Originally posted by Kaiser.:
I have dyno'd with and without cats without this huge difference of which you speak. 
The Wideband read the same with or without cats? That's not really possible since the cats scrub out unburnt fuel. Even if not 100% efficient they still would effectively lean out the mixture before the tailpipes.
Unless of course the readings were always right around stoich???
If you mean you O2 voltage readings were the same with and without cats. I believe that since it is located upstream of them.. Though it probably should actually have leaned out a little with the better exhaust flow.
