Early in line merc 4 cylinder

[mk55]

I know the first inline 4's said 25 hp but in reality were they the same as the mk 50 and 55 at 40hp. If not what changes were made to them as they evolved.

[Mark75H]

The castings and porting are completely different. The early motors have divided exhaust, the Mk50/55's do not have divided exhaust, but it came back on the 44ci Merc 500.

[mk55]

Would you consider the early sand cast merc stronger than the mk 55 or about the same and is the divided exhaust the tuning that raised the 45 hp to 50hp?

[Mark75H]

is the divided exhaust the tuning that raised the 45 hp to 50hp?

Apparently, yes.

Old sand cast 40ci motors have much weaker crankshafts and magneto drives; not worth the effort to try to change to compete with the newer 40's. For souping up fishing motors the later 40's could swap front cases with 44's and use the bigger carbs and reed block inlets without any machine work.

Mk55H's came with the larger Mk58 style fronts and bigger carbs