I would not recommend the 13X4 at all. I have had terrific results with the 12X4 and the 12.25X 3.75 APC's, depends on what you are trying to accomplish. The 12X4 with the narrow tips is very sensitive to even grass blades. The 12.25 with the wide blade can almost be used as a weed eater with great midrange pull out from a hover. Hope you have great luck with your Magnum .52. A tower .46 muffler on this engine also gives you about 300 to 500 more RPM's on the top end.