Hi Peter,
You just replied to a post that will be a year old in 3 days
I put the OS91 in the Funtana S40 and it is a perfect match, I had to get one of those BCM Pitts style mufflers for it to work properly.
The Funtana S40 has always seemed a little large for a 40 size aircraft more a 60 size but even Jims Kantana 40 was sluggish vertically on an OS61FX so the OS91FX seems right at home, Jim can vouch on how well it flys on the OS91FX set-up, it is spinning a 15*6 prop with no big issues on ground clearance.