Yes I knew the HP increase was marginal, but from what I've read here, the 53 is supposed to have much more torque. It does have a much longer stroke. It's my understanding that a longer stroke gives more torque. It's obvious from just looking at the irvine that it looks "taller". I guess the plane is just too darn heavy. I tried it again this evening with a tower muffler and still it doesn't perform as my avistar with a 46fx and tower muffler vertically.