Speedvanish

Having little background in the mechanics of my c5, I was curious about the benefits a set of after market headers, x pipe and cats might have. The person I asked, asked me not to identify him to the members of any forum. However, this individual knows from where he speaks, and has a very extensive knowledge base.
He wrote: The combination of a cam, better fuel didtribution and higher heat was the reason for headers. Headers let the engine breath and pick up volumetric effiency... especially on earlier V-8's. He also said that he would be willing to bet that a dynameter test on an LS1, correctly tuned, wouldn't improve by only adding the headers, x pipe and cats. His take was, "The factory has things pretty well figured out." He went on to say, "It actually might sound a bit tinnier, but actual "GO", I doubt it.

phoenixfoundmac

The factory designers have to take into consideration piece cost as well as NVH(noise). The factory headers are effective, but cost efficient and the entire exhaust system is restrained enough to please the average buyer. Not everybody wants their exhaust to scream. If you add headers/catbacks/x-pipe to a stock motor and get a tune(to compensate for greatly increased flow) you stand to increase hp by at least 25-30 and the sound will be way louder and way more aggresive. The type of sound will depend on which system you buy of course.

Speedvanish

The only way you can tell if there is a HP gain, is to first do a tune up, followed by a dyno . Then, remove the original exhaust manifold and exhaust system and install the new header, x pipe and cat. Then do another tune up. Then do another dyno to see if there was a HP gain. This is a lot of work that 99.5% of the people aren't going to do. As much as you'd like to see a 25 to 30 hp gain, I'd bet you that it's just not going to happen.