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Source: Nitro RC Magazine
RC car engines are designed to run with a certain percentage of nitromethane. An engine designed to run on fuel with 20 percent nitro is configured for a certain compression ratio and a fuel/air ratio that provides ideal performance. Introducing a considerably higher concentration of nitro allows a greater overall amount of fuel volume. It can result in higher compression (air is the only compressible component in the fuel mix), detonation (when fuel explodes rather than burns) and higher operating temperatures. These outcomes show the fuel exceeds the design limitations of the engine. Even if none of these symptoms are patently obvious, combustion is compromised, and that prevents fuel from being completely burned. It can create an unstable idle and erratic fuel-mixture settings. These conditions are common in engines running on higher than recommended percentages of nitromethane. Essentially, the answer to the “How much nitro?” question is that you should follow the engine manufacturer's recommendations.
There is a “window” of about 5 percent both above and below the recommended percentage of nitro you can have in your fuel before engine performance will be noticeably affected. If a manufacturer recommends 20 percent nitro, you can get away with 25 percent. Go beyond that, and you are likely to wind up with an engine that runs hotter, gets poor fuel economy and is more difficult to tune. If your engine's manufacturer doesn't recommend a specific percentage of nitro, you may have to experiment to establish the proper balance of horsepower and efficiency.
CAN YOU CHANGE THE AMOUNT OF NITRO
IN FUEL WITHOUT DAMAGING THE ENGINE?
Yes and no. Yes, you can change the percentage of nitro in your fuel safely, but you must not make a change and run the engine hard right away. Every engine undergoes a process called “hysteresis.” In hysteresis, the engine components slowly acclimate to the specific cylinder pressure and engine temperature that result from burning a particular type and blend of fuel. A change in fuel changes the whole picture for the engine. An engine that is accustomed to a certain amount of expansion using its usual fuel might now have to cope with increased cylinder pressures and the additional expansion caused by higher temperatures. Simply pouring a new fuel into the tank and “letting 'er rip” places undue stress on the engine components.
If you use a different blend of fuel, your engine has to go through a second “break-in.” Run the fuel mixture slightly rich and ease into the throttle for a few tanks before you nail it. If you fail to follow this procedure, your engine will still survive 99 percent of the time, but it will last longer and run stronger if you make a gradual change to a new fuel. There is also a break-in period during which using less nitro presents a new set of parameters for the engine.
