Originally Posted by puppypaws
The 921 is a considerably more powerful engine than the 850, it's 999 cc's versus 852 cc's, that in itself tells the story. I can't believe anyone would say they could not feel a great deal more power with the 921 over the 850. Basically what that says to me is they were not cutting anything that required additional power. You will fine the 921 loves fuel, 2 gph in reasonably heavy cutting will be the norm.
You obviously don't have a lot (if any) of time on two identical machines, one with the FX850 and one with the FX921. On two different models of one line, or different brands I can see some feeling that there's a bigger difference than there really is.
I have run both on the same exact machine with everything other than the model number and engine being identical for an entire fall season on both, doing both mowing and then heavy fall leaves with the deck in mulching mode. There is a difference, but it isn't that great. The 921 I used was averaging 1.8 gph in mulch mode doing heavy oak leaves which surprised me a little after everything i had read about how bad it would be.