Well since 1981 was the last year for the 140. I don't know what you've got.
There were basically 2 generations 140s, there was the 1972 140hp(a highly tuned Direct Charge engine). In 1978 an sightly different 140hp(Direct Charge with Power Ported Pistons) put in the line up. Rumour was that it was the same engine as the 1977 150hp inline 6. It was called a 140 because in 1978 Mercury introduced the V6 150 and apparently did not want two 150s in the line up. Thru 79 thru 1981, there were supposedly some slight changes year to year. In 1982, Mercury started rating the inline 6s at the prop and there were rumours that the crank rated 140s were the new prop rated 115s. Myself I don't think it was just a decal change, as the 115 was always had a WOT range of 5000 - 5500, while the 140 was 5300 - 5800. Comparing carbs jetting of the 115 prop rated and 140 crank ratings doesn't support the decal swap either.
In 1984 I had a 1978 90hp(crank rated) and replaced with a 1984 115(prop rated). Net difference in top speed was 10mph, which would correspond closely to a 50hp(5hp for each mph) increase. 90+50 = 140 crankshaft. I think after 1981, the 115 rating was a bit low, even for a prop rating, but it makes for a faster 115. I would guess they, and the 140s were putting out about 120 at the prop.
In Comparision, the OMC 140 only put out about 128hp at the crank, but that number was still in the 10% variance of claimed HP. On the same hull I also had a 1982 140 evenrude(crank rated), and the 115 merc had a higher top speed and better accelleration
