The COBB Tuning turbo downpipe is one of the premier modifications most owners look to upgrade on their vehicle. By opening the diameter of exhaust flow coming out of the turbo, the downpipe unleashes power where the stock restrictive unit lacks. Not CARB Legal The COBB Tuning downpipe begins with the CAD designed bellmouth that directs the air coming out of the turbo. The design permits the air to flow with very little turbulence to the single high flow catalytic converter. This metal substrate cat is highly efficient, allowing the exhaust to flow much easier than the factory unit ever would. The COBB downpipe, combined with our Accessport engine management, is one of the most easy installations for any enthusiast looking to unlock the true potential of their WRX. The engineering that went into the COBB 3" Downpipe started with the CAD designed investment-cast turbine outlet, made of 304 stainless steel for enduring corrosion resistance and reduced heat transfer to the engine bay. The shape of the outlet flange smoothly blends the turbine outlet and wastegate gases. The outlet flange is configured with an O2 sensor bung to accommodate the stock O2 sensor. A custom spun metal substrate high flow catalytic converter comes next. This one piece 400 grade stainless steel cat has a much higher flow rate and longer lifespan compared to ceramic or "brick" style cats. It also reaches operating temperature faster than other style cats for cleaner cold-start emissions. A second O2 sensor bung is located immediately after the cat to be used by the second OEM O2 sensor. All tubing used in the fabrication of this downpipe is 304 stainless steel for increased corrosion resistance. The end flange uses the stock donut gasket and factory spring bolts to aid in flex, longevity and proper fitment. This provides an improved seal compared to other methods, as well as reduced noise. An additional benefit is that it makes the COBB 3" Downpipe compatible with any cat-back exhaust that mates to the factory cat section. Subaru SS WRX / STI / FXT 3" Downpipe CAD Designed Cast Turbo Outlet for Maximum Flow 304 Stainless Steel for Corrosion Resistance Custom Spun Metal Substrate High Flow Cat GESI Two O2 Bungs for Factory & Aftermarket Sensors Utilizes Stock Donut Gasket for Optimal Sealing & Long Life