I wanted to know if it is possible to weld a precat onto an aftermarket y-pipe, or would it just have the same restriction as the stock one. I live in cali and I understand that the smog level will be fine after the engine is hot but I dont want illegal stuff on my car

 

If you can't eliminate the precat then what is the point? Since Y's remove precats there is no getting around it. Now you can put the o2 sensors before/after the main cat and still pass emissions but the visual would fail (from what I heard). Just buy a JWT/G-force ECU instead.

 

Don't worry about it. Welding a precat onto an aftermarket y-pipe is a ridiculous idea. Don't do it. Smog shops won't know the difference between a stock y-pipe and an aftermarket version...if they by a slim chance give you crap about it, just take it somewhere else.

As long as the main cat is warmed up, you are fine as far as emissions are concerned. If you don't want illegal parts on your car, then stick with what you've got.