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i have headers intake and no cat and my car is looouuddd whats better a high flow cat or a stock cat. and is high flow cat legal ??

 
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First off your car is not legal in its current state.
Witch type of catalytic converter is better is completely subjective.
I would reccomend the stock convertor to be safe.

The only way a high flow catalytic convertor would be legal in California is if it comes with a C.A.R.B. (Cali Air Research Board) certificate for your exact vehicle. If its a universal cat then its probably not legal for use on your car.

 
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I take it you got a race header? They seem to suck to me. I had a race header and nothing but CEL's. The cel can hurt your ecu. Go with stock converter it is a high flow type anyway. It flows great and plus a high flow cat is only an option for forced cars, well in my opinion. HP gains are almost non existant with converter on or off. Car likes the stock one pretty well. My car runs smoother with a converter than without!! Good luck!!