This is an age old question, but a good one. The YS designations have eluded even the best of them. You should do two things, (1) Do a search on the subject as it's been out there before, and (2) Ask the sf question in the YS forum. I know you'll get what your'e looking for there. As far as which one, it depends on who you ask. After using YS in my competition pattern days, personally I would use nothing else. They are a bit finnicky, but once set up they're flawless in performance and reliability. You need to be a bit of a tinkerer with the carb and fuel delivery system. It's not impossible, but can try your patience at times. They also tend to be a little heavier than most comparable engines of their size, but the performance often outweighs the extra ounces. Just my opinion and Happy Holidays_bob