Usually the runners are tappered from the plenum to the valves. This taper causes the air to compress inside and removes some of the air that has stopped along the runner wall.
That is not a very good description, so I will try to explain it better. The wall of the runner is not moving, so the air touching it can not move, or moves very slowly. The longer the runner the more air stacks up along the wall. If the runner is tappered the cross sectional area decreases making less space for that air to build up. The ideal taper is approximately 5% decrease in diameter per inch of runner length.
The ideal size of the plenum is 1.5 times the engine displacement. If it is any bigger than that you end up with a loss in throttle response. If it is much smaller you end up with uneven flow to the cylinders.
Hope that clears up the issue some.
