jsafety 5pts 1. To Kooks credit, they will keep sending you new cats to collapse and endanger your engine, as long as you are willing to keep doing the labor to reinstall them. Now that is service. 2. This article noted the stock exhaust as a restrictive system. Wow what an analysis! These manifolds are in fact high performance. Made with special light metals, flow balanced with an opened up casting design for high flow well beyond the motors stock needs. Wow again. You just removed an "expensive" high performance part to put in after market cats that can barely take the stock flow. Oh what a feeling. Borrowed that from Toyota, sorry, could not help it. When I re-installed my high performance light weight stock manifolds (yes it is true...lighter than the Kooks headers, of course), I simply honed out a few casting protrusions with a sanding wheel and buffed her all around inside to reduce surface drag. These "stock" manifolds are good for 600hp and with my mild prep work are good for 725hp.