The 2.5 Mercruiser engine is based on a Chevy II or Chevy Nova auto
engine from the early 1960's. The engine displacement was 153 ci which
is 2.5L. A later version was called the Iron Duke, used for some small
GM cars ln the 80's + - . The 3.0 ltr (183ci) version was marine /
industrial only. Same basic engine with a larger bore .