We have tested the GP 123ccc with i think all of the most suitable props - All Falcon Carbon props and these were our results below. We at the time were flying at higher altitudes so take note at lower altitudes you can expect slightly better performance.
At 3300ft asl, Running Putoline Mx5 full synthetic oil at 38:1- all on Stock mufflers
27x12 - too fast for my flying style, but some guys at 5000ft seem to like it. dont recall rpm. remember at higher altitude / less air, the extra pitch gives it a better "bite"
27x11 - rpm 6930 my current 3d freestyle favourite
28x10 - my favourite 2 blade for IMAC, rpm 6950 - works at at anything from sea level and up to 500oft asl - works well for 3d too, but personal preference for me it has too much bite at low rpm. seems to be a nice "constant speed" prop that is probably the mid range of workable prop on this engine.
29x9 - awesome for 3d only, rpm 6750 - but the tail authority with this much air over it is ridiculous, its like a giant foamy / indoor flyer
25x11x3. Ballistic good throttle response. Cant recall rpm - best 3 blader for great throttle response and pull.
25x12x3 - love this prop, but just doesn't have the grunt / thrust required for unlimited schedules at 5000ft IMAC comps, works fine at 3300ft and below. rpm is 6350.
26x12x3 - also great prop, worked even at 5000ft but again too fast for me. rpm was 6000 - too much load for 3d and a crisp throttle response. but pulls like a freight train!
same props but at 5000ft asl
28x11 6350rpm
28x10 6700 rpm
28x9.5 6800rpm
27x11 6750rpm
