8600 RPM, same as my Pro Zinger 15x6. Honestly I think the 16x4W is too much for my Saito .91, it does not spool fast enough, it seems to bog a little but the improvement from the 15x6 was 100 times better. It has unlimited vertical and it pulls it out of a hover with authority. I'm going to try a 15x4W instead and I think that it would be a better choice for your OS 91 as well.