have big horsepower and utilize headers. Header collectors can be 2 1/2" , 3" or larger. If you have a header with a 2 1/2" collector, there is no reason to take it down to 2 1/4. In that situation, I would install a mandrel bent 2 1/2" system all the way back.