Ok I have a 90 camaro rs TBI. I want to change the exhaust manifold therefore removing part of the emission control system connected to the original manifolds. According to my dad he researched what I want to do and the car will run the same as before. According to my uncle it will cause the car to run rich and the check engine light will come on becuase the computor wouldnt be getting correct O2 readings. I have had part of the emission control off that come from the exhaust manifolds and ive started it and let it run almost 20 times. The check engine light has never come on since then and the car seems like its running the same as it was when we originally bought it. Does anyone knopw if it will cause damage if I swapped exhaust manifolds and put new cat back on?

 

The oxy sensors in the exhaust manifolds are for fuel control, to make the car run correctly. Whether you use stock manifolds or headers, you need to have the sensors in place and working. Otherwise, the engine will be running rich, crappy, etc.
The cat is an issue if your car still needs to be inspected for a sniffer emissions test, or a physical inspection to make sure the emissions parts are still on.
If your car no longer needs to be tested by your state, it's up to you whether or not the cat "falls off". Removing it will have no negative effect on the engine, nor will any SES light go on.