According to http://www.imprezawrxsti.com/engine.htm:
    The STi engine is different from the engine that the US WRX receives in that it has a larger IHI built turbo unit, a larger intercooler with water spray, and Subaru AVCS.  Automatic Valve Control System, or AVCS for short, helps give the STi more torque and horsepower throughout the entire powerband.  It is Subaru’s version of Honda’s VTEC or Toyota’s VVTL-I, for example.  Also different is that the STi is a semi-closed deck block, which helps ensure reliability when running more boost from the larger turbo.  The STi engine also includes sodium-filled valves and other internal changes (i.e. forged internals) to make the engine more reliable at the higher boost levels.  Also, the intercooler has a pink STi logo on it, and the intake manifold is painted a pinkish-red, so that every time you open the hood you are reminded you are driving something very special.
_________________________
Bitt Faulk