Hi Ed - I dunno, just switched props, it ran OK so I flew it. I don't set the high end for absolute max RPM anyway, so I probably had a little "room" to play with. I do think that after the flight the engine felt a bit hotter than when running the 12 X 6, but it certainly performed great in the air with the 13 X 6, not a hint of being too lean. Sure impressed me! I find the Irvine a nice engine to set up, easily on par with O.S. I've got a nice tach, but didn't feel the urge to pull it out and check the RPM. I did notice (understandably) an audible RPM drop with the bigger prop.