When it was designed, the most popular engines used in it were the OS.77 and the Rossi.65. Both worked well. Then OS came out with the .91. The .65 was actually introduced after the .91.

I have run OS.77s and .91s in the Dynamax and liked them both. The .91 is more powerful. I have never had a problem with the .91 overspeeding. The OS.65 was the best of all the motors as far as reliability. It really did well in twin fan kits like the JMP F-4.