Well i went in to get my 94 z28 inspected today and failed it. They said it failed because of the cat. So looked underneath the car and who ever wielded the exhaust system did the worst job ive ever seen. Im goin spend a few hours reweilding it up and im also going to replace the cat. What are the inlet and outlet dimensions for the stock cat?

 

Before you jump to conclusions its the cat, it may just be the O2 sensors need replacing if they are very old...say 75k miles would be old. The inspection places usually don't have a clue what is actually wrong..they just know how to hook up the computer and the sniffer and they have a basic chart based on what parm failed. 8 out of 10 times its the o2 sensors. Mine's passed the sniffer without a cat before so something has to be fouled up to begin with even if the cat wasn't doing its job....and if its been running real rich for a long time, it will kill the cat too.
Hook it up to a scan tool and see what the BLMs are doing, if they are buried at 108 then O2s are the cause more than likely...a cat will have no effect on O2s (which determine what the BLM value will be) since the O2s are before the cat. A BLM value of 128 is perfect..any higher and its lean..lower and its rich.

 

Thanks 2quik6. Thats very helpful information.