Note that the Ford Racing manifold (left) has slightly less plenum area but longer runners. It also eliminates the inlet butterflies in the factory manifold. Known as charge motion control valves from the factory, they are an emissions-control device designed to increase air speed at low rpm. While customers will experience no driveability problems with eliminating this system, the car’s ECM Will need to be reprogrammed to compensate. The computer will notice the missing CMCVs, trigger a “Check Engine” light, and alter the ignition timing to compensate.