Back when I had a sentra spec-v hotshot had a header package that eliminated the primary cat, but most of us retained the secondary cat because it would not trigger the ECU for missing/malfunction of the o2 code since the first o2 and second o2 would see a "difference" being on both ends of a cat and therefore appeared to function properly. Is this the same case with the 2az-fe setup? A high-flow cat could be used too without triggering the code. My concern is being able to pass an emissions OBD scan and have the readiness monitors clear. I think the ease of remappers of being able to manipulate these in a WRX somewhat spoiled me as you could set it to anything you wanted. The look I'd get from the inspection with my 4" TXS exhaust piping yet pass a scan was priceless.

 

FWIW, just found a pic of the stock and full exhaust systems. IIRC, I had to get a bung welded just past the secondary cat where the secondary o2 was extended and installed to get a post-cat reading. Is anyone here doing this or is it too far down the stream? It wasnt a problem with the QR25DE motor, which is very similar to this configuration. I can see that if you remove the primary cat, your secondary o2 is still mounted before the secondary cat, and you will throw a cel.

 

Most of us just use an 90° O2 extension on the downstream sensor. I did that for years when running a 4-1 header, and again when I switch to forced induction. That should be all you need. You can technically go completely catless, but I don't like the smell on a daily driver.

 

Hmm weird. I would think that could be spotted by inspection or throw a code. The primary cat is probably there for valve overlap recirculation for "reburn" and clean emissions. The secondary cat should function clean enough to give a difference in the two readings. The stock configuration has the secondary o2 before the secondary cat. If you move it past that, it _should_ suffice for preventing codes. With most basic f/i, I'd still want a cat in the stream to prevent boost creep. There is some level of backpressure wanted to close the wastegate unless you're ext dumping or specifically tuned for it.

I wonder if a turbo could pass inspection if there are no cel codes thrown.

 

One of the benefits in removing the close cat in the qr25de motors is that the caustic gasoline would deteriorate the catalyst materials, which would be sucked back into the ports and eat up the rings and walls. I wonder if thats part of what was going on with some of these motors which were oil burners. There was a recall on those engines too and they were released about the same time period.

Same thing with the wrx and its 3 cats. The one that was pre-turbo was breaking up and throwing parts through the blades, which again is like sandpaper in the stream. This is why the STI and later model subarus started omitting that small unit in the uppipe.