If you ask me the Saito 100 is the best engine... however the OS91 should pull it unlimited. What prop are you using? How much does the plane weigh?

I had a GMS76 to start with on mine and even at 8lbs 5oz it was almost but not quite unlimited. Of course I didn't realize this until I experimented with different props. With a Master Airscrew and a Zinger wood prop both at 13x6 it was very limited on vertical. A guy told me to try an APC 13x6 and it went from almost no vertical to almost unlimited. It truley showed me that different props, even at the same size, can have a huge effect.