Well 2slow2matter Will you argue about mft. specs? I looked up a Saito FA50 typical 4 stroke I'd say. Rated HP .85 recommended prop for sport 11x7 to 12x6 I looked up the lowly SK 50 2 stroke for an apple to apple size. Rated HP 1.5 recommended prop for sport 10x8 to 12x5 Now 12x6 is close to 12x5 like I said almost even. Now turn a 9000 rpm 4stroke on a 12x6 and a 11500 2 stroke on a 12x5. Gee what will win the race. A 12x5 turning about 20% faster over a 12x6 about 20% less rpm. Rpm are just a close estimate.