nitro refers to nitro methane,most european countries use what is called fai fuel just 75% methanol and 25% castor oil.nitro is used to provide for a better idle and can increase top end rpm's.I choose to use low nitro fuel and use a slightly larger engine as it keeps the flying cost down in the long run.when i was running ducted fans the difference between 25% nitro and 10% was only about 500 rpm not really worth the expense.I got more rpm by going to a larger carb and a pump.