The tower .75 is 23 oz with muffler and the K&B is around 17oz with muffler(depending on which muffler). The Tower is very similar to a GMS. The difference in weight of the airplane is more than the amount of difference in engine weight. In this case the difference in airplane weight between these two engines would be around 12oz(more or less depending on where you could stick the weight) for the same cg placement. This can change the flight characteristics. My Phoenix Suchoi was extremely sensitive to cg changes going from fun to fly to spontaneous snap rolls during the fuel burn. The tank size allowed in the suc is also not very large. One would tend to think that the .75 might use more fuel than the .61. [&:]