14x8 and 15x6 are props I run on my Saito 100, I think you are running to much prop. 14x7 for speed or 15x5 for thrust on a Saito 91. You can not gain thrust and speed with a prop change, higher pitch = less thrust, more speed,...lower pitch=more thrust ,less speed. The only way you can gain speed and thrust is with more horsepower.