The Dynamax was actually designed around the OS .65 if memory serves me. Way back when Tom Cook designed the Starfire 1, he had a pair of them at the Sepulveda Jet Rally flying on the Dynamax/OS .65 combo. I also remember seeing a JHH A-4 powered with this combination at the Miramar Airshow in 1984 or 85. Nice thing is it happily soaks up the additional power from the OS .91 as well though. Never heard of an overspeed problem with the Dynamax honestly. There's just too many of this combination out there flying every weekend to say otherwise. They work, work well, and are the bargain in DF for performance/$$ ratio.