How about some empirical research on the thermal behavior of the air-cooled motorcycle engine… particularly H-D V-Twins? Hypothetically, you’ll want to use several thermocouples to measure actual temperatures at various spots around the engine and oil tank (or sump). You’ll want to find a “test track” that has a flat straight section 10 miles long or so, with easy turn-arounds at both ends. That 10 miles should be enough for engine temperatures to stabilize at various steady speeds, but don’t test until you’ve already ridden at least 20 miles to get the engine to proper operating temperatures in the first place. Oh yeah, for our purpose here, it also helps if the road happens to be out where there’s steady, high ambient temperature and little or no wind.

What you’ll most likely discover is a sort of bell curve of measured temperatures at various speeds—dependent on at least four factors—velocity, ambient air temperatures, state of tune and the capabilities of the engine lubricant.

Here are some representative temperatures at a steady speed of 65 mph in 90-degree still air:

  • cylinder head – 275°F
  • cylinder base – 230°F
  • cam cover – 210°F
  • primary – 190°F
  • engine oil “feed” – 190°F

At a steady 75 mph these temperatures will all be about 10 degrees higher. At 95 mph… well let’s just say your results might vary, but the point is higher velocities (especially with engines in a state of tune that allows them) result in higher temperatures. Logical enough when you think about it for a minute, but there’s the “other” extreme too. The one you might not have thought of so much and the one that makes this bell curve so interesting.

At the end of each test come to a stop, then watch the cylinder head temp shoot up to around 325 (or more) as the motor idles. Quite often, it will continue to rise, even to temperatures you will never see while moving—at any speed! An extended period of idling (10 minutes) could easily produce a measured 375 degrees in the heads, and 400 degrees at the spark plug—or more! And don’t be surprised if these temperatures stay over 300 up to around 40 mph, only gradually dropping as speed increases.

Dealing with hot heads?
What we could observe from this kind of test:

Cylinder heads are by far the hottest spots, sustaining temperatures 45–50 degrees hotter than the rest of the engine, no matter what the speeds or circumstances. This also points out the sensitivity of head temperatures, since they fluctuate faster and to greater extremes than any other engine component. The degree of airflow over the head fins, the air temperature, and the ability of the engine oil to carry heat away from the heads—are critical.

What we could conclude from this kind of test:

  • The conditions most likely to create engine-destroying temperature are reduced airflow over the cylinder heads (or none when idling), especially following or combined with high-speed or uphill riding.
  • The only place on the engine where such extreme temperatures can occur is the cylinder heads.
  • Oil is second only to air flow as a cooling agent—the primary defender in the absence of air flow—in addition to being literally the last barrier to damage.
  • The traditional Harley-Davidson engine is capable under extreme riding conditions of experiencing engine temperatures that can cause rapid breakdown of commonly used oils.
  • Adding a conventional oil-cooler might help, but is not insurance against thermal limits of oil.
  • Running thicker grades of oil only puts more stress on the oil pump and degrades the protection oil can offer against cold start scuffing.
  • There are three temperatures at work (and hopefully in some kind of fluctuating equilibrium) in a running air-cooled H-D engine: mechanical, ambient and lubricant. You can’t do much about the first two and most of us don’t even measure them constantly (or accurately), but we can do something about the third. All oils are not equal and some differ greatly from the “norm” in their maximum temperature capabilities—let alone the ability to lubricate.

Choice? This time—it’s about the oil not the additives
A few “cool” facts:

