I have a lot of friends that ride their quads with aftermarket pipes and no discs or core. They say it is not bad for it. Two of the screws on the back of the pipe on my 400ex broke off and i was wondering if it is bad for the motor to run it without a core. I know its bad to run it without a muffler but am unsure about this. Thanks for the replies

 

Running without the endcap on your 400 is fine. The main restriction is in the muffler body. You could actually run without a silencer, as long as you rejeted accordingly. The danger with exhaust removal is cold air entering the motor via the exhaust port during the intake stroke, which would cool the valves to quickly and cause them to warp.