In 2004 Subaru introduced the US to the Impreza STi with a USDM specific 2.5 liter engine and a nearly indestructible 6 speed transmission.   To this day, the present 2018 STi still has a lot in common with the first STi, sharing parts compatibility in many areas.  The biggest difference is that the 04-06 models have a side feed style fuel injector setup while the 2007+ STi features a superior top feed style injector setup.