Hi Jstanton
I have an asp108 red head. I broke the engine in on omega 10%. and ran it with an apc 15x8 prop. The engine topped out at just over 10K. I later learned that asp engines are actually built FAI fuel. I haven't tested this on the 108 yet but I did try it out on a recently bought, new asp 52. The instructions said to use 10% nitro and I did. With an apc 12x6 prop it turned around 10.3K. Then I ran a tank of my home brew FAI fuel and the same prop. Now the engine turns at 11.5K and that is still slightly rich. I think your 108 will perform better with the no nitro fuel. Play with the prop sizes on the engine as well as any other engine untill you get the best thrust performance with that engine. I've been practicing this and I'm now getting the best performance of a particular engine plane combo that I have. The rule of thumb is if you have a slow flying model go higher dia. and smaller pitch. This will give stump pulling power. If you have a high speed performer then go with a smaller dia. and high pitch. This will give you speed. Bottom line, play with the props and try different fuels to see which will give you the best performance for that particular model.