If my understanding is correct, O2 sensors are flow/pressure sensitive meaning that the u bend behind our cats is creating the optimal flow for the sensors reading. I'm currently designing an exhaust for the Monte that includes a universal converter by Eastern Catalytic. They build cats with higher amounts of precious metals specifically to help old GM cars pass emissions. While mine doesn't have this issue I thought I may as well change the cat when I build the exhaust, mostly to get rid of the u bend. So the question here is: what might I have to do to the O2 bung I fabricate in order to get appropriate readings and prevent the check engine light coming on?

The universal cats:
https://www.rockauto.com/en/tools/ex...ytic+converter

 

You dont have to do anything special. As long as you dont get a high flow cat, itll be fine.

 

Even if the cat is bigger than stock? Like if I installed a cat designed for a 5 liter would that count as high flow or would I need a cat designed for 3-4 liters?

 

That gets a bit tricky as liters of engine doesnt really give you any useful info about the cat.

In theory for the same efficiency, the cell size between the 3L and 5L cat would be the same, the 5L would just have a larger pipe / more cat material. If the cats are both physically the same size in both categories, I'd be concerned they're reducing cell count to help deal with the extra volume of exhaust on the larger engine (meaning it would be similar to a 'high flow' cat).

Regardless, high flow vs normal cat doesnt have an impact on O2 sensor locations / flow, the kind of stuff you're talking about. The problem is that high flow cats (especially those listed for off road use only) typically have drastically reduced cell counts (ie the holes through the honeycomb are larger) which means less efficiency, which means you run the risk of running outside of the allowable range for the rear O2 to trigger the light.

The only way to 'fix' a rear O2 light due to a cat that's not efficient enough during install is to cheat it by installing an anti-fouler (basically an adapter that goes between the bung and O2, but has a reduced diameter opening / moves the element out of the direct exhaust stream. If you've got to pass emissions, you've got to be a but careful with that as you're essentially cheating the system by defeating an emission device.

 

I definitely don't want to defeat any emissions systems, so the Eastern Catalytic built for the 3.8 is probably the way to go, even though it probably has a higher cell count seeing as it's designed for vehicles having problems passing. Do I need to worry about the o2 sensor or emissions equipment reading to low of emissions or is that not a concern?

 

Typically with emissions they just hook your car up to a computer. As long as the catalytic system is in the ready state, and a check engine light isn't on, you should pass. I can't see your car having to pass a sniffer test at the tailpipe since it's an OBD2 car. High-flow cats can sometimes throw the CEL on our cars. A lot of us just tune that code out so it doesn't report an error, you could also space the oxygen sensor on your exhaust if it trips the code.

 

by spacing the o2 sensor you're talking about moving the sensor away from the center line of the exhaust pipe so the sensor sees less of the nox in the tailpipe right?

these are the emissions checks local to me, no sniffing you're right.

Pollution control equipment check that includes the catalytic converter and other connections.
On-Board Diagnostic (OBDII) computer car emission test on vehicles 1996 and newer.
Any visible car smoke test to uncover burning oil or poor fuel combustion that results in toxic emissions.
Gas cap check to search for vapor leaks.
Tailpipe exhaustion analysis on vehicles manufactured between 1983 and 1995.

 

Yeah adding a little spacer can sometimes help if you put in an aftermarket cat and it trips the P0420 code.

 

Sweet, I think I can get a good exhaust fabed up with all this info, thanks everyone.

 

I'd be super careful assuming that across the board. I dont think the OP is after a marginal cat, so I dont think itll be a problem in his case, but for someone that is trying to sneak through a sub-par cat, you wont get away with it in all areas (and I'm not just talking CA either).

Before I moved from my hometown in IN a decade or so ago, obd2 cars got:
-obd2 plug in test
-gas cap test (its removed and put in a pressure tester)
-walk around machine sniffer for leaking gas vapor
-visual presence of the cat (typically with a mirror on a stick for regular height cars like this)

Since I've left though, I've had friends get hassled for having an aftermarket cat with either an O2 emulator plugged in place of the rear O2 or a spacer / antifouler installed. The only purpose of those two types of items is to cheat the system to get it to electronically pass. So if they're looking for that stuff and you get caught, you may end up in a bad spot.

No doubt some places still just plug it in and go, but I'm just suggesting that everyone look into their local process before assuming and getting nailed.