RE: OS 91 VRDF experts wanted
I've got 2 identical engines except 1 engine is designed to run on 0% nitro and the other engine can run as much nitro as you want. Each engine has it's own specially designed pipe.
The 0% nitro pipe is much fatter in diameter. Both pipes are roughly the same length.
The 0% nitro engine has a very small combustion chamber compared with the engine that burns nitro. Machining your own head is pretty basic work if you find that the stock head seems to let the engine run undercompressed. Undercompressed, it just wont handle much load or hit resonance on the pipe. There is a sweet spot that requires patience to find, unless the engine manufacturer already has taken the time to find it for you and has already got the right parts to make your choice of fuel and propellor work together.
There is a German website that is dedicated to RC Speed Planes. At that site you will find the most knowledgable speed freaks who can tell you where to find propellors for your engine / plane combination.