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Old 08-30-2015, 06:28 PM
Join Date: Dec 2006
Location: Fort Worth, TX
Posts: 7,234
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There really aren't exact equivalents. With 2 stroke to 4 stroke, the rule of thumb is to go with 50% more displacement. So a .46 2 stroke would become a .65-.70 4 stroke. But that's only a rough figure, because the power production between those 2 engines is so different and the props you need to run are so different. Take my .40 size Cub for example. It started with an Evolution .46 engine and flew ok with it. Top end speed was more than the Cub needed, but takeoffs required some skill and it wouldn't vertical for beans with any prop I tried. I switched it to a Saito .72 that made less horsepower but a lot more torque. Takeoffs are effortless now because that power combination pulls so much better, and with the big prop turning about 2/3 the RPM than I used to get with the 2 stroke I also see a lot better vertical performance. So the rule of thumb gets you close, but going from one power type to another means taking into account the quirks of each one.