FrankR wrote:gd, by any chance are we comparing apples with oranges now? How do the gear ratios compare between the Merc and OMC? And was your Merc a two-blade?
It is apples to oranges. From my experience, if a 40 ci Mercury runs best with a 10P prop then a 35 hp Big Twin/RD will likely need a 13P prop on the same boat. That is anecdotal based on motor/prop testing on Feather Crafts, which is also an apples to oranges comparison.
Also, I have used an automotive timing light with a built in tach when prop testing. It’s one of three tachs I use.