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Slippery Business by Royce Branch
Mazda Sport Magazine May/June 2004
http://www.mazdaevents.com
(858) 675 2076
Since friction absorbs energy, lubricity means horsepower. In a modern
engine those losses aren't likely to be huge, but since improving the
situation is as easy as pouring a bottle down a hole, this is still the
fastest and easiest output gain there is. In typical applications, the
manufacturer is generally there to help-the owner's manual walks you through
various use patterns and lists recommended oil types. Unfortunately,
choosing the appropriate oil gets harder once you start looking at
higher output applications or extended high-performance use. Once you stray
too far outside the factory's box, even finding the right grade of oil may
be tricky, as it may depend on things such as your location, season,
intended uses, personal driving habits, and the mileage of the engine.
Before even looking at that, however, there's an even more fundamental issue
to consider: the decision between synthetic and natural engine oil.
Synthetics cost more, but not without reason. On the most basic level, the
advantage of synthetics over petroleum-based oils is the ability of the
synthetic manufacturer to create a lubricant with an order of magnitude
fewer impurities than one based on natural petroleum. (It's technically
possible to refine natural oil to the same purity as synthetics, just cost
prohibitive.) And, since it's the impurities that ultimately lead to
breakdown, reducing these is important. How imporrant again depends on your
particular situation. Some other desirable aspects of synthetics include
lower. temperature fluidityat 45 below, regular l0W-40s solidify while the
equivalent synthetic keeps flowing and much higher thermal tolerances. Many
synthetics can take nearly twice as much heat as natural oils before
starting to break down chemically. Still another key factor is film
strength, a. measurement of the amount of pressure needed to force a film of
the oil out from between two flat pieces of metal. Some synthetics post film
sttength of over 3000 pounds per square inch; for petroleum oils, the
average is less than 500.
[[Synthetic base oils are assembled by connecting carbon atoms together to
make the proper-length string for the oil being built. It's very easy to
get over 99.9% pure 2-carbon-atom strings to use to assemble the base oil.
(Ethylene?) Once things get beyond C8 or C12, it's becomes much more
difficult to sort the molecules out and get just the carbon chain lengths
you need. At C2 or C4, the carbon chains are still straight. As complexity
rises, branches form and the string starts looking like a fern. The author
of the paper suggests that it's possible to get petroleum to that same
99.9%, but the chemists at Texaco say it's pretty close to impossible to get
anywhere near this level with current technology at any price. It's SOO
much easier to assemble a pure, tailored, molecule than to sift thru the
crude for just the chains you need. Even with typical fractional
distillation used for petroleum, you only get about 1/2 gallon of lube oil
from a 42-gallon 'barrel' of crude oil.]]
Of course, the real question is whether any of these advantages apply to
you-if a 500cpsi film strength, for example, isn't already enough fot your
engine. That will depend on a few key factots: How hard do you drive your
car? How good are you at following a regulat oil-change schedule? Do you
plan on doing ttack .time or adding lots of high-performance . parts? If so,
synthetics may become appealing. Ditto if you're not always right there at
the oil changer's every 3000 miles. On the other hand, it hardly makes sense
to spend a lot of extra money on oil so that your street engine might run
300,000 miles if you're trading it in at 40,000 anyway. Not all synthetics
are the same, of course, and this fact is critical to many Mazda owners.
Specifically, some are compatible with rotary engines and some aren't. Mazda
has written in the owner's manual that synthetic oils simply shouldn't be
used, though on further examination they included a self-evident caveat: If
a failure is related to one of their components 'and not from the choice of
lubricant, they'll still cover it.
If the failure is related to the lubricant they won't-so go try your luck
with the oil company. While piston engines are fairly tolerant of a
spectrum of synthetics, with rotaries it's critical that the oil you choose
be totally emulsifiable in gasoline and that it burns 100% clean. Oil that
doesn't burn totally clean leaves behind a little residue; that residue can
slowly build up on rotors, inner rotor housings, oil seals, and apex seals,
eventually leading to striations on the inside of the rotor housings, lost
compression, and broken seals.
