Okay sitting here dreaming of getting my T76 into my tahoe and am thinking about emissions. Up here in CT we are almost as bad as Cali., OBDII scan, visual cat, gas cap, and sniffer. My whippled 8.1 passes without a problem with gutted cats, but I am concerned with the tahoe passing. I am thinking out loud here so bear with me, "if" I kept the stock cats in their stock location would I have to switch the passenger side O2 sensors (gront to back, back to front) since the flow would be reversed? Also how would the cat respond to the reversed flow? or are they directional?
Next idea, would I be better off adding a single cat right after my downpipe?
I have thought about just running with either gutted cats or no cats and see if it passes, but I am still unsure if I want to deal with the exhaust smell on my DD/family wagon...Hmmm what to do, what to do.

 

I'm curious to all these questions as well. I have friends that can pass me but can't rely on that every time.

 

Well, I assume you are talking about a front mount since you mentioned a down pipe. In that case I assume you are considering putting the cats in your crossover pipe which takes place of your stock y-pipe. Realize that you have a cat for each cylinder in the y-pipe, but there is no way to fit them anywhere before the downpipe on a front mount. As far as the flow, I believe they are directional as I have seen several with a directional arrow cast in the casing. And as for you o2's, your downstream o2's(post-cat) are entirely emissions related. Your upstream's, however, are used by the PCM to control fueling. You will keep your drivers side o2 near the stock location in the crossover, and you will need to lengthen the harness for your passenger side o2 so you can mount that sensor in your log manifold. I turned the downstream's off in my tune.

So, if you want a cat on there just mount one after the downpipe, before the muffler. I wouldn't be concerned with smell as long as you don't dump it under the truck.

 

I don't know your setup, but I'd run one good flowing cat right after the downpipe. You def don't want any cat material to get into your turbo if it were to fail. Especially with the higher back pressure between the engine and turbo, not sure how a cat would like that.

 

I thought I put that it was a front mount in the title, but set-up is going to be a Turbonetics T76 with KB Racing hot pipes and intercooler w/cold pipes, 3.5" downpipe.

 

My thought is if you put them in the cross over and they happen to come apart for some reason you'll be buying a new turbo.

 

Find a 3.5" cat shell, imo

 

3.5" cat (gutted or not) at the bottom of the downpipe. I know a guy that has V bands on either end of his cat and has a blank pipe he runs 51 weeks out of the yr, and can stick his cat in for when he gets sniffed, also has a "sniffer tune" as he calls it lol.... He runs an LS9 cam with his turbo'd LS3, makes 600ish to the ground and has never failed to pass the ol sniffer

 

Ok, leaving them out of the crossover pipe make perfect sense to me now...I don't want to be destroying my turbo with a cat that came apart.
So now the search begins for a cat...any ideas?

 

I would just do a single, quality high flow cat after the DP. IMO the bigger the better