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

QSHIPQ at aol.com QSHIPQ at aol.com
Thu Apr 4 13:21:24 EST 2002


Jason:
Comments inserted below
In a message dated 4/4/02 10:27:12 AM Central Standard Time, 
janagnos at earthlink.net writes:



>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.  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.

>>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 pressure.  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).  

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

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


>>Oil, like any fluid, will find an optimum flow rate and thickness at 
>>which to operate. So from the bearings perspective it will see the 
>>same amount of lubrication.  Like I said it is the temperature that 
>>will vary which as we all know will result in warmer bearing 
>>temperatures.

Temperature has a direct relationship to aeration.  Increase aeration, the 
bearings won't see the same amount of lubrication Jason.  That is clearly 
stated in 970922.  
"At constant speed a change of the media density in the rotating bore must be 
the reason of the non linear characteristic (of pressure differences).  The 
density change is a reason of an degassing process in the lubricant.  During 
an oil pressure reduction test, there is a linear correlation between the con 
rod inlet pressure and the oil pressure at the supply bore inlet when the 
lubricant includes no free air bubbles (constant density in the supply 
bore)."



>Polymer breadown of oils is a temperature problem, NOT an interval 
problem. 

>>polymer breakdown is a shear problem, exacerbated by temperature.  
>>There are several different types of viscostity improvers (VI) used 
>>by oil maunfactures to handle different tasks.  Motorcycle oils are a 
>>prime example.  here they have to lubricate an engine, trans and in 
>>some cases the clutch, all high shear and the choice of VI is very 
>>critical.

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

>>One critical factor that is typically not mentioned is oil 
>>oxidation.  high Temps >130C make this process happen much quicker. 
>>oxidated oil does not cool, clean or lubricate as well as fresh oil. 

Agreed.  And every one of the tweeked S cars on this list, should watch that 
temp guage then.  120c is marginal, 130 is critical, higher than that you 
will have problems, it's only cycles to failure.  Audi has the same 
thermostat for ALL turbo and N/A applications for the oil cooled cars, 100c.  
That is the optimal operating temp of oil, get it there, then open all air 
ducts and big oil coolers to keep it there under the most strenuous of 
conditions.

>>Like I said before the oil will find its optimum flow rate, heavier 
>>oils will run hotter so they can flow easier. So in general lighter 
>>viscosity grades will run coller than heavier.

Hmmm, thinking of this in context to using higher visc oils to address the 
higher heat found when tracking S cars, and you put forth the oxymoron Jason. 
 Me, I'd want the cooler oil, running an oil cooler on a hot track day.  
That's not a viscosity problem, that's an oil heat exchange problem.

>>There is a myriad of things that go into designing a motor oil and 
>>the good ones are designed for specific applications.  

I think it would be better to say that a myriad of things go into designing a 
motor oil system for a specific engine.  970922 would indicate that audi AG 
has done a lot of homework, more so than many of the "let's tweek it" boys, 
that just might be saving a lot of audi turbo motors from early failure.  
That certainly doesn't exempt anyone from failure, only the onset.

Jason, before we go further, your review of 970922 would be appreciated

Scott Justusson



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