An explanation about Pumping Viscosity...

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Jul 24, 2002
Quebec, Canada
Yes, I've done a search, couldn't find anything regarding Pumping Viscosity and what the numbers actually mean. Yes, I know the pumping viscosity @ 40C represents when the oil is cold (startup), and @ 100C means normal engine temperatures. What I don't know is a higher number better or lower number better? Or does it even matter at all? Thanks, Oz
Explaination of pumping viscosity - Once an engine has been started, it needs a continuous and adequate supply of oil. The pumping viscosity of an oil determines how easily an oil will flow from the oil pump to critical parts of the engine in cold temperatures. There are two types of pumpability problems. If the oil is not sufficiently pumpable (a condition known as being flow-limited) the oil may flow but not rapidly enough to prevent progressive damage to the areas needing lubrication during low temperature starting and operation. If the oil is not pumpable at all at lower temperatures (a condition known as air-binding or gelation) the engine can be severely damaged in a relatively few minutes. A flow-limited oil may not be adequately pumped because of having too high a viscosity at the low temperature at which the engine is started. To measure the pumping viscosity of an oil, ASTM test method D4684 is used. In this test, a rotor is inserted into a stator containing the oil in question and very slowly cooled at a constant rate over approximately a two-day period until it reaches a specific temperature. A small force is then applied to turn the rotor and the rate at which the rotor turns is related to the viscosity. The unit of measurement used for reporting pumping viscosity is centipoise (cP). SAE has set specifications for pumping viscosity associated with the 'W' grade of oil as follows:
Viscosity Grade Maximum Pumping Viscosity 
25W  60,000 cP at -15°C (+5°F) 
20W  60,000 cP at -20°C (-4°F) 
15W  60,000 cP at -25°C (-13°F) 
10W  60,000 cP at -30°C (-22°F) 
5W  60,000 cP at -35°C (-31°F) 
0W  60,000 cP at -40°C (-40°F) 

Now, is it more important than the other.. let me repost something I wrote earlier which I think will give you a better picture on looking at motor oils. ------------------------ To base an oil's performance on base stock's performance is not always the best thing. Kinda like taking an musical band with all 20 or so musicians. singlarly, each one may be the top of the line in playing on time, and with precision. But then put a single drummer in that can't keep a beat, maybe even tends to drown out the instruments, then see how as a whole the band works to gether. Like the band, the pumpability is one aspect of how well the base stock is going to hold up to cold weather and as to how well the band is going to sound based on one instrument or how well an oil is going to hold up based on one aspect of the base oil is not a true picture. There is several stages to look at when looking at oil. 1.. Base stock's ability to.. a. hold up to heat (noack) b. pumpability at cold temps c. maybe flashpoint d. operational viscosity e. pumping viscosity f. shear stability 2.. detergent packages... a. how they hold up to acid resisitance (tbn and detergent levels) 3.. Wear protection... a. what kind of barrier additive is used b. what levels of barrier additives(amounts) These are just some of the things to look at, now, put it all together. If they play off key because one is higher pitched than the other(to much of one additive,not enough of the other), or off beat from the other band members(not playing well with other additives due to clash), Then you have an ok sounding band or maybe even a really sorry sounding band that only 10 beers could make them sound better(use an after market oil additive). Like many aspects of oil, it along with other parts play a small role but put to gether, like the music band, it all will sound like sweet wonderful music when they all play together (like a properly blended oil) and that my friend is how you look at oils and determine if they are what you want, but listening to the end results being played by the band(doing oil analysis on your engine with your choice of oil) is the key to determining oil quality. This is one reason I provided an area for oil analysis to be shared so that many could see real life #'s instead of just bench #'s.
Bob (or anyone else with insight), Can we use a specific weight of oil at the temp/cP in the chart with no ill effects? (assuming it meets manuf recommendation) Is there a benefit to using a grade/brand of oil that has lower than 60,000 cp at the temp we plan to use it? If so, how low is low enough? Many people focus in on oils with really low pour points even here in the South. [ July 26, 2002, 07:40 AM: Message edited by: jjbula ]
I'm suprised no one has chimed in here... For years I've been led to believe better pumping at low temp = less wear at startup. (not necessarily less wear overall) Recently I've come to the conclusion that a good barrier lubrication, moly is a good example, is more important in low temp start conditions. As long as the weight of the oil is appropriate for the temp. I'm no expert, just trying to absorb the expert opinion of others.
Recently I've come to the conclusion that a good barrier lubrication, moly is a good example, is more important in low temp start conditions. As long as the weight of the oil is appropriate for the temp. I'd have to say, you did an excellent job answering that statement as I couldn't have stated it any better and agree with it. [Coffee] [ July 26, 2002, 10:52 AM: Message edited by: BOBISTHEOILGUY ]
That's a big reason why I want an oil with moly in it for my car, since my Firebird isn't parked in the garage (that benefit goes to my wife's car) so it'll be subjected to the cold winter starts (mind you our winters are not super cold) So that moly will provide that extra protection for me. Not to mention the extra protection it provides on my many full throttle blasts! [Big Grin]
jjubla, Your statement of barrier lube is right on. If the oil (before shutoff) has left a film of zinc, moly, antimony, or other esters on the rubbing surfaces, this should protect that surface before it is subbed off or sheared off. At oil pressurization, the oil should then replenish the film-producing barrier lube.
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