Well, this has been like pulling teeth, but reading between the lines and considering the few straight answers I got, I believe I have my answer. I have been offered a .91YS or a .91OS Surpass II for virtually the same money. I can only buy one because of money constraints. I have to give my answer tomorrow or the guy will sell them to someone else. These are sold as used but I don't believe either has been run. They are from an estate and the seller has no clue about RC or RC engines. There are guys waiting in line to buy these things, but the seller is waiting for me to take one or the other. I'm going with the OS.