I may have to dig it up The person his name is Robert and he has a post on RCU is not a 16X4 W although I would like to see the tack specks with a W. And how well it will spool up. In forums type in YZ 91 fz and there is a few post on props for that engine. looks like APC 15"- 8 or 14"-10 on 20 % nitro is working the best. Good luck.