Understanding the Differences of Oil, Part 1

By John Copeland

One of the distinguishing features of 2 cycle engines is that, in most cases, the lubricating oil is carried into and through the engine in mixture with the fuel. Whether you're running a Yamaha, Comer, Parilla, or (for the most part) shifter engine, you know you have to add oil to your fuel. But the bewildering array of oils available today is enough to baffle the best of us.

On top of that, oil manufacturers, engine builders, and other reliable sources may have widely different recommendations about which oil to run, how much to use and how to mix it. Let's take a look at what's out there and try to make this process a little easier to understand.

Fundamentally all 2 cycle oils differ from their cousins intended for 4 cycle use in that they are required to burn along with the fuel. They have to provide the required lubrication to protect your engine’s internal parts and to make everything slip along with as little internal friction as possible. But when the spark ignites the fuel/air mixture in the combustion chamber, the oil that is dissolved in that mixture has to burn as well. The cleaner it burns the better. And if it adds a little extra energy to the combustion, so much the better. Of course, oils containing power additives have been around since the dawn of karting, although most are not legal for sanctioned kart racing. But that is a subject for another day. For right now let’s just look at 2 cycle oils that don’t contain those additive boosters.

We can divide 2 cycle oils into 3 basic categories; petroleum based, castor oil based, and synthetics. While a few oil companies market petroleum based 2 cycle oils, they are primarily for lawn mowers, outboard boat engines, and other low-performance applications. Petroleum as a lubricant …….. is a pretty good rust preventative. A quick look at a drop of petroleum under a high-powered microscope is enough to convince you not to put this stuff into your high-performance kart engine. Even the highest quality petroleum oil contains a surprising amount of particulate material: little bits of gritty stuff that looks positively scary under the microscope. And if that weren’t enough, petroleum oil doesn’t burn all that well and leaves a lot of debris behind when it does burn. Save this stuff for the Lawn Boy.

Castor-based oils have been a favorite of high-performance tuners for more than a hundred years. In it’s most elemental form, it is the oil squeezed from Castor Beans and its not significantly different from the vile tasting stuff your grandmother used to cure constipation. That castor oil worked because it was, among other things, a pretty good lubricant. Guess what? It’ll make your engine go too! Today’s castor-based 2 cycle oils bring some extraordinary qualities to the table. Castor has by nature incredible film strength. That means it can resist tremendous pressure between 2 surfaces without being squeezed out and allowing the surfaces to touch. This is especially important in today’s highly stressed 2 cycle engines. The point of contact pressures on connecting rod bearings in particular are almost unimaginable at high RPMs. It’s the oil’s job to maintain a lubricating film on these surfaces as well as the main bearings, and the all-important piston-to-cylinder wall interface so that they never actually touch. Another of castor oil’s endearing qualities is what oil experts refer to as “wetting ability.” Simply put, wetting is the ability of the oil to spread itself out to coat the entire surface of whatever it touches. It’s the same effect that adding detergent to the water in the kitchen sink has. In the sink it effectively makes the water “wetter” and so helps the water slip between the spaghetti sauce and the plate so its easier to wash off. Some of you may be old enough to remember when oil for your car was advertised as “Detergent” oil. Those oils had wetting agents added to help the oil coat the inner surfaces of the engine for better protection from corrosion. In our karting world, castor oil’s wetting ability helps ensure that the rapidly moving parts of your little 2 cycle screamer are always covered with a film of lubricant.

But alas, the report on castor oil as a 2 cycle lubricant isn’t all good. Some of the gums and other components that Mother Nature put in the castor bean don’t burn all that well and they can leave a gooey mess behind that can make piston rings stick in their grooves. And the parts that do burn can leave behind a significant amount of carbon on the piston crown and the inside of the head. Virtually all of today’s castor-based 2 cycle oils as described as being “de-gummed” but that is really a relative term. They have less gum and leave less carbon than their non-de-gummed cousins, but it is still something the user needs to be aware of. One other negative you need to be aware of is that castor-based oils have a disturbing tendency to “fall” out of solution in gasoline when they get below 50 degrees or so. More than one dirt racer has had the unhappy experience of finding all the castor oil pooled in the bottom of his fuel tank or in the bottom of the fuel jug when the nighttime temperature has dropped. This is not an “every time” thing and that may have to do with the various additives in addition to the castor oil in the oil bottle. But it is a significant consideration if your racing might include some cold days or nights.

