boost " ▁( the ▁" boost " ▁is ▁the ▁pressure ▁to ▁which ▁the ▁air / f uel ▁mixture ▁is ▁com pressed ▁before ▁being ▁fed ▁to ▁the ▁cyl ind ers ). ▁The ▁limitation ▁of ▁the ▁single ▁stage ▁super char ger ▁was ▁that ▁the ▁maximum ▁power ▁dropped ▁quickly ▁as ▁higher ▁alt itudes ▁were ▁reached ; ▁because ▁air ▁pressure ▁and ▁air ▁density ▁decre ases ▁with ▁alt itude ▁the ▁efficiency ▁of ▁a ▁p ist on ▁engine ▁dro ps ▁because ▁of ▁the ▁reduction ▁in ▁the ▁weight ▁of ▁air ▁that ▁can ▁be ▁drawn ▁into ▁the ▁engine ; ▁for ▁example ▁the ▁air ▁density , ▁at ▁ ▁is ▁ 1 / 3 ▁of ▁that ▁at ▁sea ▁level , ▁thus ▁only ▁ 1 / 3 ▁of ▁the ▁amount ▁of ▁air ▁can ▁be ▁drawn ▁into ▁the ▁cyl inder ▁and ▁only ▁ 1 / 3 ▁of ▁the ▁fuel ▁can ▁be ▁bur nt . ▁ ▁Two - Stage , ▁Two - Speed ▁super charg ers ▁ ▁The ▁most ▁fundamental ▁change ▁made ▁to ▁the ▁later ▁Mer lin ▁( 6 0 , ▁ 7 0 , ▁ 8 0 ▁and ▁ 1 0 0 ▁series ) ▁and ▁Griff on ▁engines ▁( 6 0 ▁and ▁ 8 0 ▁series ) ▁was ▁the ▁incorpor ation ▁of ▁a ▁two - stage , ▁two - speed ▁super char ger , ▁which ▁provided ▁a ▁considerable ▁increase ▁in ▁power , ▁especially ▁at ▁higher ▁alt itudes . ▁Two - stage ▁refers ▁to ▁the ▁use ▁of ▁two ▁imp ellers ▁on ▁a ▁common ▁driv esh aft , ▁const itut ing ▁two ▁super charg ers ▁in ▁series