On the 737, During 1 pack ops the APU must supply higher bleed air pressure to assure proper control system operations. This higher pressure requires a greater Inlet Guide Vane (IGV) open position than that required for 2 pack operations. Since there is less airflow required to operate 1 pack than is needed, a significant amount of unused bleed air is exhausted through the surge control valve. Higher IGV open position and large quantity of unused air translates into higher APU fuel burn and higher turbine inlet temps during 1 pack operations.