I wouldnt increase the displacement. You are looking for ingenuity and engineering acumen.. A clipped wing quickie 500 racer, os 46 with a pipe and and a properly trimmed glider as a delivery system is probably the norm? I would think you would want to make things more interesting..most of last years stuff looked like they focused too much thought on the speed plane not the delivery system though. With the 16000 rpm limit the planes should all be within a few mph of each other so making the delivery system stay airborne the longest is the real challenge. How many exceeded the three minutes?