chuck

BUT......wouldn't opening inlet and outlet and changing pipe between the two CATS make a difference?

The pipe between CATS is really small!

carguycw

That's the downpipe. Yes, it is VERY small. Traveler wrote an extensive thread on this topic a while back. He disassembled and measured every part of his car's exhaust system and found that the main bottlenecks were, in order of size, (1) the outlet from the primary cat and (2) the downpipe.

He was able to pick up several horsepower by grinding out the inside of the stock cat outlet and custom-fabricating a larger-diameter downpipe. The downpipe was more difficult than expected because the flange at the inlet (primary cat outlet) is non-standard and he had to fabricate a custom flange for the new pipe.

To quote TheMAN: Do a search!