▁offset , ▁a ▁mol y b den um ▁co ating , ▁reduced ▁top - land ▁to ▁cyl inder ▁clear ance ▁and ▁flat - top ▁comb ust ion ▁surfaces . ▁Im pro ved ▁off - boost ▁tor que ▁comes ▁from ▁a ▁raised ▁static ▁compression ▁ratio ▁– ▁ 9 . 0 : 1 ▁compared ▁to ▁the ▁Im pre za ▁W R X ' s ▁ 8 . 0 : 1 . ▁ ▁The ▁two ▁DO HC , ▁ 1 6 - val ve ▁heads ▁incorpor ate ▁a ▁fuel - efficient , ▁low - em ission ▁design , ▁with ▁a ▁' t umble ▁sw irl ▁air ' ▁motion ▁created ▁by ▁a ▁spe cially ▁sh aped ▁int ake ▁port ▁and ▁increased ▁val ve ▁angle ▁and ▁lift . ▁The ▁bel t - dri ven ▁cam sh aft s ▁act ▁directly ▁on ▁h ollow ▁val ve ▁st ems . ▁In take ▁val ves ▁are ▁h ollow ▁( redu cing ▁their ▁mass ▁from ▁ 5 1 . 6 ▁to ▁ 4 8 . 4   gram s ) ▁and ▁the ▁exhaust ▁val ves ▁( 4 6 . 7 ▁versus ▁ 5 0 . 5   gram s ) ▁are ▁sod ium - filled ▁for ▁more ▁efficient ▁heat ▁transfer . ▁ ▁Se qu ential ▁Tur bo ▁St aging ▁ ▁The ▁B 4 ▁uses ▁a ▁sequ entially ▁st aged ▁primary ▁and ▁secondary ▁tur bo char ger ▁to ▁deliver ▁good ▁thro tt le ▁response ▁and ▁a ▁wide ▁tor que ▁spread . ▁The ▁primary ▁tur bo ▁del ivers ▁boost ▁in ▁the ▁low ▁r pm