I always look forward to reading these YS comparison posts, and how they always get bigger and better. I don't think that I'm going out on a limb here when I say that a YS 91 does not develop the same horsepower as the 91 FX. And yes I realize that due to different mechanics and torque curves it is hard to compare a 2 stroke to a 4 stroke and vice versa. I call it the 4/3rds rule, given a certain displacement YS engine, there will always be someone who will compare that engine to another 4 stroke with a displacement 4/3rds of the YS.

Also, despite the above, this in no way is a post bashing YS engines. I have the YS 63 FZ and that damn thing might be stronger than my saito 72.