If it were me,I would go to wherever I could find the engine listed, like tower hobbies for instance and find the manufacturer recommended prop size for an engine and start there. Like your OS engines are here.. [link=http://www3.towerhobbies.com/cgi-bin/wti0002p?M=OSM]OS [/link] and the evolution can be found here [link=http://www.evolutionengines.com/]Evolution[/link] engines

You usually cant go wrong with the listed break in prop as a good starting point for any engine. In my opinion.

Here is what I found at tower about a .91 OS 2 stroke. Props: Hobby Services recommends a 13x8 or 13x9 prop (6/99)


COMMENTS: All FX series engines are covered by the O.S. two year warranty
Engine mounting & exhaust dimensions are the same as .61FX.
Engine guru Clarence Lee tached this engine w/various props
Prop Size RPM
13x6 12,500
13x8 10,400 using a 13x8 prop,
13x10 9,500 Hobby Services tached this
14x6 10,900 engine at 11,300 RPM
14x8 9,500 6-24-99
14x10 8,400
15x6 10,200
15x8 8,100
16x6 8,300