Invidia downpipes for the 2008-2014 Subaru WRX STi are designed in a variety of configurations. Every Invidia STi downpipe features 3" diameter stainless steel piping, thick flanges, and a cast upper section for optimal flow and performance. A 3" 2-Bolt downpipe to catback gasket is included with the downpipe.
Divorced vs. Bellmouth:
Divorced: A divorced wastegate downpipe has a plate the protrudes into the turbo to separate exhaust flow coming out of the turbo to give reduce turbulence. This is good for stock and smaller turbochargers; the protrusion does not work with some aftermarket turbos (i.e APS).
Bellmouth: This design has a wide-open bell at the turbo, which is better for larger turbochargers with more flow. It will work fine with a stock or small turbo, but it is designed for scenarios where wastegate turbulence is not an issue.
Invidia's catless downpipe will cause a Check Engine Light (CEL) on your STi. While this isn't harmful, it can be annoying and disables the cruise control, you can use a software solution such as the Cobb Accessport to eliminate these codes or try a mechanical fix (O2 sensor spacer). The high-flow cat Invidia STi downpipe is not emissions legal, and may still cause a CEL. The high flow cat helps to reduce boost spiking on 2.5L cars.