- cy l inder ▁dies el ▁engines . ▁These ▁engines ▁were ▁intended ▁to ▁give ▁an ▁output ▁of ▁ ▁at ▁ 1 , 0 0 0 ▁r pm ▁but ▁in ▁practice ▁had ▁a ▁continuous ▁output ▁rating ▁of ▁only ▁ ▁at ▁ 9 0 0 ▁r pm . ▁ ▁Post - war ▁ ▁After ▁World ▁War ▁II , ▁changes ▁in ▁the ▁autom obile ▁market ▁resulted ▁in ▁the ▁decl ine ▁and ▁final ▁ext inction ▁of ▁the ▁straight - e ight ▁as ▁an ▁autom obile ▁engine . ▁The ▁primary ▁users ▁of ▁the ▁straight - e ight ▁were ▁American ▁lux ury ▁and ▁prem ium ▁cars ▁that ▁were ▁carried ▁over ▁from ▁before ▁the ▁war . ▁A ▁Fl x ible ▁inter - city ▁bus ▁used ▁the ▁Bu ick ▁straight - e ight . ▁ ▁During ▁World ▁War ▁II , ▁improvements ▁in ▁the ▁ref in ery ▁technology ▁used ▁to ▁produce ▁av iation ▁gas oline ▁resulted ▁in ▁the ▁avail ability ▁of ▁large ▁amounts ▁of ▁in exp ensive ▁high ▁oct ane ▁gas oline . ▁Eng ines ▁could ▁be ▁designed ▁with ▁higher ▁compression ▁r ati os ▁to ▁take ▁advantage ▁of ▁high - oct ane ▁gas oline . ▁This ▁led ▁to ▁more ▁highly ▁str essed ▁engines ▁which ▁ampl ified ▁the ▁limitations ▁of ▁the ▁long ▁c rank sh aft ▁and ▁cam sh aft ▁in ▁the ▁straight - e ight ▁engines . ▁ ▁Old sm obile ▁replaced ▁their ▁straight - e ight ▁flat head ▁engine ▁with ▁an ▁overhead ▁val ve ▁V 8 ▁engine ▁in ▁ 1 9 4 9 , ▁at ▁which ▁time ▁Cad illa c '