I see a lot of guys here talking about bolt-on HPs and torque... I recently did some reading and found out the truth differs from what a lot of people think. Ok, ok...I am no expert in technical field, but those guys who publish articles and books cannot be too far from being right.
Here's a general idea: when you are thinking about changing your header, intake, muffler, cat-back, cat, or even any exhuast parts from stock, it doesn't necessarily guarantee that you will gain any horse power by replacing an aftermarket part for stock/OEM. Why? Hard to say, but a lot of factors come into the scene... like the flange, the diameter of the pipe, the design of float... etc. It all depends on how fine the parts combination work together as a whole.
For example, say you have stock header, cat, b-pipe, and the only thing you wanna get is a decent muffler that attracts attention. So you went ahead and bought a muffler with a 115mm tip. Now that the tip is so big you have to gas so hard to get to the speed you used to get up to, and you look at your gauge...hm... 5000 rpm...and you smile because you are happy with the power you've gained. NAH... the fact that the tip is too big and gives you no force feedback is like u trying to blow a balloon about the same size as yourself... tiring isn't it. You've got to give it all out before you can feel the difference.
Another good example is to refer to the June 2002 issue of the Sport Compact Car magazine. Road tests for RSX with different brands of exhuast pipes. I think the charts will help you analyze that bolt-on powers really vary in numerous ways.
Basically you don't gain much from bolt-on's, sometimes even a loss of power. It also vary from high-end power and low-end or even mid-range. Putting on a big muffler, again, may give more torque in high rpm but not necessarily more HP. The power you've lost from lower rpm's probably can't be made up by hitting high rpm's all the time. Sometimes the trick comes in re-adjusting torque/HP in different gear/rpm, you see the numbers move around, the there's probably hardly any HP's gained, or none.
Now I kind of regret getting an SS header, intake, and the muffler. I mean, I think a generic AEM style intake does just about the same as the real AEM CAI. As far as design, filter, weight...can't tell a difference other than what's printed on the pipe. The only reason is that they are brand names and they pay to start the product design in market, research, and pass the carb test...etc. I was glad that I got mine for $35 shipped instead of like $180. Maybe the best bang on the buck so far on my car... but I don't think I am juicing anything extra outta my engine...say...the most 5 HP? I am prolly over estimating

but oh well... it's getting late 'n I dont' know what I am talking about anymore... but I hope you guys get the idea... . Maybe somebody here can explain better than me. I have an auto maybe that makes less of everything I am talking about here... Oh well, it's an Accord, a stylish family car w/ nice ride. Data's how it got its name recognized, not meant for race anyway. I would go w/ civic/tag for race. Anyhow, correct me/re-phrase for me if you would... sorry for the long post...'n I just hope the newbies don't make the same mistake as I did back then, thinking I would gain a 50 HP easy by putting on these parts. Good luck everyone!!