Nitro is short for nitro methane, which is an ingredient in model fuel that provides some of its own oxygen, allowing more fuel to be burned than would be possible with  atmospheric oxygen only. Higher "nitro" content generally equates to higher performance as long as the engine is designed to run within the range of nitro percentages being used. Engine porting, the size and shape of the combustion chamber and several other factors determine the percentage of nitro that can be efficiently used. We recommend 15% Nitro Fuel for most applications.