Motor Oil Viscosity versus Pressure

[ahess77]

I'm looking for a good (technical) explanation of why the engine oil pressure reading goes down as the oil temperature increases. It seems counterintuitive.
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We’re generally using 5W20 oil. It has a viscosity index of 5 when cold and a viscosity index of 20 when at 100C. The 5 means a specific amount of oil flows through a calibrated hole in less than 3.8 seconds. The 20 means it takes between 5.6 and 9.3 seconds, much longer and therefore thicker when hot.<o:p></o:p>
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Therefore, why isn’t the oil pressure lower when it’s cold since it flows so much more easily? When it’s hot shouldn’t there be more resistance to flow and therefore more backpressure in the system causing the pressure reading to go up?
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If aren’t just guessing, or if you’ve found a good technical article please let me know.<o:p></o:p>
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Thanks,<o:p></o:p>

[Zack]

For the same reason your milkshake gets through a straw as it warms up.

[RR|Suki]

I think you got lost in your own explanation, becasue yes 5-20 acts like 20 when hot... but not when cold... so when cold it's "thinner" and as it gets warmer it takes on the properties of 20 which is "thinner" than 5 when 5 is cold, but thicker than 5 would be when 5 is hot. Hopefully that made sense

[magindat]

Bottom line, you left out the notion that the 5 is when COLD!!!!!! Like winter freezing a$$ cold, not 'I need a wind breaker' cold.

[CRZYBIKER]

Multi weight oil ratings are more of a stability rating rather than a viscosity rating like straight weight oils.

Take a straight 5 weight and a 5W20, cool both to 32 degrees, they will both thicken equally, they will flow the same (same stability)

Take straight 20 and 5w20, heat both to 200 degrees, they will both thin, they will flow the same (same stability).

In a nut shell, 5w20 is as good as a 5 weight in winter and as good as a 20 weight when hot.

[RF Overlord]

ahess, the "5" in the equation is the relative viscosity of an oil compared to a reference standard at 40°C, while the "20" is referenced at 100°C. When there's a "W" after the first number, it means the reference is at 0°C.

A 5W-20 has the same viscosity as a straight 5-weight oil would when measured at 0°C. but will have the same viscosity as a straight 20-weight would when measurecd at 100°C. So a 5W-20 is still going to get thinner as it gets hotter, it just doesn't get AS thin as a straight 5-weight would under the same conditions. You could also put it the other way: a 5W-20 will get thicker as it gets colder, but won't get AS thick as a straight 20-weight would.

*EDIT* Damn my slow typing fingers...I see CRZYBIKER just said the same thing above^^^ :)

[ahess77]

A 5W-20 has the same viscosity as a straight 5-weight oil would when measured at 0°C. but will have the same viscosity as a straight 20-weight would when measurecd at 100°C. So a 5W-20 is still going to get thinner as it gets hotter, it just doesn't get AS thin as a straight 5-weight would under the same conditions. You could also put it the other way: a 5W-20 will get thicker as it gets colder, but won't get AS thick as a straight 20-weight would.

OK, That's the part I missed when reading the oil viscosity measurement info I could find. So, an oil with a viscosity index of 5 is still thicker at 0C than an oil with a viscosity index of 20 is at 100C. Oil does get thinner as it gets hotter just like you would expect, multi-viscosity oils just don't get as thick when they get cold as a standard weight oil would.

I appreciate the explinations. As I re-read my origional post I hope nobody was put off by my "no guessing" request.

Thanks,

[RF Overlord]

So, an oil with a viscosity index of 5 is still thicker at 0C than an oil with a viscosity index of 20 is at 100C.Bingo. :up:


You asked for a good technical explanation, so I'll add this:

A multi-grade oil is actually a single-grade base stock that has been treated with VIIs (Viscosity Index Improvers). These long-chain polymers are all coiled up like tiny Slinky toys when cold, but they "relax" and unwind when heated. This allows them to intertwine and "thicken" the oil. The problem is that they also get "cut" into smaller pieces during normal flow through gear-type pumps and extreme pressure bearing surfaces. This "chopping up" of the VII polymers is called "shear" and is what accounts for many conventional oils' drop in viscosity as they age.

One of the beauties of synthetic oil is that it can be engineered to be multi-grade without the use of VIIs, which is one of the reasons synthetic oil can be left in service longer.

[gnxtc2]

I'll throw a twist into this equation even more.

With higher pressures, there is more heat at the bearings and causes the engine to run hotter.

The idea is to have an oil pump that is efficient. High volume with lower pressure, just like the Nascar guys. To have an efficient oil pump costs $$$$. Manufacturers will not spend the money for these kind of pumps. They just increase pressure to compensate for the volume.

http://www.johnsonsoilpumps.com/product_frame.htm

Billy T.
gnxtc2@aol.com

[Sharky]

I'm looking for a good (technical) explanation of why the engine oil pressure reading goes down as the oil temperature increases. It seems counterintuitive.It doesn't get much more technical than this: Motor Oil 101 by A.E. Haas (http://63.240.161.99/motoroil/).

If you start at the beginning and read each chapter in sequence it makes sense and is VERY informative. Dr. Haas is also one of the resident experts on the Bob is the Oil Guy (http://theoildrop.server101.com/forums/ubbthreads.php?Cat=0) forums.


About the author:
Dr. Haas is a physician and surgeon. He graduated from the University of Florida with a degree in biochemistry with honors. He studied motor oils since high school where he did independent studies on this topic. He studied the properties of viscosity.

When he was a general surgery resident in Chapel Hill he studied the flow mechanics of human blood. Today he continues his research by discussion of oil products with chemists in the field and chemists from the oil manufacturers.

He has personal racing experience in Formula Super Vee. He is his own Lamborghini and Ferrari as well as Mercedes mechanic.

Hope this helps! :cool:

[RF Overlord]

^^^what Sharky said^^^

Doc Haas is highly respected on BITOG.