I am flying my new Ecstasy, which is a 7-1/2 lb. proto-type with a 90 ST, Mac Long Header and Tuned Pipe & an APC 15x4W prop. It has 1255 SQ. Inches of wing area that hovers just over 1/4 throttle. An OS91 FX should be great on this aircraft. I would question the 91 - 4C. With my personal taste, I think I'd go the 120 4C if I was going the 4C route. The 91 4C will fly it for sure, just not quite enough HP for my personal taste

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Kevin