I have swapped the OS .61 and .91 back and forth on a couple of airplanes, and the .91 is much stronger. It has almost 1 HP more power, and the torque must be a lot higher too. If you take the center section out of the muffler on the .91 I think the weight is the same. It runs fine with the center section and baffle removed, or just use the .61 muffler. Bob