Because I tried nova, OS and sirio 火咀 before. Sorry can't remember what mode it is as both Nova and sirio are not very stable. But the OS is very stable and quickest out of all of them. And I had been using OS since. BTW: Older STS engine follow Novarossi but the new one's are following RB as STS send an engine to RB for modification and they copy it for the new S series engine.
