Re: "theoretically, the additional 17 inches would mean an additional 12.75 horses"
No. 1/8" offset yields 1/8" more stroke, not 1/4". Final stroke: 3.656". Displacement: 259"
Additional displacement always results is less power per unit of displacement, and adding stroke is the least effective method.
Rough rule: (new stroke^.5) ÷ (old stroke^.5) = power increase.
In this case:
3.531^.5 = 1.879
3.656^.5 = 1.912
Power added: 1.8%
