If im not mistaken, The OS 91 weighs several ounces more than the Saito 91. If you already have the engine go for it. You shouldnt run into too many problems. If not go for the Saito 91 or 100. Both are light with a great deal of power. Hey prophanger, what plane are you flying with a Saito 72 that weigs less than 4 pounds. My edge weighed over 4 pounds more than the heaviest plane you recommend for the Saito 180 and I had plenty of power. Any more would have been a waste. Anytime you are worried about overspeeding the plane in a vertical line at half throttle, I think maybe you have too much power.