STIEVO - I owned a WRX for six years and invested $15,000 in engine and suspension mods while retaining a cat, so please don't jump to calling someone a noob. In the future I'll steer clear of your threads to avoid your alienating know-it-all responses to peoples' posts.

 

^Look bro, there's no need to get all butt hurt. What you said was factually incorrect and showed a general lack of knowledge and experience, and I called you on it. Get over it. The fact that you had a WRX with a bunch of work done to it doesn't mean jack. I know a lot of people that have heavily modded cars that don't know squat. The bottom line is this: you have a little bit of experience with a WRX, but you have a lot to learn. The fact that you supply a link to a test where a little NA honda motor barely made any power with a test pipe compared to a high flow cat is utter proof of this.

NA engines and turbo engines are night and day different when it comes to exhaust upgrades, and the gains that can be expected from these upgrades. Since a turbocharger's performance largely depends on the pressure differential across the turbine wheel, reducing backpressure in a turbo car produces larger gains than it does in an NA car. Also, a WRX is a bad example to use because unlike the STi, it has a cat in the header system (specifically the up pipe) AND the exhaust. Putting a test pipe in a WRX won't yield anywhere near the gains that it will in an STi or any other car because you still have another cat suffocating the turbo before the exhaust even begins.

For someone with as much experience as you seem to think you have, you should know this. So again, I'm not trying to be ugly, but you are noob, man. And that's ok, we all have to start somewhere. Hell, we're all noobs compared to someone. I know a good bit, but compared to some people, I'm a total noob cake. I like to look at it this way: thinking you're an expert will only prevent you from learning new things. Anyway, here's a link to an article that explains some of this, and I think it will help you. It's a good place to start learning. http://www.sportcompactcarweb.com/te...ems/index.html

 

I am still lost as to why a 3 inch test pipe won't work for you? Won't that help reduce backpressure even more? I don't claim to be an expert in this by any means, just a thought from your above statement.

 

^True, a 3 in test pipe is still an improvement over the stock cat. The issue is the diameter change; going from 2.5 to 3.0 back to 2.5 again creates a cavity where turbulence and backpressure build up. So again, while it's still an improvement over the stock cat, it's still not ideal when compared to a constant diameter setup. Read the link that I supplied, imparticular the part where it talks about gutting your stock cat and why that's not a good idea. It's sort of the same concept here, just not quite as bad.