i have a carsound cat. the only difference i have noticed with it on is the car being ALOT smoother at shifts(auto) i have the IHE combo too. my accord had a small pipe (5inches long?) that has the o2 sensor bung on it. its a factory part of the car. it is between the header and cat. that little pipe is the only part of the factory exhaust i have left on my car. maybe i should weld a new bung onto a larger diameter pipe...