I'm pretty sure the 151's and 181's were the 100 and 140 hp models.. my dad had the 181/ 140 HP in his '73 Starcraft, used to push that rickidy aluminum boat to a top speed of 53-54 MPH, and it did it quite well... we have a fiberglass hull 23' Imperial. with the 488/ 3.7L and it does lower 40's planed out, but it's a smoother riding boat and takes the waves better