A hotter plug generally works better in colder weather. So does a higher nitro content, but don't bother with that until you are ready to risk damaging engines.

Prop size and effieciency is controlled by many factors. Some of wich are engine displacement, rpm, weather conditions, type of airframe, weight of the prop, and many, many other factors. Your planes best performance is going to be found by experimenting with different sizes and makes of propellers. the manufacturers suggested prop sizes are exactly that, suggestions only and a good starting point. One size prop will give different performance if used on the same engine that's moved between two different planes of the same weight and similar size.

For all of you flying the Ultra Stick, add a piece of 1/2" triangle stock to the bottom side of the wing hold down plate if it doesn't already have one. Under extremely high wing load conditions, the wing hold down plate has been known to separate from the fuselage in flight, taking the wing with it.

The OS and Irvine engines are both very good engines. The older OS engines (SF, FSR) were, in my opinion, better than the current generation of OS engines. Those of us who had been using the older engines had the most reliable engine that money could buy at the time. I don't believe that to be true today.

Enjoy,
Silversurfer