Re: Hto make a 50 into a 55 2cyl
The 55-horse twin appeared for 1976. The increase in horsepower came via an increase in displacement from the 50?s 41.5 c.i.d. to 44.99 c.i.d. The increase was by way of a larger bore, increased from 3.06? to 3.187?. The stroke remained as before at 2.82?. Another change was chamfered exhaust ports.
To me I always thought it funny that the 50-horse had a 41.5 displacement while at the same time the 40, based on an older design, had a 43.9 displacement.
It sounds to me like the most practical swap would be the actual complete 50-horse motor. If the compression is anywhere as good as you say, the displacement deficit would be fairly negligible. Don?t forget to perform the proper precautionary maintenance on it before putting it into service!
BTW, Johnson did offer an optional high-compression head for the 3-cylinder loop-charged running mate to the 50, the 1975 Stinger 75 (also the 1975 Evinrude Hustler 75). The really high-octane gas began to peter out in 1975 as refineries shut down production of it to add refining capacity for the new unleaded gas. I don't think OMC promoted the high-compression head after that.
