Would it be smog-legal, in the US, to put dual exhausts on my '80 Chevy, if I Also add a cat. converter to new exhaust? ?

Question

Update:

Yes, they Certainly do. When I went to get smog check in 2010, they said Every bit of smog eqt. it came with has to be there and working, "even if it cost $1,000";  they didn't care. My state even smogs  1969 cars...used to smog Motorcycles, too.  

Update 2:

I also Said:  ".. Also add  a cat. to new exhaust..". "Add"  means  Additional; means, There is Already One (or more).I  cant think of a car made after '78 Without a cat.

Update 3:

No, no O2 sensor. This is an '80, oldest I ever saw with one was an '86. And, it had a Carb.

Update 4:

Again; all cars  after '78: I Know;  have a Cat on Each tail pipe. If I  cut bypass and added second exhaust; logically; that new, Separate exhaust should have a cat, too,. I am Not simply adding on After the cat. Little  advantage, sound the same.

Answer 1

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Ask a smog test station in your state.

Answer 2

In your state they may say the original exhaust design is the one approved for your vehicle. 

Personally if the exhaust is one or twin I cant see how that will alter what is leaving it. 

If a cat is not already there,  don't put one in. 

Answer 3

As long as you have 02 sensors I don't see a problem.

Answer 4

emissions testing laws are set by each state and sometimes vary based on what county you live in. What's legal in rural Montana isn't the same as what's allowed in Los Angeles.

In terms of federal laws, I don't think they apply to vehicles that old. You are not required to add a Cat if the vehicle did not originally come with a Cat.