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Kinematic viscosity

Kinematic viscosity

What is motor oil kinematic viscosity?

Kinematic viscosity is a measure of motor oil’s resistance to flow and shear due to gravity. Here’s an example, fill two beakers with the same volume of two different viscosity motor oils and pour them into a pan. The thicker oil will take longer to empty the beaker. Kinematic viscosity is what you see in most motor oil company’s ads. They usually chill oil and show how long it takes to pour out of a beaker. Kinematic viscosity is measured in seconds-to-empty. In other words, it’s the time required to flow a fixed volume of oil by gravity through a calibrated orifice or capillary at a controlled temperature. That value is then converted into centistokes (cST) or square millimeters per second.

But kinematic viscosity is very misleading since engines pump oil under pressure, not gravity. So kinematic viscosity is really irrelevant when talking about starting an engine in cold weather. To get an idea of how motor oil performs in cold weather, you want its absolute or dynamic viscosity. That’s a measure of how well an oil performs during cranking and pumping; the two most critical measures when talking about how oil performs when cold.

Kinematic viscosity is measured at 40°C (100°F) or 100°C (212°F), while absolute viscosity is measured at a different temperature for each SAE viscosity grade.

In other words, Kinematic viscosity is a measure of oil when hot (minimum 100°F) not when cold. So, when oil is expressed as Xw-XX, the XW is it’s absolute or dynamic viscosity and the XX is its kinematic viscosity.

absolute viscosity

©, 2020 Rick Muscoplat

Posted on by Rick Muscoplat



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