I can't say that I have experience with the f800x, but I do with the vanguard engine and the icd deck. Good mowing deck, and very good engine. Vanguard is the only b&s engine I would recommend. Believe it is made by daihatsu. My dad has the vanguard big block on his snapper pro s200xt. Plenty of engine for the 61" deck. Haven't been on one with the 72" deck though.
