I have a 2005 Subaru Impreza Outback Sport and I am trying to find which exhaust system will fit best. Will any system with just the Base model work? WRX? STI?
The 2.5i in your OB Sp isn't restricted sufficiently by the OE system to result in anything but noise and possible CEL from O2 misplacement if you add aftermarket. I'd be careful swapping in a system made for the 2.0t (WRX) unless you're sure it'd not be a waste (npi). If you wanted a little more pep from the system you should've gotten a 2006-2007 clone which has the variable-cam system that's punchier off the line. OTOH if you want more sound just goof around with the muffler options, like maybe removing yours and then adding minimal ones until you're happy. But I'd leave all CAT-forward stuff OE. Cheaper that way too.
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