it keeps your car from throwing a code for not having a cat on it. You may not need it...but you might. It goes in the secondary sensor (post cat) and basically just makes it read less emissions by getting it out of the stream of exhaust. They're about $15.
I can't speak for any other state, but a lack of a cat will give you the big fat emissions FAIL for most counties in PA. For OBDII vehicles they do hook it up to the machine and record everything - and this data is sent to the State for records and evaluation by the machine. Hello Big Brother.
Older vehicles they use the tailpipe sniffer, but they can use it on newer models if they think something is goofy. My Saab 9000 failed every year with all of the fuel and boost mods I had on it.
But, and this is key, they do perform a visual inspection to ensure all of the parts are there - gas cap, evap stuff, and yes, a cat converter. You might be able to get away with hogging out the guts of the cat if your emissions inspection is pretty soft, but I wouldn't even bother. Get a $99 performance cat and be done with it (or used perhaps as mentioned). You could weld an O2 bung just about anywhere.
Thankfully there are exemptions if your vehicle has driven less than 5,000 miles since last inspection. Comes in handy sometimes.
I'm jealous. Once your legislative body figures out that they can gouge everyone for another $50 a year for another b.s. sticker on your car, it will be game on. If Pennsylvania could figure out a way to tax you for blinking, they sure as heck would.
Good luck Nike, if you can exploit your connections/situation, by all means.
im not trying to exploit, im just trying to save money on not buying cats, plus maybe gain a little in gas mileage and performance. i cant seem to figure out what to use...ive read about non-foulers, but what size? and how do you think the range rover electronics would deal with it?
the electronics will be fine. The pre-cat sensor runs the engine...the post cat-sensors just tell the computer if the cat is in working order or not. (to my understanding anyway). If you don't want to eliminate the cats all together, put in some universal high flow race cats.
For me personally, I don't like the smell of running no cats, so when mine go I'll probably throw some high flow cats in there.