Someone should make a stickey of this stuff already...
Back pressure is evil, plain and simple. There is no need for a little, a lot or any...commen misconception surrounding the backpressure theory...its like the bermuda triangle.
reason most people see a loss in performance or "low end" when swapping an exhaust system over is they increase the pipe diameter size to much. generally one size larger than stock is acceptible in stock applications to mild performance.
when one increases the exhaust size to much they slow down the exhaust gas flow which results in a low power feeling at low rpm. they slow down the exhaust gas velocity at lower rpm, which hurts exhaust scavanging and also disrubts the flow. this IS aided at higher rpm because the amount of flow is greater to meet the larger pipe diameter.
u can run nothing but straight pipe and the engine will perform excellent if the pipe is proper diameter. Suffice to say u can not totally eliminate back pressure for a production engine, so most of this theory is jaded at best. what we can do, is design an exhaust system to the best of ones ability to take advantage of what we have. this includes running the proper amount of cats, a high flow muffler and proper pipe diameter and lenght. if u wont to take this one step further u can add a set of headers, to aid in the scavange of the exhaust becuase they are tuned for maxinium scavanging and use of the exhaust pulse (when the cylinder fires it creates a pulse).
all these things (headers, proper pipe diameter and lenght) will help smoothen out the exhaust pulses, scavange the exhaust and eliminate the backpressure completely. on a street driven vehicle we can not use this set up so we need to design our systems to co-encide with the catalyist and muffler.
also, why so most cat back exhaust systems increase hp/tq? becuase they take advantage of a high flow muffler, eliminating the back pressure a little.
here are some links to aid in my post: