A)Most oil manufacturers work out the viscosities etc. in a laborotory in
glassware and lab apparatus. This can lead to differences in the rating of
the oil and its performance under real conditions. The bottom end of an
engine is a pretty stressful place to be.
B)As the engine heats up, metal parts expand, which I would have thought
leads to gaps (bearing spaces etc.) becoming narrower, but the oil pressure
guage suggests that the opposite is occurring.
>From: "D. Michael Scott" <dms@snip.net>
>Reply-To: "D. Michael Scott" <dms@snip.net>
>To: Richard Shipman <mbelect@mindspring.com>
>CC: Jeff Boatright <jboatri@emory.edu>, spridgets@autox.team.net
>Subject: Re: Oil Pressure-Cause for concern???
>Date: Sat, 23 Oct 1999 20:51:49 -0400
>
>***Maybe this is a little off the thread. But, what would cause the car to
>have
>good strong oil pressure on start up until running hard for one to one 1/2
>hrs.
>and then the pressure gradually drops to lower than recommended running
>levels
>(with 20-50weight)? Or what is happening when oil pressure only gradually
>drops to
>lower than recommended levels when running?
>Thanks, D. Michael
>
>Richard Shipman wrote:
>
> > ----> If the flow remains the same, but the pressure drops, this is
>because
> > the thinner oil is leaking past the bearing surfaces faster. The oil
> > pressure is monitored at the main distribution oil gallery. From there,
>it
> > takes a long circuitous route through the bearings, oil transfer holes
> > drilled throughout the crankshaft and block over to the cam bearings and
>up
> > to the rocker arms, and finally seeps out at each bearing surface and
> > returns to the sump via gravity. So, if you are measuring low pressure
>at
> > the main gallery, and each bearing is weeping more oil, and the pressure
> > drops to subsequent bearing surfaces further down the oil circuit until
>at
> > worst case, the pressure reaches zero before reaching the furthest
>bearing
> > and you can guess what happens next.
> >
> > Richard
> >
> > At 03:53 PM 10/23/99 -0400, you wrote:
> > >I guess the other side of this is, what is necessarily wrong with
>using
> > >lower weight oil, even if pressure drops, if the flow remains that
>same.
> > >That is, the bearings would still be lubricated, right?
> > >
> > >---
> > >On 10/23/99, Andy Webster wrote:
> > >>The oil pump provides flow...not pressure.
> > >>Pressure is provided by the resistance to the shearing forces applied
>as oil
> > >>is forced through small spaces, such as between bearing surfaces,
>through
> > >>filter media and down small galleries. So as bearings wear and the
>spaces
> > >>get bigger the oil flows through with less restraint,like sucking your
> > >>thickshake (or partially gelatinated non-dairy gum based beverage)
>through a
> > >>wider straw.
> > >>So really thick oil is bad news for a tight tolerance (new) engine and
> > >>really thin oil wont build any pressure in an old worn one.
> > >>Andy
> > >
>
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