When you say the new one is worse, what do you mean? Your MPGs are lower? Worse smell?
What I changed my cat over to the CatDaddy universal one, I had no problems at all - if anything, it was better - but thsi was also in conjuction w/ new muffler and 2.25" pipe.

Personally I do not see how using seafoam engine treatment would do much "good" for a cat, but I'm probably missing something.

With 200+ Billion electrical parts, the world most complicated machine is inside your own skull.

Question Reality.
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'89 Rnr DLX "SR4.5", 32s w/ 5.29 locked f/r blah blah