I have a basic question on the general benefits of higher nitro in a fuel. I do understand that higher nitro offers more power.
Just as an example (say in an OS 25LA engine), if I can achieve a desired ground rpm with 5% nitro and achieve the same ground rpm with 10% nitro (with associated needle adjustment), is there really any additional benefit to the higher 10% nitro from a power perspective?
Potentially - because you are running it richer in the second case, putting in a different place in the response curve. Generally this will make it more responsive in flight. Whether it is better or not depends on where you were before you changed it. The in-flight response is critical and any successful system requires a load response in the maneuvers - 4-2 break, medium 2- hard 2, fat 4 to medium 4, something like that.
An example, 1999 NATs (I think). Ted and I were running 10% SIG, just like at home. We get to Muncie, and while we were getting the right in-flight speed, with our setup, the engine was running about 50-50 4 and 2, and I melted the front end of my pipe. I had to turn the needle in about 1/2 turn. In-flight performance was rather wimpy compared to what we were used to. We were getting by, until Thursday afternoon, Ted said, "hey, lets try some 15%". Tried it, immediately, the needle went right back to the original position, fuel consumption was back to normal, the engine ran in a 4-stroke almost the entire flight, and it damn near pulled my arm off. At a later NATs, 2001, I think, someone complimented me on my engine run, saying my 4-stroke ran better than everyone else. I had to break it do him it was a PA61 - ran the entire flight in a thundering 4-stroke.
For the 25LA, I would start with 10%, and make sure that at a satisfactory flying speed it would never fall into a 4-stroke, because its too responsive running that way. If it went over the top lean in the maneuvers, I might try 15% to richen it up.
Brett