I'll keep this brief. My neighbor's experience with getting his 96 SE smogged. HC was just a little over the test limit even with the cat hot.

Gutted pre-cats
Y pipe mod
Hi flow aftermarket cat
SVT cat-back
Passed smog check two years ago equipped as above
Did not pass this year
Evidently smog specs are tighter this year
Tried adding stock cat behind hi flow cat - didn't help

At this point he researched and found that he needed an OBD II cat. I have not heard of them before, but he learned that it is more expensive and nore effective. He found a source for one that is speced for use behind an engine up to 5.7 liters so it should flow well and be big enough to clean things up.

Results with this cat by itself and the rest of the exhaust as above - it easly passed with very low readings. HC was down to something like 12.

Engine seems to perform about the same.

Interesting, I assume there is a major price difference between the regular cat and this OBD-II cat?

What was the brand of the OBD II cat?

I read about OBD I cats but those were the ones with only the bung for the upper O2 sensor and no lower O2 sensor bung.

I'll see if I can get a brand and part number. Yes, it is much more expensive, but as you can see, it works.

The brand name is Sound Design. A Google search shows that Sound Design is a Magnaflow brand for cats. I did not get a part number.