An Exhaust System is not just about noise! Another misconception about Exhaust Systems is "Bigger is Better!" With so many exhaust manufactures out there, where do you start? After reading this hopefully you'll have a better understanding and appreciation for exhaust systems.  I got started at a young age, actually picked up my first torch when i was around 9 or 10. My old man got me into it; my pops started in 1969. So when i say I have experience, believe me, "I have experience!" Everything from splitting inline 6's, lake pipes, bellflowers, flame throwing kits, electric cut outs, glass packs, custom headers, "X" and "H" pipes.  http://www.magnaflow.com/05news/magazine/htr.asp 

Lets start with the basics. An Exhaust is exactly what it says. EXHAUST- To empty by drawing out the contents. In this case we're referring to the pressure created in the cylinders. With this being stated maybe you can realize why pipe diameter is such an important part of any exhaust system. These are the recommendations according to Flowmaster Technologies.

Notice that I said "recommendations." A good example would be on some of your 30's and 40's vehicles where its necessary to run the exhaust through the supplied holes in the frame. This is where a qualified exhaust builder/installer can be of some assistance.