Nitro Fuel, what percentage to use...?

This issue is no longer available. and so it is also fitting the be in the premier issue of the RCAJ News letter.
The topic of RC nitro fuel sparks intense debate: how much nitro should you use? How much and which type of lubricant should the fuel contain? Will you get more power with fuel that has a higher nitro content? Can you switch to higher-nitro-percentage fuel without harming your engine? Is it a bad idea to switch fuel brands? These are legitimate questions, and a shortage of information adds to the confusion. I enlisted the help of Jerry Conley of Wildcat Fuels to shed light on fuel-related issues so that you'll be able to make an informed decision when you buy your next gallon of fuel.
Jerry has degrees in mechanical and aerospace engineering from Florida State University. He was involved in the development of fuel systems for the Air Force and the automotive industry and has consulted for the USAF, NASA, Nissan and Toyota. Jerry is president of Wildcat Fuels; his experience in the hobby industry spans 27 years.

The methanol used in most RC fuels is very pure and of a consistently good quality. Methanol is a fuel like gasoline but is less volatile yet produces more power; it also costs a heck of a lot more!

The nitro in RC fuel varies in quality; very pure nitro runs cooler and more cleanly than less pure nitro, and the difference can be significant. RC nitro fuels have a certain percentage of nitro mixed with methanol and lubricant. A gallon of fuel with 25 percent nitro is one quarter nitro; methanol and lubricant make up the rest of the mix.

FUEL BASICS
RC fuels differ most with respect to their lubricants. The lubricant (oil) is essential to keep the engine's moving parts working freely and to remove heat. Lubricants used in RC nitro fuel are either purely synthetic or a blend of synthetic oil and organic castor oil. A coating of oil prevents metal parts from rubbing against one another, and avoids heat buildup and excessive wear. The type, quantity and blend of lubricants in a fuel are the most closely guarded secrets in the business. A few manufacturers disclose lubricant content, but most of them leave you guessing.
Conventional wisdom says the castor oil promotes cooling while the more dynamic synthetic oil protects engine components from wear. Few fuels contain only castor oil; most contain either both types of oil or only synthetic oils.

The flash point—the temperature at which a lubricant will burn—varies with lubricant. Lubes with a very high flash point tend not to burn at all and leave the engine awash in oil. This certainly helps keep an engine well-lubed, but it can cause excessive oil discharge in exhaust. Lubes with a lower flash point tend to burn with the fuel, and this leaves a cleaner combustion chamber for the next cycle.

WHICH NITRO PERCENTAGE SHOULD YOU RUN?

This is an “It depends who you ask” question; in fact, no fuel is ideal for every application. Nor is it a given that a higher percentage of nitro results in better performance. The correct percentage of nitro and its effects on engine performance are rather complex issues, but I'll try to cover the basics. To understand how nitro content affects

NOW, HOW MUCH NITRO SHOULD YOU RUN?

Maximum performance means an engine is running at maximum efficiency with maximum horsepower. The bottom line is that using high-nitro fuel that far exceeds the parameters of your engine may only net a slight gain in horsepower, but overall performance is compromised; so, more isn't necessarily better where nitro content is concerned.

When engineers design an engine, one of their primary considerations is the type of fuel to be used. The compression ratio and the timing and duration of the intake and exhaust ports take fuel composition into consideration. For example, Fedération Aeronautique Internationale engines—popular in model airplane competitions—are designed to run on pure methanol. The competition rules specify that fuel must not contain nitro. Adding nitro to the fuel used in these high-compression engines usually results in poor performance and high running temperatures.

Similarly, 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.

STORAGE

Methanol-based fuel is hygroscopic. In plain English, that means it sucks in moisture like a sponge. If you leave the cap off a methanol fuel bottle for a couple of minutes, you'll see moisture accumulate inside the bottle, especially in humid conditions.

Since methanol also evaporates very quickly, it's important to keep the cap on your fuel and filler bottles. Even with the bottle capped, dramatic changes in temperature accelerate evaporation and moisture accumulation. If you have to store fuel, even for a short period, keep it out of the trunk of your car and off the cold basement floor. Try to keep fuel in an area where temperature remains relatively constant. If you must store it in a basement or garage, insulate the bottle from the floor with a piece of scrap carpet or lumber. If you are careful, and avoid conditions that make fuel go bad, you'll be able to safely store fuel and use it up to two years later!