Thanks for your input, 1QwkSport2.5r. I haven't had a chance to run the engine with the smaller props. In the meantime, I heard back from Mr. Josef Svajda of MVVS and this is what he had to say, "

for standard muffler the 14x8 prop is probably too much. Each prop mark has another air-resistance, the RpM for the same marking prop from various are up to 1000-1500 different There should be 10000RpM on the groud at least Use 13x7, or 13x9 prop(APC) Or use the tuned pipe 3250 (3255N is NOT tuned muffler)"

Thanks again for the input and in the meantime I'll try to play around with different props, pitches and nitro content.