SPMRC Club

2-&4-Stroke Fuels

The following is the forth in a series of articles exploring all facets of model engine fuel.

The writer is Don Nix, founder and former owner of POWERMASTER FUEL.

Well, what do you think? Is there really a difference, or is this merely a big hype by the fuel manufacturers to sell more products? Let ’s see a show of hands - ah, yes - about evenly divided. Well, let ’s explore the facts.

Fact: Most 4-stroke model fuels contain less oil than comparable 2-stroke fuels. The most common response to this is, “But 4-stroke engines have more moving parts, they should need more oil, not less!” Well, that sounds reasonable, but it doesn’t stand up under close examination. The number of moving parts has nothing to do with it. What is important? Think about it.

Fact: With rare exceptions, 4-stroke engines run at substantially slower RPMs than a comparable 2-stroke engine, most in the under-10,000 rpm range vs.12,000,13,000 or more for a typical 2-stroke of the same size. They are engineered to deliver maximum power at slower rpms, with bigger props. What does this have to do with it? One of the main factors used in determining the proper oil content of fuel is heat. To use the well-worn term, it doesn’t take a rocket scientist to figure out that the more slowly an engine turns, the less heat it generates from friction. If you don ’t believe that, rub your palms together slowly, then as fast as you can. So, lower rpms = less heat = less need for oil.

Fact: 4-stroke engines only fire every other stroke, vs. every stroke by a 2-stroke engine. Firing, or combustion, burns fuel, which creates heat. Logically, it may be deduced that if there is fire in the chamber only every other stroke, the engine has time to cool off a bit between combustion cycles. Let ’s take that a little further. Using a hypothetical 4-stroke engine turning 10,000 rpm = 5,000 combustion cycles per minute, vs. a hypothetical 2-stroker turning 13,000 rpm, with the same number of combustion cycles per minute, the gap widens. The 2-stroker has 160% more combustion cycles than the 4-stroker. Even though this is partially offset by the fact that at least some 4-strokers have a higher exhaust gas temperature, the message is clear: 4-strokers remain cooler, and need less oil.

Fact: Oil doesn’t burn (or shouldn’t)- methanol does. Using a little logic, we arrive at the conclusion that a properly made 4-stroke fuel will deliver better performance than a 2-stroke fuel in the same engine. Why? Remember …the 4-stroker is only firing every other stroke. This results in the plug element wanting to cool down between strokes, resulting in a “colder ” plug.. Excess or unnecessary oil, constantly dousing the element, is going to make it more difficult to achieve a slow, smooth idle. Those who contend that, “Well, using too much oil can’t hurt anything ” are wrong. In addition to causing undue friction in the engine, keeping the metal parts from properly mating, etc., too much oil in 4-stroke fuel is constantly trying to cool a plug element that is already having problems.Sort of like pouring a bucket of cold water on a poor guy whom is already shivering. Again, since oil doesn’t burn, it’s doing nothing to help us develop power, it simply lubricates and goes right out the exhaust and all over everything. However, suppose we don’t put unnecessary oil in the fuel, and replace it with methanol, which does burn. Well, what do you know, greater top end power! Hey, I think we ’re on to something here! Remove unnecessary oil from 4-stroke fuel, and we get a “twofer ” -- two benefits for the price of one, a slower, more reliable idle plus greater top end power!

Conclusion: For reasons that should be clear above, a properly blended 4-stroke fuel should deliver better all-around performance in a 4-stroke engine than a regular 2-stroke fuel in the same engine.

While it ’s not going to actually harm anything to run 2-stroke fuel in a 4-stroke engine, never, ever run 4-stroke fuel in a 2-stroke engine. It’s not going to have enough oil. Now, for those of you will say that you have done it with no problems, I’ll agree. If you have a real good ear and keep the needle valve “fat ” (rich), it will probably work just fine. But the official word is DON’T! It reduces your margin of error unacceptably.

Finally: Because engine manufacturers have been burned in recent years by some fuel makers’ attempt to lower the cost of their products by using either too little oil or a cheap grade, most manufacturers today are recommending that you run a 2-stroke fuel only in their 4-stroke engines. Or they will specify what would seem to be an abnormally high oil content (and it probably is). Who could blame them? Since they know they have no control over the oil used in someone else ’s fuel, they ’re just trying to cover their fannies. So would I.

Note: I believe it ’s commonly known that the manufacturers of YS engines, among the most powerful 4-stroke engines available, mandate that only fuels containing oil contents in the normal 2-stroke range be used. Their engines are unique, and the manufacturer’s recommendations should be followed, although, as with anything, there are exceptions.

Next Installment: Storing fuel for maxi-mum shelf life.

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