Yeah , its not like they stretch a 50hp up to 70hp , they build a 70 hp and then detune it down to 50hp . Some things are hard to change ....like port timing or sizing , primary compression .
Actually the opposite was the case for decades, more hp was squeezed out of a given displacement, just about every model yr. Some examples of many were; OMC's 70.7 inch block was originally, a 50hp, later becoming a 60, and finally a 65. Same went for their 2bbl 89.5 inch block, 75hp, then 80, finally 85. A 4bbl version of the block, 90hp, then 100, and as a limited edition 115. The 49.7 inch 3 cylinder Looper started as a 55, then 60, 65, 70, and 75.
Mercury also did the same. the 99.9 inch inline 6 started out as 125hp, later 135, 140, 150, and 115. The 4 cylinder 66.6 inch started as an 80hp, later becoming an 85. Prop Rating dropped it to 75hp. Merc's 3 cylinder 49.6 inch block started as a 65hp, became a 70, and a highly modified version was rated at 90hp