So let’s see, castor oil “wets” very well, but it leaves a lot of carbon and gummy residue, you can’t run it when it’s cold out, and it tastes bed (trust me!). So why is it so popular with tuners and engine builders? Two words: IT WORKS. For consistently squeezing that last bit of performance out of your engine, it's pretty hard to beat castor oil as the lubricant in your fuel.

But what about the 3rd category of 2 cycle oils; the synthetics. As the name implies, these are lubricants not derived from petroleum, or castor beans, or anything else that occurs in nature. They are the result of exhaustive research in the laboratory looking for compounds that offer excellent "lubricity", surface-wetting properties, and, in the case of 2 cycle oils, clean burning. These oils were originally based primarily on a family or organic chemicals called esters. By comparison to castor oils, these "new-generation" oils offered a number of advantages. Their solubility in gasoline and methanol made them easy to mix and to keep mixed. Unlike castor they did not deteriorate over time, either mixed with fuel or still in the bottle on the shelf. They also left fewer deposits behind when they burned. They were, and still are, an excellent choice for less experienced karters whose attentions need to be focused on developing driving and tuning skills more than getting that last 1% out of the engine.

The energy crisis, with its lines at gas stations and newfound interest in fuel economy, focused a lot of attention on synthetic lubricant research. Sunoco and Mobil Oil in particular made tremendous strides in developing oils that A: didn’t require petroleum as a raw material, and B: provided significant friction reduction to increase fuel economy. Their research spawned thousands of new companies developing and marketing a vast variety of lubricants and/or additives. These products greatly expanded the old ester-based synthetic lubricant sphere.

By and large these new oils have a lot to offer for karters of every level. They have outstanding lubrication properties, they are highly soluble in both gasoline and methanol, and that solubility is not temperature sensitive, and they are exceptionally clean burning. That does not mean that every synthetic product can do everything it promises. We’ve heard some fairly outrageous claims and tested dozens of products. The good ones are really very good, but there is also plenty of “snake oil” out there.

(My favorite was the super lube additive that the salesman claimed was the product of Soviet military technology. Knowing what we now know about the now-defunct Soviet Empire, I’m not sure I’d view that claim as a positive sales strategy.) But if you stick with one of the top brand-name synthetic oils, you’ll likely be very happy with the performance.

The cleaner burning properties of these synthetics mean that the ritual cleaning of pistons and rings that is part of the castor- based world is a thing of the past. Synthetic oil users also are spared the daily hassle of scrapping built-up carbon from the piston crown and the combustion chamber just to ensure passing the cc test in post-race tech.

So how is a karter to decide which oil is right for his application? First of all, ask your engine builder/tuner. Assuming the track or organization you race with doesn’t dictate a spec oil, complying with the engine builder’s wishes will usually be the best choice. Certainly if you race in cold conditions you’ll want to think twice about using castor-based oils. And if you’re not too hot on doing lots of maintenance at the track between races, the synthetic products are an attractive choice. On the other hand, if you’ve maxed out every other element of your engine performance package, lots of top-flight tuners will tell you that you can get just a bit more by using castor. This is particularly true of air-cooled engines. The water-cooled 2 cycles offer some unique considerations, oil-wise. But the air-cooled engines will usually crank out just a bit more on castor-based lubes.

I’m sure I’ll hear from synthetic oil manufacturers, dealers, and proponents of all stripes that I obviously don’t know what I’m talking about. They’ll insist that, if you’ll only use it according to their instructions, if you’ll only build and tune your engine to their specifications, if you’ll only drive your kart the way they think you ought to, their oil will outperform anything else available. They might be right. But in my experience, most karters are unwilling or unable to redesign their entire engine program to accommodate some hoped-for oil performance increase.

Next month we'll take a look at fuel/oil ratios. There always seems to be some confusion about how much oil is too little how much is too much. And how much oil do I add to a 5-quart tank to get a 16:1 mixture like the oil bottle says? Next month we'll work out an easy to use system to get the right ratio every time. See you then.