http://planetsoarer.com/resonator/ResonatorsAcoustic.htmThis link deals with intake plenums (aka series resonators).
The middle of a page shows 3-axis graphs that relate volumetric efficiency (lv) (vertical axis) to rpm (n) (horizontal axis) to cylinder/plenum volume ratio (Rv) (z axis).
Two graphs are shown for various pipe length/area ratios.
As you can see, for large values of cylinder/plenum volume ratio, there is a very large bump in volumetric efficiency (which is closely related to torque)
As the cylinder/plenum volume ratio decreases (aka larger plenums), the volumetric efficiency curve is flattened (a good thing unless you like to drive in a narrow rpm band). Notably the single peak is replaced by two peaks around the original, but the amplitudes are reduced.
In simple terms, bigger plenums give more high rpm torque, but low rpm torque may suffer. It all depends on the volumes, lengths and areas. Unfortunately, I dont have the calculations for these so I cannot even suggest appropriate sizes.
As for parallel resonators... my experience with these has been the same... they remove both noise and torque.
