Retarded valve timing causes loss of power just like retarded ignition timing. At low rpm a small valve opening is used for good velocity to aid mixing and good combustion to maximize torque. Valve timing is set to achieve maximum combustion pressure since flow is fairly efficient. As engine speed increases a larger opening is used to offset the shorter time to fill the cylinder. Normally an increase in lift helps midrange and higher cylinder filling. For very high rpm operation, lift and duration are increased.