lift will have a greater propensity for engine knock referring now to fig3 a a graph 64 illustrates a normalized cylinder pressure trace of the exemplary six cylinder engine 12 in an hl mode and a corresponding knock signal 66 a plurality of pressure signals 68 are shown for the cylinders 18 a pressure signal 70 highlighted in bold represents a particular cylinders 18 e g cylinder 6 in the firing order the knock signal 66 indicates small levels of background noise and or small levels of knock activity these small levels of knock activity do not create audible knock and occur during normal engine operating conditions referring now to fig3 b an exemplary graph 72 illustrates a normalized cylinder pressure trace of the engine 12 operating in the hl mode with one of the cylinders 18 operating with a differential intake valve lift more specifically one of the cylinders 18 includes the intake valve 20 operating in the hl mode and the intake valve 21 operating in the ll mode a plurality of pressure signals 76 are shown for the cylinders 18 a pressure signal 78 highlighted in bold represents a particular cylinder 18 e g cylinder 6 in the firing order with a differential intake valve lift the pressure signal 78 has increased in magnitude as compared to the corresponding pressure signal 70 in fig3 a this increase in magnitude is the result of the differential intake valve lift a corresponding knock signal 80 indicates periodic engine knock activity in accordance with the cylinder experiencing differential valve lift although the plot 72 indicates that an increase in engine knock activity occurs when an ll failure exists during the hl.