It is the Delta D=differential pressure not over-pressurization when Discharge Valve and over-press valve are functioned on.
Flying higher would result less efficient in engines to produce sufficient pressure to maintain cabin altitude thereby inadvertently increasing the diff press. regardless of your selected cabin altitude.
Below 8.7 Diff, Discharge vlv is operating, Above 8.7 Diff, both Discharge vlv and Over Press vlv are operating. Above 9 diif., cabin press is decreasing regardless what setting you selected.
Last edited by Kuchan; 24th Mar 2012 at 12:31.