I've flown several Q-40 models with the Thunder Tiger 40 motor and 9 x 6 APC prop.

They fly very well with that combination.

The issue I had to learn was landing. These turn into a glider after the engine is shut off.

I don't think a standard 25 with muffler would be the best combination.

A Viper Q-500 does work fine with a 25. Your area is flying Q-25 models and they do work well.

Scott Hartman
Fritch, TX 79036