As noted previously, inlet metered pumps characteristically retard the start of pumping at light loads. To correct this condition, several forms of light load advance have been developed, two of which are described below.
โข The first of these, used in speed advance (used only in the 78-81 5.7L diesel), uses a smaller diameter advance piston to retard the full throttle advance curve with respect to the light load advance curve. The actual advance position at full throttle depends on the difference between the flow into the power piston chamber between pumping events and the reverse flow through the bleed orifice during the pumping event. The smaller diameter advance piston raises advance chamber pressure during the pumping event because of its small area. The flow rate through the bleed orifice is therefore increased. The smaller piston area also increases the advance motion for a given orifice flow change.