Anyone remove the cats for a straight pipe?
 

someone was telling me that when this is done the car throws a ses light and doesn't run right and basically throws itself in a limp mode... i wanna do this to my x5 but im unsure if it will harm the truck

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Why do you want to skip the cats?

You would need to do something with the Oxygen sensors fitted after the catalytic converter. These sensors are a critical part of the engine management system and would be reporting something very odd as far as the engine was concerned which is why the vehicle probably goes into limp mode.

As far as I know there are different types of sensors for before and after the cats, so it may be as easy as replacing the rear ones and a tune-up.

You would need some O2 simulators so the ECU thinks that the Cat is there and so it does not throw any check engine lights. There's probably not a big performance increase to do this on the X5. Race cars do it to remove weight and some restriction, but the BMW cats are pretty free flowing.

of course the o2 sensors will still be used because there will be spots welded on the straight pipe for the o2 sensors to screw into. but im asking if anyone has done this and what are the problems if any that occured? thats what im trying to get at. the only reason i wanna do this is because i use to have a pretty raced out vr6 before my x5... 89 jetta coupe with a 98 vr6 swap with alot of work spaking e46 m3's with mods and i had a straight pipe but it didn't even throw a check engine light and was loud as hell and flowed the exhaust pretty well... so thinking its a v8 with more air flowing and better breathing motor i thought what would be the cons/pros of this being done... anyone who has done this please chime in.

Your old car was OBD1 and didn't have the advanced emissions controls. If you remove the cats on a newer OBD2 car and keep the o2 sensors, it will definitely throw a Check Engine light.

98 motor = obd2... i had obd2 in my car and check engine light was never on

Did you retrofit the all of egs/emissions? Did you have 2 or more O2 sensors? Not trying to pick a fight, but when motor swaps are performed, the OBD2 emissions stuff is usually not installed.

i get busy wtih vw's... i tricked the ecu to think the emissions were there but no check engine light...