Hi guys,
I have a .91 FX mounted on my Extra 330S airplane, and was wondering what the ideal prop would be for this engine ? I used 14x6 for a few flights, but some people said that this could be too much for this engine. I am using Bisson pitts stlye muffler and the engine is cowled. I was being recommended to use a 13x8, so before making the switch, i thought to ask everyone here.
Thanks for sharing !
-Gaurav

Seems to be fine.

From Towerhobbies on 91FX:

           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               8,30

Gaurav,
You need to judge yourself whether the prop performed as intended, if not then you can experiment and see what you like.
A 14x6 is in the middle of recommended size and hence should neither be underloading nor overloading the engine.