[torsen] Re: [s-cars] Re: The dreaded oil thread

janagnos at earthlink.net janagnos at earthlink.net
Thu Apr 4 15:22:08 EST 2002


Comments interlaced as well. 

>>Yes, but part of the API requirements for all oils is the ability to 
>>sustain foam. All oils will foam and it is up to the manufacture of 
>>the particular oil to decide how resistant to foaming an oil is 
>>beyond the minimum requirements. Most high ester based synthetics 
>.will lose the foam rather quickly. In most cases a high vis oil might 
>>hold foam longer, but that is not always the case.

>A couple of points to consider Jason. "Besides the dissolved air, especially at high speed condidtion, the lubricant contains entrained aire in form of small bubbles. The oil aeration increases clearly with engine speed. The reason is the high back oil flows at high speeds and therefore shortened time to degas the air within the oil sump"

>The biggest difference in turbo motors is that you have a super high temp oil feed to the block FROM the turbo center bearing, which adds huge potential oil aeration problems. 

Absolutely agree.  Any bend in the system will also add the the likelihood for aeration.

>The reason the turbo feed line is so much smaller than the turbo return line, you want it at the lowest pressure possible entering the sump. This in turn helps to keep aeration to a minimum (adding the extra oil heater = turbo). It doesn't eliminate the problem however.

To a point, but the flow rate of a volumetric oil pump increases with angular velocity. If the pressure of the oil pump reaches the opening pressure of the by-pass valve, the valve opens and the flow rate decreases accordingly. We have volumetric pumps in these cars right?

>>Higher pressure does not equate to more aeration. If the system 
>>entrains air in the oil it will do so in any viscosity that you run 
>>through it.

>That statement is not supported in 970922, exactly the opposite is true, in fact it's a linear relationship, aeration v 

A thought. In the oil gallery, the viscosity of aerated oil is less than that of pure oil due to the presence of air bubble. But, in bearings, where a high shear stress exists because of the shaft rotation, the non-dimensional viscosity increases with rpm. As the inlet pressure in the bearing increases with increasing engine speed, even when the same amount of air still exists in the oil, the viscosity decreases. It has been shown that the viscosity gradually decreases to the same level as or below that of pure oil. This can be explained by the reduction in pressure gradient due to the increase in bearing inlet pressure. Shear force becomes less. Meanwhile, in the oil gallery, the viscosity is lower than that of the pure oil due to the air content. As the pressure in the oil gallery increases, the viscosity gradually increases as expected.

>Aeration is affected by mass, temp and viscosity, as well as oil return circuit (here we are a special case, cuz in addition to turbo return, you also have piston squirters as well). 

Squirters are designed (ideally, go VW/Audi!) in such a manner to be “cavitation” free, and should not contribute to the aeration other than when the oil nails the backside of the piston.

>> thicker oils do lubricate as well as the thinner oils. 

>That statement in and of itself would need some major qualifiers, IMO.

OK Here:
Based on an understanding of domains of lubrication, hyrodynamic, elstohydrodynamic and boundary.  At a given speed and load the viscosity of a lies at optimum value and lubrication occurs under the conditions at the edge of the hydrodynamic and elastohydrodynamic regimes of lubrication with a full fluid film being established between the two surface, the best place to be.  If the viscosity is higher you move to increase the fluid friction.  If the viscosity is lower than optimum, it cannot maintain the fluid film and you enter boundary lubrication, not a good place to be. For a given viscosity the same arguments can be given for speed.

Your comment ” Specifically, polymers shear (breakdown) as temperatures increase. These alone are not "deadly", but most of oil polymer breakdowns are in direct correlation with the abilty to reduce aeration. 

The polymers used to build viscosity have nothing to do with reducing aeration.  Aeration (foam) is controlled through anti foam agents, which more often then not silicone complexes that reduce the surface tension of the liquid and inhibit the foam’s ability to stay around.

High amounts of zinc is found in motorcycle oils, which is the last resort to bearing wear. Still a temperature problem, and avoidable.

The amount of zinc in motorcycle oils is no higher than what is found in typical SG oils.  There are infact several other “last resort’ compounds used to inhibit bearing wear.

Scott, basically my post was my back assward way of saying (apparently not very clearly) use the lightest weight oil possible in ones engine to achieve good lubrication and keep oil temperature low when at the track. In normal driving mode it won’t make a lick of difference.  As much as the oil cooler is not great in these cars (should be larger for sure) the oil viscosity will have an effect on what temperature the oil operates at and should therefore be as low as possible. Less fluid friction equal less heat.

Jason "I am not trying to beat a dead horse just add a little pespective" Anagnostis

94 S4

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