Abstract: A series of four-stroke engine manifolds for providing improved power and torque over a wide range of engine speeds are described. A two-mode two degree-of-freedom resonator model for manifolds is described that predicts two individual torque peaks. The model is shown to be valid only when two described dimensionless parameters, computer from the dimensions of the manifold and the engine to which it is attached, have minimum values. If the two minimum values are not maintained, the correct resonator model collapses to a one degree-of-freedom system which provides only one, generally lower, torque peak. By requiring minimum values for the two parameters, the resulting manifolds provide the desired improved power and torque. The manifolds comprises two, three or four primary runners each connected at first ends to an engine cylinder and together at second ends to a secondary runner. A primary volume is defined as the volume of an engine cylinder with its piston at midstroke.