a 13x6 is a pain in the rump to tune on that engine.....because that engine cant turn a 13x6..it overheats.....i bet that 12x4 will solve all of your problems
your engine wont scream on a 12x4....14000-15000 rpm isnt screaming, 16000+ is screaming
you should prop that engine around the 13000-16000 rpm range...the perfect seems to be about 13500-14500 on the ground to get the most amount of power
you will also have a huge improvement in fuel economy