what gain do i get for doing a cat back system opposed to jsut replacing the factory muffler with a flow or maggie?

 

This is what I have done. Replace the converter with a high flow converter with a 3inch outlet and a flowmaster with a 3'' inlet and outlet. finish it off by running the rest of the exhaust with 3'' to the back. This setup is not so much as loud but a deep rumble. when you step on it, it sounds sweet!!!! Maybe a 10 to 15 hp gain.

 

With a cat back or custom system, you get the larger exhaust tubing with smoother bends. The muffler is probably the single most restrictive thing in the stock exhaust, but I'm sure there are losses from the rest of the tubing also. When I took my old stuff off the stock stuff looked like a sick snake, contorted, bumped and mangled. The new stuff had mandrel bends and less of them.

To some extent, you'll get what you pay for. I've seen muffler shops do both great work and hack up jobs. With a cat back you'll have a known quality control element, and if you put it on yourself you'll save labor, thus making the purchase price the total price.