The new GM VORTEC gasoline engine has high fuel efficiency and low cyclic variability of combustion- atributes which resulted from fast burn. Organized mixture motion, in the form of swirl, is an element essential for fast burn. The focus of this study is on in-cylinder air motion and its importance to combustion.Quantifications of in-cylinder air motions resulting from the intake process (swirl) and from piston motion (squish) were made. Swirl was determined experimentally for both straight and helical intake ports. Squish velocities were derived analytically. To determine the relative importance of squish and swirl, the amount of swirl-flow energy dissipated was compared with the amount of squish-flow energy generated. The comparison showed that swirl was the dominant in-cylinder flow.The impact of swirl on combustion was quantified, and the minimum amount of swirl needed to measurably influence engine operation was determined. These results also explained why helical ports dramatically improved VORTEC engine smoothness and utilization of 91 RON fuels.