I have to confess that I ran about 10 gallons of green cruelpower through my K&B 4011. I quit and went to Sig, because it was turning my airplane green, and my friends were making fun of me. I flew my K&Bs on my Agile Arrow, at 720squares, and 52 oz., and on my Humongous, same area and weight. I have several K&B 4011s. The only problem I had was one day there was black around the drive washer and the front seal was gone. I swapped for another engine, and in a couple of flights, the same thing happened to it. On to a third engine, again same thing. I was losing enough fuel (all this with the Humongous) that I could not get through an OTS pattern. I put in new front bearings from Boca, and solved the problem.
I'm a midlevel Advanced flier, and my K&B 4011s ran to suit me. I did manage a 17th place in OTS with the Humongous at the 1999 VSC.