I've been looking for a cat back for quite a while and finally decided to go custom. I've researched and found lots of different opinions on the sound of different systems. So I decided that I need to learn some basics about exhaust systems so that I can make a decision for myself. Instead of asking how a system sounds I want to know why it sounds and flows the way it does. Where can I go to learn this information? The manufacturers web sites aren't very helpful, and dealers only tout the brands they sell. I want to know what is in all those different cans, cats, and resonators. How much differnce does the shape of the can make? The type of material from which they are made? How much difference does a straight through design make? Basically I need to find a beginners primer on exhaust systems. Can anyone help me out?