Hi,
I am going to ask for a few more opinions before I head out to the exhaust shop to have, what I think is, the original 1 3/4 " exhaust replaced on the '72 Valiant. The current set up is getting really weak around the muffler inlet and outlet and I don't want it let go while driving. Presently the car is a stock 225 with 80 rear wheel HP. My goal, for this car, is to double this figure using the stock exhaust manifold. So taking this into consideration what size system should I install to support this?. I want it to be single outlet and quiet. I value your opinion.
Thankyou, Nico

2.25" would be all you would ever need and even 2" should really be fine. The quality of the bends and pipe joints is important. A well done 2" system will flow much better than a 2.25" system with crimped-up bends and lousy joints.

At the level you are looking at you would give up little, if any, peak horsepower with a good 2" system and muffler. It will be quieter too.

As far as mileage goes, you do not need a huge system. For best mileage the car is running at part throttle and high vacuum. You do not need huge flow capacity, just low restriction at that level.

Myself I like the Walker Super turbo Hemi's (long case) for good flow and fairly quiet performance. For max performance I like the Hooker Aero chambers, louder but they flow extremely well and don't drone or sound like crap. Stay away from the "most popular performance muffler".