I am building a 1/2a proto too, I bet the 4.2 apc will be good for that because you can whip the plane for take off then you have 1.5 laps to get into the pylon or center circle. I ran a 4.6x3 glass prop on an AP and 40% fuel, it spun up to 29k then broke about 2 minutes later. I replaced the carb with a venturi. If I would have used milder fuel, maybe it would have made a few runs.
I will use the GZ .049 for competition or the Cyclon, Problem with that Cyclon is the lack of substantual beam mounting lugs for use in 1/2a proto
I think a "Club" class event that only allowed the AP or other non bb engines would be a blast to play in on the local level.