Afternarket catbacks tend to be designed with performance, efficieny, and sometimes even sound projection :down:

Anyways, stock exhausts tend to be made with sound supression and warranty in mind.

They can also have different bends (my GReddy has 115 degree bends, not 90 as with stock), be made out of higher grade steel (Or aluminized steel sometimes. If they salt the roads where you live or if you're really close to the beach, avoid alminized steel like the plague. Still can't understand why GReddy uses aluminized steel over stainless... lighter maybe?), more optimum placement of the resonators.. etc.

Catbacks eliminate problems with restrictive piping, can position resonatorss to work best with the muffler, and a bunch of other stuff that make the exhaust system work better together.

Mandrel bends= Bends in tubing that do not compress the tube. Normally you have to pinch the tube and reduce the diameter of the pipe when you bend it, but Mandrel bending eliminates any "crunches" in the pipes.

Stock muffles are "baffle" designs, that are like a maze for sound waves (Like if you sawed your stock muffler in half it would have a whole bunch of weird metal plates that catch the sound). Straight through mufflers dont place anything in the flow of the air, but due to the nature of sound waves, catch them as they flow by.

I hope that made sense.... I've been typing all day