I've been running these engines hard several years, mostly in the vertical position (ie hover) I found the 13x4 wood and 12.25x3.75 APC to produce the most pull by a good margine, but if the plane is under, say, 4#10oz or so, the 12x4 is much better. It will spoolup and chage speeds RIGHT NOW and seems to be the perfect "granny low", very linear and clean. But for a 5 pounder you really will want one of the others.

also, the 12x4 is REAL weak. The tips break so easy, so I quit useing them until I quit hittin the prop all the time.