If you want more noise, get an intake. I've noticed no difference in power from ones with stock to ones with any kind of intake, whether it's underhood or down in the fender.
Just way too noisy.
I think many people think because it's so much noisier, it's got to make so much more power. I can only think of a few handful of cars/trucks that have so much intake restriction in the filtering area that can make a big difference in power output with a changed setup.
The manifold on the '03 is already a steel tube "header" anyway. Don't get me wrong, it's probably not as nice as a properly done true header, but will you notice anything? I don't really know. Does NY allow exhaust changes ahead of the cat for their inspections?
My Integra GSR did pull a little better with the DC header I put on, and it was actually quieter as the A pipe had the infamous broken dual wall pipe. I ran it with the cover off the airbox (it used a cone style filter in the box) and it was so ungodly loud, people would stare as it passed under wot.
I wish RSR would still make an exhaust for these cars, I had one on that, it was a little deeper and audible at light throttle and idle, no louder cruising, but full on it just screamed. No more though.
