1 6 7   k W ) ▁at ▁ ▁( crit ical ▁alt itude ) ▁at ▁M . S . ▁speed , ▁using ▁+ ▁ 1 5   lb / in ² ▁" boost ". ▁The ▁F . S . ▁g ear ▁required ▁approximately ▁ 2 0 0   hp ▁( 1 4 9   k W ) ▁to ▁drive ▁it . ▁As ▁a ▁result , ▁the ▁maximum ▁power ▁generated ▁by ▁the ▁Mer lin ▁ 6 1 ▁in ▁F . S . ▁was ▁ 1 , 3 9 0   hp ▁( 1 , 0 3 6   k W ) ▁at ▁ ▁using ▁+ ▁ 1 5   lb / in ² ▁of ▁boost . ▁The ▁Mer lin ▁ 6 6 ▁used ▁in ▁the ▁L . F . ▁M k ▁IX ▁produced ▁slightly ▁more ▁power ▁but ▁because ▁of ▁the ▁use ▁of ▁slightly ▁different ▁g ear ▁r ati os ▁driving ▁smaller ▁imp ellers , ▁the ▁critical ▁alt itude ▁ratings ▁of ▁the ▁super char ger ▁stages ▁were ▁lower , ▁ ▁and ▁ ▁respectively . ▁By ▁contrast ▁the ▁Mer lin ▁ 7 0 , ▁which ▁was ▁optim ised ▁for ▁high - alt itude ▁flight , ▁had ▁critical ▁alt itudes ▁of ▁ ▁( M . S ) ▁and ▁▁ ▁( F . S ). ▁( Un like ▁the ▁Mer lin ▁engines ▁the ▁Griff ons ▁used ▁super charg ers ▁which ▁were ▁designed ▁to ▁achieve ▁maximum ▁performance ▁over ▁a ▁wider ▁alt itude ▁band ; ▁as ▁such ▁there ▁were ▁no ▁Griff on ▁eng ined ▁L . F . ▁or