Stock size is mostly 2.5 inches I believe with a smallish section of 2.1 or 2.2 " OD. Any turbo application vehicle, even a 2.0 with our punt k03 will benefit from a larger exhaust/better flow....to a point or course.Because it's a tiny little 2.0 engine. Any increase you would get from 2.5 or 3" pipe would not be noticed until a turbo upgrade. Which means 2.25" is adequate for 99% of ST owners. And it weighs less. And it's cheaper for Flowmaster.