Some cases, two strokes are better, others, 4-strokes are. I don't know why people are not judging the VERY best 2 strokes (OS), against THE VERY best 4-strokes (ie YS). I know I praise them alot on this forum, but it is true, YS are simply the best 4 bangars. Anyway, if you look carefully, the YS DZ will be very near to the OS 160 in terms of revs, seeming as thats all everyone wants here. Yet, it is smaller.......For 3D, it has a pump, it is damn reliable, throttle is smoother than silk, and it has torque in all ranges. The two stroke, isn't! Throttle isn't as linear, alot of feul is bypassing the whole combustion, and just runs out. I noticed that the exhaust on twos is much wetter, than 4 strokes, which is just smoke. Also, when you look at the smaller two strokes, with people running them at about 15,000 RPM or more, alot of power is being wasted. A guy put it like this. Air, is like ice. Put a dodge viper and a mini on tarmac, and bring them both to full power, who will win? now, put them on ice, and put them at full power, now who wins? The dodge, (which is the prop in our case), has too much power going through, and it is sliding all over the place. So don't be too sure that your prop spinning faster is translating into more air over your surfaces faster. Lastly, look at weights of engines, you'll find that four strokes have a larger displacement, but are the same weight or lighter than small two strokes. Take a YS 63 and an OS 60. The YS is bigger, but lighter. Infact, it is nearly the same as a 46FX, (but still heavier), yet has FAR MORE RAW POWER. So it is not fair to compare a 60 2, with a 60 4 stroke. The YS 110 or the saito 100 are lighter than the 91 FX, (I think!) Yet, have more power. Lastly, for hovering planes at lower throttle, the torque of a four banger is IMO alot better to live with!