That sounds pretty bad. On the other hand, synthetics have some qualities
that are ideal for use in rotaries. Rotary engines are quite susceptible to
cold-start oil starvation, which occurs in the few moments upon startup
before pressurized oil reaches all the bearings. While all modern oils tend
to cling to internal surfaces prettyotenaciously, many synthetics do a
better job of cushioning coldstarting parts. Where synthetics really start
to look appealing in a rotary,- however, is that previously mentioned
thermal stability.
The ability to tolerate a lot of heat is critical for a rotary's oil, since
these engines experience spots of considerably higher temperature than
typical piston engines. In the latter, for example, exhaust temperatures
tend to run around 1250 degrees Fahrenheit; in a rotary, it's closer to
1900. Also, where the reciprocating parts of a four-stroke piston engine go
through three unpowered (and therefore cooling) strokes for every powered
one, a rotary's parts only have two per cycle. For both of these reasons,
heat-and more specifically an oil's ability to tolerate ithas a lot to do
with the engine's overall lifespan.
[[I would suggest using a synthetic 2-stroke racing oil to mix with the
fuel. The synthetic racing oils start with a heavier base oil that is
designed to protect in much hotter engines. The detergents and dispersants
are selected for higher temperature use as well. The difference is most
obvious when recognizing that standard petroleum 2-cycle oil is completely
washed from the cylinder wall each cycle and must be constantly replenished.
Synthetic racing oil is strong enough to stay on the cylinder wall up to
FOUR cycles.]]
In general, synthetics tend to start suffering thermal breakdown between 600
and 700 degrees Fahrenheit; for natural oils it's more like 350-400 degrees,
which is cutting it fairly close for an engine design that may see internal
hot spots of 330 degrees in normal use. (This is why it's. imperative for
rotary owners to monitor and change their oil religiously; no matter what
type or brand they've selected.) Synthetics are even more appealing once you
start to talk about turbocharged rotaries, which may see localized
temperatures of 450 degrees in normal operation and up to 650 degrees right
after- shutdown. Additionally, synthetics not only tolerate high
temperatures, in some cases they even reduce them. The higher lubricity of
synthetics tends to minimize friction, which in turn reduces total heat.
[[Synthetics also transfer heat to the engine or gearbox case more
efficiently. Petroleum oil has a variety of molecule sizes. The different
size molecules tend to stratify - and form insulating layers. Synthetic
oils' uniform molecule don't stratify - they remain turbulent - and carry
the heat from place to place.]]
Inner rotor seals can be scored by degraded lubricant. At the tips of a
Wankel's rotor(s), apex seals close off the chambers to maintain
compression." If either the seal or the inner housing acquires a coating of
burned oil residue, the resulting surfacing can initiate chatter, scarring,
and eventually seal failure-hence the need for clean-burn oil.
Finally, of course, there's the issue of extended drain intervals. While all
situations differ, synthetic makers generally recommend that rotary engines
using their oil can go 7500 miles without an oil change, though filters
should be swapped every 2500 miles. This is a lot better (and possibly even
cheaper) than full changes every 2500 miles. Of course, if you don't do the
filter swap yourself and therefore must go to the garage every time, that
feature is unimportant. It's just one more case where use determines the
choice. So let's go back now to a subject we touched on earlier: What
happens if an engine problems arises during the warranry period when
syntheric oil has been used?
[[Extended drain intervals: The base oil must be capable, and the additive
package must be as well. The additives are the more critical part here.
They are 20-25% of the oil and first to be destroyed or depleted. Once the
additive package is severely degraded, the gentle rise of the line charting
the oil's useful life goes nearly vertical. The additive package includes
the detergents, dispersants, extreme pressure additives, viscosity improvers
(if applicable), rust and oxidation inhibitors, antioxidants, and others.
Most manufacturers - including all but one synthetic oil company - mix the
additives in petroleum oil. The additive package in its petroleum carrier
are then mixed with the base oil.]]
