I'll try to explain the comments in my prior post a little better. For optimum performance a fan needs to be matched to an engine much in the same manner that we pick the right diameter and pitch for a propellor plane. If the propellor is too big you load the engine too much, the rpm is low and you loose performance. If the propellor is too small the engine will over rev, use extra fuel, loose performance, run hot, and put a lot of extra wear and tear on the engine.

If you've ever seen an engine performance curve you know that as rpm increases hp increases until you reach the hp peak, after that power drops off as rpm increases. For best thrust you would typically end up with a larger diameter/lower pitch propellor that probably runs at less than hp peak rpm. For best speed (air velocity) you would typically pick a smaller diameter/larger pitch propeller that runs very close to the hp peak rpm.

In the case of a ducted fan there other factors associated with air flow and ducting that enter in, but everything else being equal you basically still want to run the engine close to it's horsepower peak for best velocity.

The information that comes with the OS91 engine says that the engine puts out 4.8 hp @ 22K rpm. Now 22K is pretty close to what it will run with a Ramtec fan. This fits with the story that the Ramtec was designed to match an OS91. The Dynamax was designed back when 65-77 sized engines were available, so it doesn't load an OS91 as much as a Ramtec. Consequently a Dynamax will let the engine rev up to about 24K rpm, producing about the same amount of thrust as the Ramtec does at 22K. Some that have run the Viojett/OS91 combination tell me that the OS91 over revs (maybe 26K) and the engines don't last very long. The only way around that is to run overly rich to slow the engine down, thus loosing performance. That would suggest that a Viojett/OS91 is not well matched.

BVM does not advertise their engine perfomance numbers, but I would guess that the BVM 91 is about 5.5-6.0 hp at 28K, and pretty well matched to the Viojett. These engines will hold up well running at much higher rpm that an OS91.

Based on all my years of engine design and development (automotive), I know that for a given displacement the kind of things you do to boost high rpm hp typically degrades output at lower rpms. There is no free lunch, aside from efficiency improvements you basically tilt the hp curve trading off low rpm output for high rpm improvement.

This is speculation on my part since I have not seen the hp curves for each engine nor have I tested them, but I would expect a BVM 91 to have less output at 15K and 20K that a OS91.

If that is true then a BVM91 would not perform any better, and probably worse, than an OS91 in a Ramtec fan. A Dynamax fan would be a better match for a BVM 91 since it would allow the engine to rev a little higher, but I doubt you'd see any noticeable improvement over an OS91. Neither combination would be up to par with a BVM91/Viojett.

Again this is speculation on my part. The rpm numbers quoted above are estimates, your actual results may vary. It would be interesting to hear from someone who has tried a BVM91 in either a Dynamax or Ramtec fan.

Joe