Quote:
Originally Posted by CSBM5
Wow, this guy is far from a tribology expert. His statement "All liquids are incompressible (that’s how hydraulics work, including brakes with their watery thin brake fluid), so it does NOT matter what the viscosity of any liquid is. Thick oil or thin oil will create the same incompressible liquid oil wedge." is 100% false.
Viscosity plays a large role in how the oil wedge in a hydrodynamic bearing behaves, especially "local" viscosity since there is a large temperature gradient in the oil wedge in a hydrodynamic bearing. Actual viscosity of the oil in the wedge is the primary fluid variable in the load carrying capacity of bearing.
In fact, the use of high levels of polymer additives in wide-range multi-weight oils creates a non-Newtonian behavior of the oil which becomes especially important in the behavior (and load carrying capacity) of the oil film under high strain conditions. The normal linear relationship between strain rate and sheer stress of the oil becomes a non-linear relationship under high rates of strain whereby you see a fall off in viscosity with high strain rate -- which immediately comes back once the strain rate declines. This is the situation in the oil wedge of the bearing -- extreme rates of strain; hence localized viscosity is profoundly important to the successful operation and load carrying capacity of the bearing.
I would strongly suggest the writer of the material spend some time studying Tribology prior to making such statements. A good reference is "Engineering Tribology" by Stachowiak and Batchelor.
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I think the point that we should note above, as it has been voiced in other threads as well, is that the S65B40 does not have enough oil flow at high engine speeds. This, as shown by the results posted by AutoTalent, is primarily due to the tight tolerances chosen by BMW. A stop-gap could be to use thinner oil to increase the oil flow across the bearing surface (at what other consequences I have no idea). The best solution: change the clearances to best-practice standards (rod bearings, main bearings, side clearance) and use the thinnest oil that provides proper protection at the highest RPM/temp that the engine will see. This is not cheap and certainly impractical for 99.9% of S65 and S85 owners.
We can at least address the rod bearings without a total tear-down...
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