Anything larger than an OS 52 is really a waste on the LT-40. There is a point of diminishing returns on these planes. The 65 is a bit more than the plane needs, the 91 is WAY too much for the plane. The LT-40 is a 40 sized 2 stroke plane. Mine flies great with the 52, and now that the engine is finally broken in, I get really good performance out of it.