Nowadays, stock exhaust systems and pipes don't win any beauty contests (and they rarely did in the past), they weigh much more than their aftermarket counterparts, they sound horrible, and you never reach the bike's full potential with them. Whatever the reason for replacing the exhaust pipe or the entire system, if you remove the exhaust valve, the dreaded fuel injection light will appear. Install the ESE to fix this, so that the fuel injection light comes on only when it's needed, i.e. when there's a problem with the engine.