Re: 75hp Chrysler Switching from a 1 to 1 ratio lower Unit to a 2 to 1 off a 90hp????
The 75 is a 1.76 ratio and the 90 is a 2 to 1. If the engine is currently running within the recommended RPM range, start out by going up 2 pitches. That is, from a 15 to a 17.
Since you already have the 19 pitch prop and since the 18 spline props are getting more difficult to find, try it. However, if you don't have a tachometer, buy one. As with any voyage: "You cant get there unless you know where you are starting" and unless you know what each prop is turning, you simply can not evaluate the changes.
The bright spot is that the Motorla ignition uses the standard 12 pole Teleflex tach which is still available