Thank you. That is how I guessed it would be but I am loath to argue
until I am sure. On an other BMW I have the fan could be spun easily
by hand when the car engine is not running whereas the one I have now
seems to drag and is not as free and of course I can hear it spinning
when the engine is accelerating. Now I can argue and be sure of my
Ummm... well... no.
See, even though multiviscosity oil has a higher number for the hot
viscosity (2nd number), that number is comparing it to a straight weight
oil of that viscosity at that (high) temperature. So, even a 0W50 oil
would still be considerably lower viscosity (thickness) when hot than
OTOH, whatever magic stuff they use inside these fan clutches would have
to actually increase in thickness as the temperature increases. This
would have to be a pretty rare substance to behave that way...
In actuality, there is a thermostatic spring valve that varies the
fluid's flow path depending on temperature.
Here's a decent lowdown of it:
Sure. Oil is *relatively* thin at low temperatures, but still shows the
effect of its "viscosity modifiers". I thought this was the basis of
the fan clutch, something filled entirely with this backwards fluid.
But I like the spring idea. Thanks for the link.
I still don't think you get the point. Oil (and pretty much every other
liquid in the natural world) gets *thinner* with heating. It can't be
The "viscosity modifiers" that you (and the oil purveyors) mention only
makes the oil thin out less with an increase in temp than it would if
they were a straight weight oil.
So that 0W50 oil we were talking about, starts out as thin as a 0 weight
oil when cold but it still gets thinner when heated. However it doesn't
get as thin as it would if it were a straight ) weight. It only gets as
thin as a straight weight 50W oil would. But that is still thinner than
the 0 weight (or itself) when cold.
Hope that helps clear it up some.
Most modern cars with viscous couplings use the temperature sensitive sort
- but older or cheaper ones may not.
The clue is to look at the front of the coupling. If it has an obvious
spiral spring, it's temperature sensitive.
*42.7% of statistics are made up. Sorry, that should read 47.2% *
Dave Plowman firstname.lastname@example.org London SW
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