harmonic resonance and volumetric efficiency

how is possible to obtain 2.5L of air  from a 2L engine?

plenum and runner section

All motors will make the most torque at a specific rpm. This is due mainly to the length of the runners in the intake manifold. When the valves are open the air is flowing through the runners. When the valves close, that flow is backed up and a pressure wave reverses back up the intake runner into the plenum. Then the pressure in the runner is lower than in the plenum and a wave is sent back down the intake runner. This oscillation happens multiple times. By changing the length of the runner, you change the distance the pressure wave travels and how frequently the oscillation happens.

This is optimized to have a pressure wave moving down the runner just when the valves are opening
again to have the air rush into the cylinder. By doing this you can for example get 2.5L of air into a 2L of cylinder volume, This is why engines have more than 100% volumetric efficiency. This is why torque will peak at one certain rpm for car engines.