There's no one answer to cover all the contingencies. With the development
of extended-drain motor oils (both synthetic and petroleum), most major
carmakers have relaxed their once-rigid compliance requirements. On an
individual basis, of , course, the first order of bus iness is to determine
the cause of the ?lilure. If it's found to be a factory flaw-a finding that
some dealership techs may not be eager to leap tocomplialJce is generally a
moot point anyway. If the failure is found to be oil-related, the situation
gets stickier. Most reputable oil producers will stand behind their product
and cover the repair costs, but it's never an easy situation. Fortunately,
unless you start buying something that was brewed up in somebody's basement,
oil-related ?lilures are a rare occurrence III any case nowadays.
Okay, want to get down to the techmcal nitty-gretty? The following
definitions are based on Red Line synthetics-which we've tested extensively
and found perfectly suited to rotaries (even turbocharged rotaries) under
extreme conditions-but generally they apply across the board.
VISCOSITY: The resistance of a fluid to flow .freely; in layman s term,
think of it as the oils thickness. Viscosity is measure by a scale called
weight; 20-weight (20W) oil is less viscous than 30W, and so on.
[[There's no such thing as 'weight' - and the 'w' has nothing to do with
viscosity - it's about temperature. Single viscosity oil will be labeled
something like 'SAE 30'. Multi-grade oil might be labeled '20W-50'. Each
grade of oil falls within a range of viscosities. For example, an SAE 30
must measure between 9.3 and less than 12.5 centiStokes (cSt) at 100 degrees
centigrade. Multi-grade oils are tested at two temperatures - 40 degrees C
and 100 degrees C. The 20W in the oil above denotes that the oil is an SAE
20 (between 5.6 and 9.3 cSt at 40 degrees C), and an SAE 50 (between 16.3
and 21.9 cSt) at 100 C. For W - think 'winter', not 'weight'.]]
MULTIGRADE OIL: Oils that exhibit varying viscosities based on conditiom. A
10W40, for example, behaves like l0W oil in some circumstances and 40W in
others.
STABILITY OF VISCOSITY: An oils ability to maintain the appropriate
viscosity in different circumstances. Multigrade petroleum oils are usually
a combination of a very light natural oil with chemically formulated
thickeners; the goal is generally low viscosity at low temperatures/low revs
(for good flow rates during startup and cold weather) and high viscosity at
high temperatures/high revs. The thickeners in petroleum-based multigrade
oils tend to be less effictive at stabilizing viscosity. than the inherently
multigrade structure of synthetic oils; most synthetic 10W-40s, fOr example,
have greater true high-temp/highrev viscosity than "heavier'petroleum-based
20W-50s.
[[Viscosity improvers - the 'thickeners' - are usually the first component
of an oil to break down or physically shear. As this happens, the fluid
that begins 10w30, becomes a 10w25 then 10w20 then, if completely killed, a
10w10. The viscosity improvers are like springs that straighten out when
cold and puff up when hot.]]
THERMAL STABILITY: A lubricants ability to tolerate heat without breaking
down. The thickeners in multigrade regular oils are particularly vulnerable
to high temperatures, tending to break down into varnish deposits that can
cling to hot engine parts. The lack of chemical thickeners is one key
component of synthetics' higher heat tolerance.
[[The main benefit of synthetic in high-heat service is the significant
reduction in oxidation and nitration. Synthetic fluids are naturally
saturated - there aren't many open bond sites for oxygen or nitrogen to
connect to. Petroleum oil is unsaturated by nature and has lots of open
bond sites for oxygen and nitrogen to bond when hot. The main destroyer of
viscosity improver additives is physical shearing.]]
VOLATILITY: Essentially, the lubricants propensity to boil (aka evaporate).
At 400 degrees (bear in mind, this is fir hotter than oils should get in a
properly running engine) petroleum 10W-40s typically lose 22% of their total
weight; good synthetics will only lose about four percent.
[[This benefit of synthetic fluids is because a synthetic is pure of lighter
fractions with lower points of evaporation. Petroleum fluids are an
'average' soup made of lighter, medium, and heavier fractions. The lighter
fractions evaporate off and become pollution, or are sent to the engine to
be burned by the PCV system. Viscosity of the remaining oil increases.]]
Synthetics now go far past oil.