I replaced mine because I heard a tapping sound coming from the head, and suspected the intake air bypass valve diaphragm to be faulty. Needless to say, that wasn't it, but I'm happy none the less. The stock manifold is heavy, takes up a lot of valuable space, and the whole Intake Air Bypass system is full of parts that could potentially break. Its also easier to replace your oil filter, and fiddle with your wire harness once you go aftermarket. :thumbup:

The stock manifold also has a single plenum, but just dual runners. Its a nifty idea for adding a little zip and a little mileage at once, but its impractical for high performance. You're diverting air (which causes resistance) across both runners, and then that air collides on the other system near the manifold which causes resistance. BMW or Mercedes is doing an infinitely variable intake tubes for one of its big engines to get the same result. Good milage and torque low, a little more power up high... no valves or air diverters to worry over.