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they aren't mandatory they just help keep back pressure in the engine i know that because my friends cut his muffler and catalytic converter off and put pipes so it doesn't have enough back pressure on the engine so it chokes the engine and it vapor locks

THIS IS A RESPONSE TO THE PREVIOUS ANSWER:

I am not sure when the became mandatory but they are in fact are. I believe it started in 1975. They are required to help with pollution. The newer one have an added feature that helps lower oxides of nitrogen which is a key factor in smog. the older ones help convert unburned hydrocarbons (gasoline), and carbon monoxide(
a poisonous gas). These are required by federal emissions laws and you can get in serious trouble if you are caught removing them and driving said vehicle on public roads. So the previous responder was very very wrong by saying that they aren't mandatory

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15y ago

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