I've been looking at used DF engines and have a question. What is the main differences between the BVM 91 R and S engines and how can you tell by looking at them? Which would be the best for general sport flying?

The "S" version has milder port timing and will be more reasonable in hot/humid conditions. The "S" version has two dimples on the sleeve as viewed from the side of the engine between the head and the case. The "R" has only one dimple.

Good luck.
Jim

Thanks Jim, I'm way down south south right know and that's good to know about the S version. What's the general consensus on the 91S in a Viojet versus OS in either the Viojet, Dynamax or Ramtec? I know I'd be somewhat limited on applications with the BVM fan, but I also have a motor mount to put a BVM 91 in a Ramtec. Will this engine work in a Ramtec fan? I've been told it wouldn't be a good match for a 91R but what about the S.

Sorry, My only experience is with the viojett/fan with the 91R. As many will tell you, this is a very reliable combination.
Good luck

Jim

Reliable once you get the induction cylinder properly installed


LOL (Inside joke for Mr. Gibb!)