This will replace your factory o2 housing and downpipe with our high flow ZZPerformance package. It is made of stainless steel so it will outlast the car. The ZZP unit is two pieces just like the factory parts and they are joined by a 3" VBand clamp assembly. This package will bolt up to the stock turbo and cat back exhaust with no modifications needed.The gains are hard to measure because the Regal adjusts its boost by the amount of airflow the MAF sensor is seeing. So as you add mods the car sees the added airflow and reduces your boost to bring the power back down. So we included a dyno sheet to show the gains. Our o2 Housing/Downpipe package, ZZP tune, and ZZP high flow intake tube bring the average HP and torque gains of 70 whp and 70 ft lbs of torque across the entire pull which is huge power gains! The peak gains of this package were 89 whp and 77 ft lbs over stock! Please make sure to select the correct model because the GS has different hanger locations and a larger outlet flange. Please note: the factory o2 housing uses a ring style gasket. that is left out when installing this product. you can use a small amount of red high heat RTV at the turbo to o2 housing connection. Fits 2011-2013 Regal 2.0TOur high flow catalytic converters are EPA certified. If your state requires CARB compliance, these do not meet that CARB standard. Please contact customerservice@zzperformance.com if you have any questions.We are currently offering $100 in store credit to purchase your stock catalytic converter back from you. If you're interested, take a photo of your stock downpipe and email it to Henry@zzperformance.com. We will provide prepaid shipping labels for domestic (U.S.) shipments.