Doing a conversion, 9cc equals to 0.54cuin. From the OS Engines website, ([link]http://osengines.com/[/link]) the OS55Hyper is 8.93cc. This engine has been uses sucessfully in several 50 sized helicopters. it was designed to be a direct replacement.

Can you narrow down your question? Which helicopter? Can you post a link to the engine in question?

Rafael