I did a search and didn't find anything that exactly answered my question. I was on testpipes.com They have several sizes of diameter testpipes, 2.50, 2.25, 2.00. They say that all of the sizes are direct bolt ons for stock connections. Does this mean that the outer flanges wil match stock connections and only the pipe diameter is varied? Will all these diameters fit the stock flanges?

If so, 2.5 seems to be the obvious choice. I have a bone stock 96 GSR, so the OBDII needs the 2 O2 holes in it right? How much louder will it be with stock exhaust in place?

Thanks for your help, and please save the anti-testpipe speech.