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Have you checked out this thread:Originally Posted by Jonney Knuckles
Exhaust FAQ
As for an AP... ehhh I'm a cheap bastard. I went opensource. Look up "Tactrix".
There has been debate on brands, I myself am on the "its just a friggen 3" pipe" Cheaper the better.
The newer WRX's have a tendency to boost creep when ran uncatted, but there are remedies to fix that IIRC.
If you are mechanically inclined, which you are, a DP swap should take no more than an hour. Just remember to flash the stock/stage 1 map back.
I'm in Cali strict-ass emissions, yet half the modded subies smell like a mobile gas station.
A new downpipe should have an o2 bung. The AP, or just any tune for a downpipe will tell the ECU to expect different numbers from the O2 sensor.
From the thread mentioned above:
Good luck and have fun man!Downpipes
Most aftermarket downpipes replace the section of exhaust between the catback and the turbo. Only '02-'05 MYs need to worry about the few "shorty" downpipes on the market (e.g., Bosal, Oakos) which only replace the section closest to the turbo (leaving the rear cat pipe in place).
Modification of emission control devices (e.g., catalytic converters) is a violation of the Federal Clean Air Act (§203(a)(3)(A)), and therefore is illegal in every U.S. state. Some downpipes include high-flow cat(s), while others are completely catless; a downpipe which utilizes HFC(s) is just as illegal as fully catless. When determining whether or not to run a catted/catless downpipe, it is up to the end-user to research their local/state emissions laws. On vehicles with stock or mildly upgraded turbos, a HFC will not impact the performance that much (minimal enough to be equated to "dyno error"). Some owners (mostly 2.5L) have had boost creep issues with catless downpipes, and will switch to a catted exhaust to provide some backpressure; some cars have boost creep, some cars don't.
There are three styles of downpipes you can purchase:
Bellmouth
Divorced
Flat-plate
As for which is the "best" design, you want a downpipe to be as free-flowing as possible. The flat-plate design blocks the wastegate, and is generally considered an "inferior" design on an internal wastegate turbo (this is negated when an external wastegate comes into play). The difference between a flat-plate and bellmouth/divorced downpipe has a direct correlation with the flow of the turbo. When comparing bellmouth/divorced, in theory, separating the exhaust/wastegate gases with a divorced-style downpipe is ideal, but the difference is minimal enough to be equated to "dyno error". Pretty much any brand will perform the same (any difference you see by switching downpipes is minimal enough to be equated to "dyno error").
The main thing to consider is whether or not the downpipe tapers or not. Most downpipes on the market are a full 3", but a few taper to 2.5" to mate with an OEM-style catback.
You usually need to modify the OE heatshield to fit aftermarket downpipes. If the heatshield is missing, you can usually pick them up cheap from the classified section. If you also need the bracket to attach it, you want Subaru P/N 44021AA012.
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