Cat cell count

When I had my stock cats off, on the inlet side, the side attached to the header, there is a "deflection plate". The plate is right smack in front of cat matrix on the back/inside of the inlet hole. The way they have it welded in appears that the designers are attempting to reroute the exhaust so that it fills the entire cat chamber. But looking at it you can see that it creates a blockage of the exhaust flow, and causes turbulence. Not only that but, the cats are so big that as the exhaust enters the cat, and as it expands to fill the chamber, velocity of the exhaust slows down. So adding everything together it makes sence to me why the stock cats don't perform as well as a set of free-er flowing, smaller, cats.

If I can manage to find where I put my cats in the garage I'll snap a pic so you can see what I'm talking about.

 

Here ya got Drew.

Dug out my stock cats and snapped a few pics of the inside. You can clearly see the deflector (or what ever it's actual name is), and how it's blocking the exhaust flow. One other thing I forgot to mention is, on 348s the O2 sensor sits right in the middle of the flow. But on the 355's they put the O2 sensor back further, so that it's more inside the cat matrix chamber, just before that plate we see here in the pics. And on the 5.2 355s you have TWO O2 sensors. One on the inlet side of the cat, and one at the exit = even more disruption of the gas flow.

If you have to keep your stock cats you may want to try getting a pipe spreader in there and open that stupid thing up. Or some how crush it, cut the spot welds, then remove it all together. Either way that plate it not doing anything to help keep the speed of the exhaust moving fast.
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355 OEM cats are 400 CPI as was told to me by my mechanic, my BM high flow cats have 200 CPI.