RE: Prop Suggestion for OS .46 AX on an Extra 300S
You need to break the motor in, it takes a while. OS engines run great out of the box, but they do continue to make power as they get used. I had a .46AX that eventually spun a 12x5 at around 12,000 rpm.
If you like 3D flying, try the 12.25x4.75 or whatever it is. If you want speed, try a 10x6 or 10x7. 11x6 is a good all around prop.
A lot of it depends on tuning and fuel too.