No muffler? Mine figure is with stock muffler w/o internal baffle. I think the stock muffler reduces power a little, even with the baffle removed. I do think 9900 is a little high to be revving a YS this size. I am going to try a Bambula 16x6 to see if I can get the rpm down while getting more thrust.

I think YS 140 is too much for this 60" bird. I think the YS 110 is adequately over-powering this bird. But then, I haven't even built my Harrier 3D; I'm only using UCD3D 60 as a comparison. In my opinion, things get to a point when the additional power does not justifiably benefit the extra weight, fuel consumption, and vibration, especially for 3D.