Illegal maybe but is it worthwhile?
If I'm not mistaken, there is a law in all 50 states requiring a Cat be installed on a vehicle if it came from the factory with one.
Only situations I can think of where you don't need one:
Race vehicles that are not streetable
Farm vehicles
Move out of country
there is one other if it is older then a certain year i think it is 85. the kicker to that is vehicles came with cats and they provided a ton of back pressure and horrible mileage. they can be taken off but only you can take it off a muffler shop cannot. my parents have a 83 econoline van (love the thing) it came with a cat but my dad was able to take it off with no penalties. he said that the mpg difference was big 10 with it on 16-18 with it off.




