This is all true if we assume that air behaves as a perfect gas, inlet pressure and temperatures are constant, and there are no abdiatic processes in effect. Also we must disregard some degree of fluid dynamics. Oh and I almost forgot accoustical effects.Toykilla,
The cyclinders will intake a volume equal to their displacement, but they can only intake from the supply that is available. There is a given quantity of air in the plenum. Think in terms of moles of a gas, in this case that gas is air. If we increase plenum volume, the pressure drops and the moles of gas expand to fill the volume. For ease of computation lets assume that the cylinder displaces 1 liter. If we have 100 moles of air in a 1 liter volume of the plenum above the cylinder intake, then that cylinder will intake 100 moles of air. If we increase the volume of the plenum above that cylinder to 2 liters we get a pressure drop and the 100 moles of air expands to fill the 2 liter volume. The cylinder only displaces 1 liter and now that 1 liter volume only holds 50 moles of air. By sloping the plenum we get to keep consistant pressure rear to front and a more even distribution of air to all cylinders.
A good analogy is a person exercising at sea level and then at a high altitude . There is greater atmopheric pressure at sea level than at high altitudes. Regardless of which elevation you are at your lungs will pull in the same volume each time you breath, but because of the higher atmopsheric pressure at sea level there are more moles of oxygen per given volume. This is the reason people fatigue earlier at higher elevations