I would say that you would be in the ball park at the same RPMs as the FP-20, or at least a good starting point. I ran a FP.25 on my P-Force and would take off at 10,500 RPM with an APC 10-4.
   Good luck and have fun,
   Dan McEntee

So you were running a 4-2 cycle Dan?
Also, I was wondering if the .25 "should" have a larger prop than the .20; 9-4 is commonly said to be best/largest for it.

  "Best" and "largest" are definitely NOT the same thing. Back in the old days, the only way to harness more power was to use a bigger prop. That's not at all true for these engines. The 25FP would probably swing a 11.5-4 fast enough to fly a Twister-sized airplanes, and I have seen people use 10-4 Tornado 3-blades on 20FPs with adequate performance (on a Ringmaster, of all things). For a Twister, for example, I would suggest something like a 10-4 Rev-Up (inefficient so it wastes some of the excess power).

    A 25FP with a 9-4 probably wants to run about 14,000 on the ground to wind up in the right setting in the air. Then see what you get. If you get too much speed (likely, depending on which model we are talking about), then you might want to look into using an air filter or start restricting the venturi. That's why I think the 25FP is too big for these smaller 35-sized models, you have to choke them down excessively.

      Brett