GJAUBIN,
What you need to understand is that a prop with an 8" pitch will fly the plane faster at any given throttle setting as compared to the 6" pitch that you had previously. Pitch designates how many inches forward the plane will move with one revolution of the prop. 8" pitch is not the best choice for your trainer, it will bog the engine down forcing you to give it more throttle which in turn will fly the plane faster than it should. Go with a 10x6 and run the engine a little on the rich side.