I'm running Xoar 14x6 with stock DJI motor mixer and it works well - upper spinning CCW and lower CW. Getting 6 minutes with 4x 5000 6S, Epic, Ziess 25mm, downlinks, batteries, etc - 24lbs total. Been searching around and so far have come across the below regarding X8 - I still need to test: smaller props w/ more pitch are more responsive, larger props w/ less pitch are more efficient (I'm opting for more response due to a battery issue I discuss below). The sweet spot prop based of Ecal running 40-60 ESC & Tiger 3515 is: 14x6, 14x7, 15x5, 15x4.7, 16x4 So it seams the options/configurations are: - same size props - lower motors spin 10% faster - same size props - lower prop with more pitch - motors spin the same - same pitch props - upper are 1in larger - motors spin the same - upper prop 2 in larger low pitch 16x4 - small prop with high pitch 14x7 (for example) I'll be testing all configurations out in the coming weeks - ultimately it sounds like you want the bottom prop to spin faster than top. I will say from my experience make sure to get good batteries! Zippy's are crap, Nano Tech A-Series 65-130c is good - I'm now using Giant Power Nano 65-130C 6S 5100Mah (I'm a distribution now if you need batteries). Make sure you get a high C rating since the volts fall off curve does not just drop off like cheap batteries. I've had an Epic fall from the sky with 23Volts at full power with 40C rated batteries, lucky after 100 foot decent at full power it slowed enough not to damage anything except some landing gear.