I found that fuel with 10% nitro works well. Fuel with higher nitro levels than that makes the engines run hotter with out any real increase in power and the higher price is just wasting money. I tried some a couple of times and don't bother with anything but 10 % now. Whats also important is to make sure that the fuel has at least 18% oil content. Some fuels out there have only 12 or 16 % oil. Have no idea what these fuels are good for but the RC engines don't like it. [8D]