Almost 1 year ago I started a topic on the diydrons forum in wich I asked on how much efficiency loss I will have on converting my quad to an X8. Almost no reaction to my question. Nobody could give some figures wich I could use for my calculations. The only helpfull answer I got was from somebody who said that the efficiency loss depends on prop size and distance between the props.
Time has past, other projects are closed so I'm starting with the build of a Y6 multirotor. And the first thing before building the frame is a thrust test of 2 motors in sequence. Result is a 10% efficiency loss. See full graph and info on my blog at http://wipo-y6copter.blogspot.be/
This figurs can by handy for everyone who is building Y6/X8 copters. Currently I'm building the frame, but I will try to post more efficiency loss tests for other prop sizes asap.