  • The “flash point” of oil is the point at which it begins to vaporize (Burn!)
  • The higher this flash point, the better the oil will hold up in high temperature environments.
  • Pure synthetic basestocks have much higher flash points than traditional petroleum oils.
  • Synthetic oils are made up of particles of uniform size, thus they have less “internal friction” than petroleum oils.
  • This uniform size lowers the temperature of the oil, improves the cooling
  • Synthetic oils have smaller particles which all flow freely through the center of the oil galleries within your engine.
  • Larger, more irregular particles in petroleum oils will be pushed to the “outside” of the oil stream—next to engine components—remain there and do not distribute heat from engine components
  • Heat carried away lowers engine temperatures by as much as 20 to 50 degrees F.
  • This significantly extends the life of engine components.
  • Less stress is put on the oil, which extends its useful life as well.
  • Legally, the word “synthetic” is a marketing term and refers to properties, not to production methods or ingredients.
  • Most of the “synthetic oil” you can buy today is actually, more often than not, made of a new (since about 2000) highly-distilled and purified dino-juice called Group III oil. Group III base oils cost about half as much as the synthetics. By using a blend of mostly Group III oils and a smaller amount of “true” synthetics, the oil companies can produce a product that has nearly the same properties as the “true” synthetics, and nearly the same cost as the Group III oil. (Castrol Syntec is one such oil.)
  • So-called semi-synthetics usually have less than 30 percent synthetics and the rest is Group III basestocks.
  • Pure synthetics are now available in their “pure” forms only in more expensive and harder to obtain oils. Delvac-1, AMSOil, Redline, Mobil One, Motul 5100 (and possibly Syn3) are oils made from pure traditional synthetic basestocks.
  • Only one—Redline—is not made up of Poly-Alpha-Olefin (PAO) basestocks. Redline is composed of (are you ready?)—Polyester! Don’t laugh—this isn’t where old leisure suits from the ’70s went to die—this is generally acknowledged as the best protection of all from high temperature operation.

Heated discussions
Let’s take a minute to get something on the radar (and off my chest) right here and now! The best-known 20W-50 motor oils to Harley riders are obviously HD360 and Syn3. The Motor Company has chosen not to use conventional ratings (API, JASCO, etc.) on their oils. This can be looked upon as both the good news and the bad news about the stuff.

The good news is H-D uses their own system of “tested certification” to come up with their own rating—which is expressed in hours of use, hence the “360” moniker on their regular dead-dino oil. Seems to me, this makes sense insofar as it differentiates their oil from other motorcycle oils, not to mention car oils. And, believe me when I say, now more than ever, there are huge differences.

The bad news, on the other hand, is that using this as an excuse, the factory offers virtually no further information about the detailed specifications and standards of their lubricants and doesn’t even rate their two “recommended” lubricants the same way, even though they may meet the same standards in “certified testing.” (One could rightfully ask, tested by whom, and certified to do what?) Harley simply isn’t talking—as much as they are brain-washing. They want it taken on faith that their product is superior for their machines—simply because they tested theirs and didn’t test anyone else’s! Fact is, it may well be. The issue I have with that is they don’t make their oil and they aren’t forthcoming about its properties and protections. So, it might not be!

The maximum thermal specification of any oil is the flash point. This is the temperature at which the lightest ingredient will vaporize enough to burn, and is a damn good indicator of the peak temperature limit of the oil. Most major-brand “conventional” 20W-50 oils have a flash point of 425–440 degrees F. (Some breakdown and oxidation occurs at lower temperatures so a few manufacturers also quote a maximum continuous service temperature of say, 275–325 F, like Syn3’s rating of 300°F.) A little research will quickly reveal that there are oils that have somewhat higher flash points:

  • Amsoil Synthetic Motorcycle 20W-50: 449°F
  • Havoline Formula 3: 465°F
  • Pennzoil GT Performance: 460°F
  • Castrol GPS (20W-50): 414°F
  • Mobil 1 V-Twin (20W-50): 518°F
  • Redline motorcycle oil (20W-50): 490°F
  • Redline motorcycle oil (20W-60): 490°F
  • Bardahl American Classic (20W-50): 500°F

To name just a reasonably well known few.

There are more brands (Motorex, Motul, RevTech, Drag Specialties, etc.) that may have acceptably high(er) flash points and certainly there is more to lubricants than flash point alone! But all else being equal, flash point is arguably the determining factor in protecting our engines. To put it another way, unless you let the situation get a little too hot to handle, you can use a lot less oil and never break down.

You see, unless they get hot enough to burn, the base oils will actually last as long as your motorcycle—many hundreds of thousands of miles. The reason we change our oil in the first place is that the additive package (roughly 3 percent of the volume in a quart) wears out, ironically, from heat. If you could imagine a device that pulled out your oil, gave it a very thorough cleaning, replaced the buffers, detergents, and viscosity improver molecules, and put it back into your engine (a sort of oil dialysis machine), you’d never “change” oil in the regular sense of the word. But we don’t have this option, so we dump out our entire four quarts of oil because 3 percent of the oil is used up. It’s really just a lazy, expensive and wasteful way of getting a few suspected contaminants out of our engine, most of which are actually trapped in the truly excellent filters we use these days. We can do better, and not so damn often, if we try. If you can simply avoid the waste by beating the heat, that would be really cool!


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