Originally Posted By: Shannow
Originally Posted By: demarpaint
I was told by a very reliable source in very simplistic terms that a "thick" oil will shear more in an engine specifically designed to run on a "thin" oil. It also depends on just how "thick" the oil is, and how "thin" an oil the engine was designed to run on. This came right from the top tech guy at one of the boutique oil companies. His explanation made a lot of sense at the time. Take it FWIW.
Yep, Honda in particular make reference to reducing radial clearances, increasing journal diameters and widths, particularly with respect to the 16 grade oils coming.
Increase in journal diameter increases relative speed, which increases film thickness. Increasing diameter and length increases surface area, which reduced the apparent pressure on the wedge and increases bearing film thickness. Decreasing radial clearance increases film thickness.
The diameter increase, and clearance reduction increase the shear rate across the oil, and will push non-Newtonian fluids into their high shear behaviour.
40 under temporary shear will still be thicker than a 20, and the polymers may be worked more (but in a bearing, they aren't being cut, like in some of the injector tests)...40 will run hotter average working temperature within the bearing.
So yes, as manufacturers start to actually design for thinner oils, as ipposed to test thinner oils in existing designs and declare them satisfactory, thicker oils will be worked harder, and run hotter.
Which is interesting, because all of the tricks that they are using, closing up clearances, increasing journal diameters, and bearing lengths lead to increased frictional drag...which sort of defeats the purpose of the thinner oils in the first place.
Thanks for taking my overly simplistic explanation and bringing up a few levels for the more tech savvy people here. I was afraid I'd be flamed, by the way I stated it.
