RE: Clipped Wing Diamond Dust / OS DF 25
General rules of thumb....add castor oil to bring the % up to 20% for break in. I would run it with a 7x5 prop for break in. Personally, I believe the engine should be broke in open exhaust [coolest possible configuration] and use a high pressure latex bladder to feed the engine. Once the engine is able to run a 2 oz load with a steady needle, then it is time to play with the pipe.
Get a baseline open exhaust tach reading with a 7x6 and 7x7 prop, then you'll have something to go by when adjusting the tuned pipe length.
If you can get the engine to hit 24,000+ with a 7x6 or 7x7 prop, the DD's speed should be pretty impressive...