GH wrote:
>Someone correct me if I'm wrong, but I thought that one of the
reasons
>for a bypass valve was to maintain oil flow when starting a car
in cold
>temperatures, when the oil would be thicker and more difficult
to force
>through the filtering material. Some engines might see 70-80psi
or more
>at startup in cold weather.
The higher pressure isn't (or shouldn't be) caused by forcing oil
through the filter. At least not through a clean one. It's all
the other restrictions - orifices in journals, bearing gaps,
etc. - that cause higher pressure when oil is cold and thick. The
oil pump will usually have a pressure relief valve that deals
with this. When you crank a cold engine (that's not slap wore out
;<) the oil pressure stabilizes at a point determined by the
relief valve. As the oil warms up and thins, the relief valve
closes and then your oil pressure is determined by the various
restrictions in the lubrication system and the flow volume. If
the oil _filter_ constitutes a significant flow restriction, then
you need to get a better one. Or at least a clean one.
;<)
>Could a bypass valve also serve as an anti-drainback valve, or
would
>that likely be a separate feature?
I think they're usually separate. The anti-drainback valve is
usually just a rubber flap on the inside of the top plate. When
oil pressure from the pump is present, the flap is easily pushed
away from the holes in the top plate, allowing oil to flow in the
correct direction. When pump pressure disappears, the small
backpressure caused by oil attempting to flow downhill is enough
to push the flap against the holes and block reverse flow.
Jay
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