I looked around and asked about the 98 impreza 2.5RS to see if I could find a motor from one of them. He said that they don't exsist.
the 98 Impreza RS exists...but it's the same EJ25D motor that is in your 99 outback. why would you remove one motor and install the same motor?
I want to keep the 2.5L but not have turbo.
that's exactly what you have in the car now.
you need to clarify exactly what you're trying to do.
it sounds like you want the gains of a turbo engine on your non-turbo engine which is not physically possible.
you have two options:
1. minor modifications to your engine. do whatever you want, the gains to be had are very small so it doesn't much matter. get a pulley set, header, intake ,spacers, whatever you feel like trying out...and you'll gain 11 hp and be like "i think that's faster". it'll still be slow on the track.
2. complete full on engine swap to a turbo. block, wiring, harness, ECU, exhaust. then the options and gains are vast.
one is pocket change the other is a full on project requiring lots of skill or lots of money.
turbo engines can be modified for enormous gains - non-turbo's can not. in general you'll be disappointed and never satisfied if you try to "bolt on" to a non turbo engine and hope for big gains.
Did the WRXs start having 2.5Ls back in the late 90s, or early 2000s?
do you mean turbo or non-turbo? impreza's/WRX just got the 2.5 liter...same engine that's in your car starting in 1998. the turbo engines come later - looks like previous poster covered that....
if you want the "fast one"....WRX...then that means a turbo engine. those are extremely expensive and it's a huge project- completely engine, crossmember, engine wiring harness, body side wiring harness, ECU, or aftermarket controller.