Dr. Mayf,
I did some Tribology work many years ago, so that along with many years of
racing have given me a small amount of knowledge.
The oil's main purpose is to keep moving metal parts from touching. It's
secondary purpose is to remove heat. A crankshaft rotating in a plain
bearing generates much heat, some conducted down from the combustion
chamber, the majority from the oil itself as the hydrodynamic boundary
layers shear. A big part of the viscosity question centers on pumping
losses through each bearing race. There is both the static pressure and the
dynamic pressure that builds up, so the amount of oil that can be pumped
through a given journal at a given clearance varies as the temperature and
RPM increase. Generally most engines I build are "clearanced" to use a
10W-30 oil. This means each journal is measured and clearances are set to
flow a given amount of oil at a given pressure/temperature. You always want
some pressure build-up inside each journal, otherwise the oil would not
suspend the rotating mass. However, too much resistance to flow and the oil
stagnates and overheats. The balancing act is between the static oil
pressure, the load, the running temperature, the journal surface area and
the viscosity of the oil. I have to make some assumptions first, mostly
that I am restricted to 60 PSI maximum hot static pressure at the pump, and
that the journals all behave in a uniform manor. For most American V8s
there is so much collective knowledge that simple look-up tables are
available to help with clearances. For more exotic engines some testing
would need to be done. You want to be able to support your dynamic load and
also move enough oil through to keep everything cool.
If the oil viscosity for a given bearing clearance is too low you will
create heat from an insufficient film thickness which results in
metal-to-metal contact, generating even more heat until the bearing
surfaces braze together. This is amplified when impulse energy is
introduced, such as power-shifting. Too high a viscosity will also allow
heat to be generated, this due to the internal fluid friction and shearing
created within the oil. This localized heating will thin the oil out inside
the bearing so it pushes out quickly, but the cooler, thicker oil behind it
cannot flow fast enough in to replace it so the bearing starves and you
wipe the bearing. Another problem with too-high viscosity is cavitation.
The bearing creates both high and low pressure zones, these occur on each
side of the oil's minimum film thickness, and can cause oil cavitation in
the bearing from the rapid expansion of dissolved air, water and/or fuel.
The bubbles will implode and you will lose the oil film, again wiping the
bearing. This is called cavitation pitting, and it happens in racing
engines that run high-viscosity oils.
OK, so Dr. Mayf you say that you are all OEM clearances, but the OEMs
generally designed for oil temperatures around 130 C or so, and that is
10W-30 territory. Do you monitor oil temperature on a run? A LSR engine
will be pulling WFO at high RPM for a long time, so localized oil
temperatures will rise much higher than that. Are you running an oil
cooler, I hope? A cooler will help stabilize the system making it more
predictable.
The synthetic oils designed for racing will remain in their viscosity class
for much longer at elevated temperatures (200 C+) before breaking down, so
for that I recommend that you start with a top-line synthetic like the
Mobil 1 Racing 0W-30 or Red Line 30WT Racing oil. These oils also have a
lot more various zinc additives to further protect the rotating shiny
parts. A thicker oil like the Mobil 1 Racing 0W-50 may not flow fast enough
in your bearings to dump heat as quickly, but if the ambient is high (40
C+) then that would be a good choice. As to using a high-flow oil pump,
there is no need if the clearances are good and the right oil is used,
otherwise you are just creating parasitical power loses by moving extra oil
through the pressure-relief by-pass. Besides, most after-market racing
pumps flow more than an OEM pump anyway. And I recommend Wix filters, they
seem to have remained consistent in quality.
Could be worse, the Formula 1 guys pay over $500 a liter for special Mobil
1 Racing oil!
Hope this helps.
Bugz
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