The impeller is taller which moves more oil at a faster rate.. I would turn my motor with the ign off until I saw the pressure gauge move.. Do to the remote oil filter and oil cooler The HV pump would make the gauge react much quicker. You really don't need much pressure on a solid lift motor. Harleys only have about 3 lbs. Pressure will just wash the bearings out on a babit engine. But you need constant oil flow to fill the clearances. This is one reason a HV pump is a good replacement for a motor with high mileage and worn with wider clearances.