I doubt this. On some sites they claim that with 4-2-1 headers, accompanied by BOTH a Cold Air Intake Induction system, and a catback exhaust gains can be around 45 bhp (or as mentioned, 37 hp at the wheels) I know that CAI/headers may not do a lot, but maybe with the addition of a catback would that really increase the gains?

My estimation is more like eh... 15-20 hp perhaps.

your estimations are more accurate.

although, since these are usually tuners first mods, it FEELS like you're getting more power out of your car. Hell, when I took out the stock air box for the CAI it felt and SOUNDED like I had 20 more horses, but in fact is was more like 5, lol.

-P

ya probably around 15 whp. ive heard 5 for cai, 5 for header and 10 for a catback, i doubt its that high so i just say 15 total

I wouldn't dare to throw a number in, because it depends on the bolt on itself (design and quality), your own engine base power, and your other mods.
However, I think the most gains in a bold stock RD are from two mods:
1) removing the OEM intake and replace it for whatever you want, much better if it's a CAI, but the OEM is so restrictive you could even gain something with a short ram intake.
2) 1.8 intake Cam

With those two, you would have invested like $250 (if you don't go for the most expensive CAI) and you will have very nice gains, but nowhere near 20HP.

After that, every other bolt on will perform better, catback and headers.

Depends on the car... on my Corvette, doing my exhaust system (custom headers, 4inch side exhaust) netted me probably around 35whp. If they are talking I-4s, then they are talking out there butt.

Wow, that's still pretty impressive. 15whp... with just my CAI I get 3whp and you can really feel the power, so that's like fivefold the gain. The car must pull a lot harder with just that setup.

there is no real set number. hell too b16's could both get i/h/e and one might walk the other.

bigger engines typically respond better to just i/h/e like hamhead said about his corvette.

you're right to think around 15-20

only real way to find out would be to dyno your car after every modification to find out how much each one gave you.

I agree with you. But most certainly I'm going to feel the difference in the way the car DRIVES, which is most important.

So CAI might've only given me 3whp...and I can really feel the difference. That's THREE ponies.

At this point I'm more interested in torque gains than flat out horsepower. I know the headers will really help.

Bah, low end sux0rs with auto.

i have a 4-1 header...definately notice it top of second and up.

the 4-2-1 just didn't appeal to me.

The main thing with bolt-ons like an intake, etc. is that they increase horsepower all the way through the powerband, not just at the top. Tacking on 1PSI on my GSX only really adds after I start making boost @ 4000RPM. An intake on a Tib starts racking up power after 2000RPM, and adds a lot (8.6whp @ 3400RPM I believe) to the lowend.