I use them because they are more efficient. Four strokes generally use have the amount of fuel and have 75% of the top end power. Four strokes are becoming more powerful.

Ys is really powerful. The ys 63 is suppose to be more powerful than the saito 72 and the ys 91 is suppose to have more power then the os 120.

I just got a os 91 with the cap 232 that I got. I like that engine. I think I needed a bigger engine to balance the plane anyway, so I can not complain about extra weight in the nose.