RE: 4-Pitch propeller and engine tuning
I run an APC 14x4W on my Magnum .91 2C Runs GREAT and turns about 11K on the ground (from memory [sm=confused.gif])
I have also run an APC 15x4W. Doesn't pull any better, spools up slower but larger prop wash, comes down to personal preference.
Maybe ST engines don't like small props [sm=confused.gif]