In the foreground is an intake manifold designed and ported to work with the Dart 9 Degree head. In the background is a Cup manifold. Notice how much smaller the plenum area on the manifold in the foreground is. The plenum is the open area inside the manifold before the runners start. A large plenum is necessary for a high-rpm motor but can kill low-end torque. This is why the 9 Degree manifold has a much smaller plenum area with visibly longer runners.