The first problem you have is that YS motors use a different induction system, so comparing them to anything else would not work. comparing 2 stroke to 4 stroke would make more sense if you compared an OS 2 stroke to a an OS 4 stroke. Same quality of motor built for a similar application. You will find that prop size generally follows displacement size as rule and that bigger props at a lower rpm are more efficient. Because 4 strokes and 2 strokes motors due the same job at comparable weights and a 4 stroke uses less fuel per unit of output. 4 stroke would be favored for most general applications, but cost is a big factor as well. What are you trying to prove here is my question. Big props more efficient then little one? 4 strokes are more efficeint then 2 stroke?, etc. This is why your doing this class so you can sort these things out. good luck