STI is Subaru Technica International. They are a company in Japan that makes performance parts for Subaru. Kinda part of Subaru, kinda seperate. It's still Fuji Heavy Industries. When you buy a WRX, you get a Subaru WRX. When you buy a STI WRX, it has STI parts. Suspension, interior, breaks, wheels, transmission (6speed!) etc etc etc. More horsepower, blah blah. Not really like 'Type-R' or 'SI' like you said, more like TRD like on Toyotas. And the WRX first came out in 1994, everybody thinks its just the bug-eye ugly things. Its not. The GC8 (older body) just weren't sold in US, only 2.2 L's, 1.8 Brightons, and starting in 1998 the 2.5 RS. Those all have essentially the same body, just different engines and such. All Imprezas however. My eventual goal is to convert my car into a Version 4/5 STI WRX. I MIGHT boost my 2.2 liter engine, and get a widebody kit (flared front/rear fenders, ala the 22B impreza) or swap out my engine for an older STI 2.0 liter WRX turbo engine. Already got the v.4 wing. I just need the hood and v.4 or v.5 front bumper.
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be aware, ski with care