Switch_639,

its a trade-off, the prop size... If you install a larger prop, you will get less RPM and a smaller prop will give more RPM. But this not mean you will get more thrust from the smaller as compared to the larger and even the reverse may not be the case. Rather than evaluating by the sound of the engine, you should see how the airplane feels flying with different props. Try them all and select the one you are most comfortable flying with. For example, I have following props: 11x9, 12x4, 12x8, 13x4, 13x6, 14x4, 14x8, 15x6, 15x4 and just two operating engines: 55AX and 91FS. I am only using 13x4 and 15x4 respectively on my airplanes as both are 3D. Remaining are stock I have tried and kept aside so I can use on my pattern ships later.

You may want to note that the lower RPM on a larger prop will be better for bearing life but will increase spool-up time marginally. Note that prop pitch also plays a role in this. Also, if you under-prop your engine, you risk overspeeding damage. So select carefully. For the record, i feel 12x8 is too small for that engine

Ameyam