I had a Magnum 91-4S in my WM Intruder. It ran pretty well when new. I put a gallon of 15% CP 4 stroke fuel then switched to CP 20% heli, still OK. Then the SPA 30% in my OS hit me and on the second flight the Magnum started to sound funny and on tank #3 the funny noise got loud and the compression got soft. I now have the OS 91but I will not spend 350 on the 81 and see how long it will last. It's original size planes and 2 strokes for me now. Maybe an old OS 70 (not the POS 70FL) in an original size machine but no more big boys.

Peace

Mark O