Nitro fuel (sometimes referred to as glow fuel) is a pre-mixed blend of nitromethane and methanol in a base of either castor oil or a blend of synthetic and castor oils used to power glow plug-ignited R/C engines. The methanol serves a dual purpose, acting both as a volatile and to react catalytically with the platinum element in the glow plug to keep it "lit" between power strokes. The base oil also serves a dual purpose as both a carrier for the volatiles as well as a lubricant for the engine's internal components.
The fuel is labeled with a percentage number which refers to the amount of nitromethane present in the blend. 20 and 30 percent nitromethane fuels are excellent all-around fuels for racing; most fuels for general use contain from 5 to 15 percent nitro. Ultra-high performance aircraft fuel may have as high as 60 percent nitro content.
Oil percentages are listed as well, but not as boldly. Generally, oil takes up between eight and sixteen percent of the mix. 12% is a good all-around number found in quality fuels. Fuel with extremely low oil content, sometimes as low as five percent, is used in highly competitive surface model racing to aid in bringing the engine up to operating temperature quickly. Fuels of these types should only be used by experienced racers and tuners to prevent engine damage.
Naturally, the higher the nitromethane content, the more power (and the more heat) the engine makes. Engine displacement has a bearing on nitro content as well. Larger engines actually require less nitro in some cases. As the amount of nitro goes up, the amount of methanol goes down to compensate. One should not use fuel with a high percentage of nitro without retuning the engine with a colder glow plug and necessary carburetor adjustments, nor should one substitute fuels intended for model aviation in surface vehicles and vice versa since air and surface models have their own unique operating parameters and the fuel is carefully blended as such. However, some blends suitable for helicopters may be used in boats; these fuels will be labeled as such. One company, Powermaster Hobby Products, does make a universal sport fuel with 18% oil content and a nitro content of between 5 and 20 percent.
Regardless of the brand of fuel used or the type of model being run, some engines cannot be tuned to accept high-nitro blends and must be operated per the manufacturer's recommendations.
See also: Nitro fuel myths