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long and healthy life, and you decide to consult a doctor for advice as to how to accomplish this. When you come to the subject of food, you say, "Tell me, Doctor, if I wanna still be healthy and virile at 90, how do I eat?" The good doctor replies, "M'boy, if you will eat two pounds of food a day, you'll be fine!"
My guess is your response would be something along the lines of, "What kind of food., Doc? After all, no two are exactly alike. Is that two pounds of lettuce or two pounds of pork chops?" If he replied, "It doesn't matter. Just as long as you eat that two pounds every day, you'll probably outlive your kids." You'd probably run, not walk, out of that quack's office!
Why then do we blindly follow someone's word when they say, "Thou shalt use no fuel that does not contain XX% oil." It makes no sense to me, nor do I think it will to you, if you stop to think about it. All foods are different, so are oils.
If that's true, why do the instructions with my engine specify a fixed percentage of oil? Simple: to protect themselves. All engine manufacturers have been burned in recent years by "bargain-priced" fuels containing either inferior oils or insufficient amounts of oils.
Everyone that I've talked to will admit off the record they know fuels containing good oils won't need as much as their instructions say. But they also say they have no control over that, so they are going to print a high number, in hopes the amount of even a cheap oil will be sufficient. Frequently, it isn't.
So why not just put a lot of oil--at least 20% or more--in fuel and not worry about it? There are several reasons. For example:
Any more oil than is necessary makes the engine run really badly. Think about it. Methanol burns, oil doesn't, or at least it shouldn't. Common sense tells us the less oil (non-burnable) we can safely use (to an irreducible minimum point, of course), the more methanol (burnable) we will have in our combustion chamber. More burnable ingredients equals more power. One well-known magazine writer, with more than 50 years engine experience, tells me that for every 1% oil removed from model fuel, the effect is about the same as adding 1% nitromethane. And it costs a lot less!
By the same logic, the less oil we use (to the predetermined minimum, of course), the less the oil is going to be dousing the glow plug element, and we should be able to achieve a lower, smoother idle.
Next to nitromethane, oil is the most expensive ingredient in model fuel. By not using an unnecessary amount of oil, the manufacturer can keep the cost of the fuel down, putting a smile on modelers' faces. Remember, even another 25 cents in manufacturing cost translates to an additional dollar or more at the retail level.
So, what is the right amount? It depends on what kind of oils, in what combinations, with what additives, etc. For what use (sport airplanes, racing, helicopters, boats, cars, ducted fans)? What size engines? (As engine size increases, they need progressively less oil. Why? Simple mathematics. Surface area of the combustion chamber increases at about half the rate as the displacement increases.) Most people know that the big Tournament of Champions and Unlimited racing engines use oil in the 4% to 5% range.
Ducted fan and helicopter engines typically need more oil, 4-strokers or less. It surprises most airplane fliers to know that top competition model car engines use fuel with oil contents in the single digits, even though they are turning in the 40,000-50,000 rpm range, and have no fan in front to cool them! As matter of fact, they will hardly run on regular airplane fuel.
from the newsletter of the
Itasca R/C Club
Gail Lane, editor
Grand Rapids MN
Don Nix is the president of GBG Industries, Inc. The article was reprinted in the Itasca R/C Club newsletter with permission from PowerMaster Fuels.
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