In my case my YS 1.10 or my OS .91. In the last decade or so the difference between the two and four strokes has narrowed too much to bother with the old school thinking. The two stroke will always pull more and faster RPMs but the torque of the four stroke puts them about the same. I was running a SK .91 two stroke in an Extre 260 and changed that out with an OS .91 FS and other then a little top end speed the plane flew the same. Gene, keep in mind I only use the better makes of four strokes and the cheaper brand of two strokes. That makes a difference. The SK is a low RPM high torque two stroke engine, not a big RPM ball of fire. I also don't run Magnum four strokes. Only four strokes I have on hand at the moment are the YS and OS brands. Something to keep in mind.