RE: DUCTED FAN JETS
Gents, if I may add my 2 cents. I had flown ducted fan for many years (since 1988) and have been through quite a learning curve. I have transitioned to turbines; however, miss the many jet flyers that were in the hobby since the price of turbines has eliminated the common sense pricing of the ducted fans. Personally, I have been thinking of reverting back to Ducted Fan.
On the subject of breaking in the OS-91. I believe the secret here is the fuel. I use a fuel with 22% oil. With the RPM's of these engines reaching 23,000 RPM, oil becomes your friend. We have access to Ritches Brew 10% nitro, 11 / 11 oil (22%). Which means 11% synthetic - 11% caster oil. The price is right at $10.00 a gallon. I run the engine rich for the first few flights and break the engine in while in flight.
I would also recommend using an "in flight" needle valve sold by Jet Model Products or BVM. You can richen or lean your engine out in flight using a spare channel / servo on your radio. The secret here is to look for a good exhaust trail from your plane while in flight. If the exhaust trail gets too thin while in flight, simply adjust your servo controlled "in flight" needle to richen up the engine.
On the subject of Fan Units, Jet Hangar hobbies offers the Turbax I for .46 DF engines. For the .65 to .82 DF engine I would recommend the Jet Model Products Dynamax unit or BVM Viojet. The .91 DF engine I would primarily recommend the Ramtec fan unit and secondly the Dynamax unit or BVM viojet. The reason for the Ramtec over the Dynamax / Viojet is due to the transitional design of the units. The Ramtec fan blade pitch was designed to be used with the .91 engine. It properly loads the engine to limit the RPMs while producing more thrust than the Dynamax / Viojet units. The Dynamax and Viojet units were designed when the smaller DF engines were available (i.e., the .65 to .82 size DF engines). Just from experience, I can note that the Dynamax and Viojet units require the .91 to turn at 22,500 to 23,000 RPM to product the same amount of thrust of the RAMTEC running at 21,500 RPM. In short, you will have fewer problems with your engine self destructing due to the stress of the higher RPMs required by the Dynamax or Viojet. Once again, this is my opinion based on using all three types of fan units.
Hope this helps!