To add to SleeperZ's response...
It helps throttle response and idle smoothness. From what I've read, Ford couldn't make the intake manifold big enough. When you nail the throttle the engine tries to pull in as much air as possible to fill the cylinders. When you just have a 3" pipe mounted to the top of the throttle body, it doesn't hold enough air to completely fill the cylinders. With the plenum box, it adds a ton of volume to the intake. When you nail the throttle all of the air in the plenum is instantly sucked out. The engine doesn't have to pull it through a little 3" pipe. It has a big-ass 16"x8"x6" box full of air ready to go.
The idle smoothness comes from the plenum box softening the IAC valve's pulses. If you just have a small pipe the pulses go back through the intake piping to the MAF. The MAF senses the air going in reverse and it leans things out. All causing lumpy idle.
I hope that makes sense.
BP
