So I have been looking for this. I did not find it in the “Taxi Study” often referred to. However, there was a paper from the 1990’s that may shed some light - Properties of Engine Bearings Lubricated with Low HTHS Viscosity Oil, 980702, Ono et al.
www.sae.org
Experimental, not fully formulated oils, were used having API SG quality additives mixed into paraffin mineral oils. The variable was the amount of a viscosity index improver to adjust HTHS between 1.8 and 3.0. The kinematic viscosity ranged from 5 to 11. The test engine was an in-line gasoline engine with an unbalanced weight on the shaft that varied the load by RPM, around 6,000 RPM.
Oil feed (sump) temperatures were approximately 150C during the runs and had to be artificially heated with the thinner oils to keep up the temperature. Bearing temperatures were a maximum of 180C.
Results:
Wear was steady from a HTHS of 2.5 to 3 but doubled when the HTHS was lowered below 2.5 to 1.8.
Wear decreased with decreasing clearances of the bearings. Of the several bearing composits tested, the aluminum-tin-silicon variety had the least wear. (We already know that smoothness is associated with less wear as well. These sorts of things contribute to the production of engines “designed” for thinner oil usage).
They concluded that the HTHS viscosity rather than the kinematic viscosity is the only aspect that determines wear. (I have problems with this statement as they are closely and directly related).
My thoughts:
They used oil formulations not specifically designed for the emerging engines running 20 grade oils.
They used non fully formulated experimental oils using SG rated components. Newer specifications have come a long long way since then. Base oils alone can account for superior performance.
Who drives around with sump temperatures at 304F? A constant sump temperature of 302F is one heck of a severe and certainly unrealistic condition for any passenger car. Even then, if for a short period of time your sump did get that high, the wear rate according to this study, would only double. It would not be 10x or 100x more. And all this for a HTHS of 1.8.
Running the engine in a severly unbalanced condition is certainly much worse than in any balanced engine. So whereas a HTHS may show elevated wear when at 1.8 it would unlikely do so in a normal engine.
And if HTHS is the only number that counts, let’s use 20 grade oils with the higher HTHS values. The thinner kinematic viscosity will help us gain some HP and fuel economy. Who doesn't want this?
People have stated they use oils having HTHS above 3.5 to give them headroom. I believe that anything at or above 2.5 already has plenty of headroom given todays lubricants. Sure, if you have 20% fuel dilution your oil may drop below a HTHS of 1.8. But even then, todays oils should save your ...
Away with 30 and 40 and 50 and 60 grade oils. Use oils based only on HTHS. One of my favorite oils is Renewable Lubricants Inc. where their 20 grade oil has a HTHS of >2.9 accoring to their own specs. And we know it will perform well in a Ferrari Enzo that specs a 60 grade oil, in non track use.
Viscosity index improves of that time were not as good as those of today. Early, excessive thinning of their experimental oils was an issue for certain.
ali
Properties of Engine Bearings Lubricated with Low HTHS Viscosity Oil
Properties of engine bearings were investigated with different bearing materials and different HTHS viscosity oils by means of both an engine test and a rig test.The rig test well simulated the bearing wear which occurred in the engine test. Lead-bronze bearings with lead-tin-indium overlay gave the
Experimental, not fully formulated oils, were used having API SG quality additives mixed into paraffin mineral oils. The variable was the amount of a viscosity index improver to adjust HTHS between 1.8 and 3.0. The kinematic viscosity ranged from 5 to 11. The test engine was an in-line gasoline engine with an unbalanced weight on the shaft that varied the load by RPM, around 6,000 RPM.
Oil feed (sump) temperatures were approximately 150C during the runs and had to be artificially heated with the thinner oils to keep up the temperature. Bearing temperatures were a maximum of 180C.
Results:
Wear was steady from a HTHS of 2.5 to 3 but doubled when the HTHS was lowered below 2.5 to 1.8.
Wear decreased with decreasing clearances of the bearings. Of the several bearing composits tested, the aluminum-tin-silicon variety had the least wear. (We already know that smoothness is associated with less wear as well. These sorts of things contribute to the production of engines “designed” for thinner oil usage).
They concluded that the HTHS viscosity rather than the kinematic viscosity is the only aspect that determines wear. (I have problems with this statement as they are closely and directly related).
My thoughts:
They used oil formulations not specifically designed for the emerging engines running 20 grade oils.
They used non fully formulated experimental oils using SG rated components. Newer specifications have come a long long way since then. Base oils alone can account for superior performance.
Who drives around with sump temperatures at 304F? A constant sump temperature of 302F is one heck of a severe and certainly unrealistic condition for any passenger car. Even then, if for a short period of time your sump did get that high, the wear rate according to this study, would only double. It would not be 10x or 100x more. And all this for a HTHS of 1.8.
Running the engine in a severly unbalanced condition is certainly much worse than in any balanced engine. So whereas a HTHS may show elevated wear when at 1.8 it would unlikely do so in a normal engine.
And if HTHS is the only number that counts, let’s use 20 grade oils with the higher HTHS values. The thinner kinematic viscosity will help us gain some HP and fuel economy. Who doesn't want this?
People have stated they use oils having HTHS above 3.5 to give them headroom. I believe that anything at or above 2.5 already has plenty of headroom given todays lubricants. Sure, if you have 20% fuel dilution your oil may drop below a HTHS of 1.8. But even then, todays oils should save your ...
Away with 30 and 40 and 50 and 60 grade oils. Use oils based only on HTHS. One of my favorite oils is Renewable Lubricants Inc. where their 20 grade oil has a HTHS of >2.9 accoring to their own specs. And we know it will perform well in a Ferrari Enzo that specs a 60 grade oil, in non track use.
Viscosity index improves of that time were not as good as those of today. Early, excessive thinning of their experimental oils was an issue for certain.
ali
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