Even the Tee Dee .09 would not be happy with an 8" prop, it ran a 7x3.5 . The AP engines are much happier reving up, so I expect that a 6x4 or 6x5 would be the best bet. Recheck the instructions and go to the lower end of the prop sizes, especially for break-in. Tee Dee engines produced a lot of torque to swing relatively large props, they were intended to pull free-flight models straight up. Yes they would rev, but not like a modern Schneurle engine.