RE: OS 91 VRDF experts wanted
I've never held a DF engine in my hand, especially not .90 sized so I've got no specific knowledge about this engine.
What I do know is if the exhaust timing is set for high rpm on a tuned pipe, the exhaust timing will not allow the engine to develope as much torque as a regular sport engine of the same size.
A 11x8 prop might not be feasible, try a 10x8 to see if it makes a big difference with how the engine hits the pipe. You are trying to match the load to the engine to get an explosive jump in rpm.
I would find out what the optimum high rpm range was for this engine as a ducted fan model and then try to match that rpm with a carbon fiber speed prop.
To really be prepared for a tuning session with an unknown engine, you need to have the ability to experiment with various compression settings, various nitro percentages, various props that meet some sort of target speed you are trying to acheive and of course the ability to adjust the length of your exhaust system [if the exhaust system isn't already "pre tuned" for a specific application].
If I'm playing with an unknown engine, I start with the lowest compression, lowest nitro and least amount of load...then gradually work up from there.