Robin Ream wrote:
Hello Paul,
I know this is a rotary engine forum, but I've got what I think
is a
simple car question you can probably answer very easily. I'm
installing an
Auburn limited slip differential in a two wheel
drive pick-up and the manufacturers recommendation is to use a
proprietary additive with heavy weight petroleum oil rather than an
over the
counter heavy weight synthetic. The question: Are they
trying to insure repeat sales of both additive and differentials,
or is
the synthetic oil really not good to use in the limited slip
differential?
It looks to me like they're trying to approximate
the quality of the synthetic by using the additive, but as Dennis
Miller
says "That's just my opinion; I could be wrong." :-)
Thanks,
Robin
Robin,
as far as I understand this, synthetics are too "slippery" for the
"limited"
part of the differential, essentially not providing enough friction
between
the friction plates. I am sure they could build a limited slip d. for
synthetics if they could ensure that every customer uses the correct
oil. If
built for synthetics and used with regular oil, the limiting action may
be
to harsh and you can get driveability issues ( and or liability
issues....).
I suggest to use a g-rotor typ limited or torsen-typ limited if they
exist
for your application.........
Thomas Jakits
David Arbogast wrote:
My excursion diesel limited slip and F-250 SD I had with limited slip both
had synthetic lub in the differential. Ford has a "friction additive" that
is added to the 80w90. It smells funny. The lube
is still synthetic, so I think it retains the high temp qualities...
Robin,
I use and sell AMSOIL synthetic lubricants and work primarily with
commercial fleets. I also sponsor a VW club filled with folks putting more
money into their cars than I've put into my house in the past few years. :)
I'll pass on what I've learned from both the classroom and the field.
I read the Auburn instructions. They make it clear that the limited slip
diff has been "...extensively tested with high quality non-synthetic 80w90
hypoid oils treated with GM or Ford friction additives..." They go on to
say that use of other oils or additives MAY cause differential clutch
chatter. The operative word here is 'MAY'. There are many combinations of
gear lubes and friction additives and they probably won't live long enough
to test them all and pass-on to their customers which ones work and which
ones don't; so they pass on what's worked for them.
Synthetic gear lubes and petroleum gear lubes are both hydrocarbons. One
is a pure, tailored, base oil with uniform carbon chains made in a
controlled lab environment. The other is a soup of lighter and heavier
elements that average out to the viscosity and properties necessary. The
uniform nature of a synthetic base oil means less friction and better heat
transfer from the gears to the case. The net effect is a reduction in
operating temperature of 20-50 degrees F; longer gear, bearing, and
lubricant life; and more power transferred to the wheels (or better fuel
economy - depending on what one does with their right foot!).
Gear lubes are about 10-15% additives. The biggies for differentials -
hypoid gears - are the extreme pressure and anti wear additives. Sliding
pressures and shock loading require the use of high levels of extreme
pressure additives - the highest treat rates of all gear lubes - GL4 and
GL5.
Friction modifiers are another additive that appears in both engine oils and
gear lubes. These are either chemical (organic fatty acids or organic
phosphoric acid esters) or suspended solids (PTFE / TEFLON, graphite, and
molybdenum compounds). They are used to make the oil 'slipperier' -
generally for a bit extra fuel mileage.
As a general rule, synthetic oils do not need friction modifier additives to
make them slipperier, while petroleum oils do. In the specific case of this
limited slip diff, they recommend using a petroleum gear lube (which
contains a friction modifier in it's additive package to make the lube
slipperier) plus an additional friction additive to counteract some or all
of the extra slipperiness so the clutches don't chatter. What? Too
slippery? Add some sand... Limited slip additives are like the sand...
Limited slip units usually have the same performance with either petroleum
plus a friction additive, or a synthetic with no additive. Some extreme
cases call for a synthetic with a modifier. Some examples: 2003-2004 AM
General Hummer uses 75w90 synthetic with no modifier. Most 2001-on Chevy
and GMC trucks use synth 75w-90, most without modifiers. Dodge trucks are
factory filled with 75w-90 or straight 90 petroleum with a modifier only in
the limited slip rears. Ford uses mostly synthetic 75w-90 in both their
standard and limited slip diffs. Jeep uses synthetic front and rear with an
additive for the limited slip rear.
One of my clients runs a highly modified VW GTI. 2.0 liter four cylinder,
bigger heads, larger intake and exhaust plumbing, T04 turbo, larger
injectors, and performance programming in the computer as a start... He's
also running a Quaif limited slip aftermarket differential. We ran about a
dozen petroleum and synthetic gear lubes before finally stopping the chatter
with a synthetic lube and a limited-slip friction additive.
Stock Ford trucks call for 4-8 ounces of additive in the rear ends depending
on size. Auburn's recommendation is about 6 ounces, assuming a 4-pint
capacity. This doesn't sound too out of the ordinary.
Here's my recommendation: Fill the diff with your favorite high-quality
synthetic gear lube of the recommended viscosity. Drive on it awhile. If
you don't have any chatter, you're done. If you do, grab the friction
additive and treat at 1/2 of Auburn's recommendation - 3 ounces to two
quarts. Drive around, drive a series of figure-8s to move the fluid and
additive around, and see how it feels. I'd be surprised if you'll need it,
but if you have any remaining chatter, use the rest of the additive.
Andy Hecker
San Antonio
ahecker at satx.rr.com
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