I had always thought that the emmission equipment was supposed to last the life of the car. I have driven many >200k mile vehicles with the original cat with no apparent degradation of performance. As an aside, it is against the law to remove and replace your catalytic convertor if it is still working. I think the point that Fats is making is that it does not matter if a high flow cat or test pipe is able to flow more, if this is not limiting the flow the engine(air pump, if you will) is able to pass through. I have always had a philosphy of upgrading parts when I replace because of failure or wear, but if the part is not worn or impeding performance, I would just spend the money on good beer:cheers: