All,
Two things that help in this situation (to my understanding) are synthetics,
and oil coolers.
An oil cooler will keep the oil cooler and thicker, resulting in a higher
pressure when warm. A synthetic oil flows better as it is slipperyer (is
that a word) than conventional oil. As such, you get increased flow for a
given pressure. WHen I was breaking my engine in after the rebuild, I used
20W50 for about 3K miles. Then I switched to Castrol 5W50 Syntec (for the
winter). I saw an IMMEDIATE 10 psi drop in oil pressure. Now I get 25 psi
warm and 80 psi cold. That's because there's less resistance to the
slipperyer (is that a word) oil for the same flow rate.
I do not have a cooler yet (well, I have the parts, but I didn't install
them yet), so I expect my warm (hot) pressure to increase a bit when I do.
On a side note, does anyone know of a remote oil filter setup for the 1500
block? I'm tired of coating the engine with oil every time I change the
filter. I'd like to install it away from the engine, where it can drip
right into a bucket.
Michael
'78 Midget
-----Original Message-----
From: Douglas Braun & Nadia Papakonstantinou [mailto:doug@dougbraun.com]
Sent: Tuesday, May 15, 2001 11:42 AM
To: Richard B Gosling; growe58; spitfires
Subject: Re: Dealing with pressure - or lack thereof
My understanding of this subject is this:
It's not really oil PRESSURE that lubricates the bearings,
its oil FLOW. The Spit's oil pump (like virtually all of them)
is a positive displacement pump, which means that the flow is
mostly independent of the pressure at a given RPM.
The oil pressure that you observe is related to the flow rate,
the resistance to that flow resulting from the oil being
forced past the bearings, and the oil viscosity. As the bearings
wear, the resistance goes down, and thus the pressure. If
the oil pump itself is not badly worn, the flow rate will
not drop a whole lot, even if the pressure is way down.
Note that changing to a heavier grade of oil in this situation
will increase the pressure, but probably the flow rate will
not change. But at least a thicker oil is a better lubricant.
In a nutshell, low oil pressure is more a SYMPTOM of engine
wear than a problem in itself. Of course, if you suddenly
start reading low oil pressure on a engine in good condition,
there could be a totally different problem, like a clogged filter
or oil passages (or lack of oil!).
Doug Braun
'72 Spit
At 07:30 AM 5/15/01 , Richard B Gosling wrote:
>Oil pressure is important! Yes, it is true that much of the engine is
> lubricated by splash, but the bits that REALLY matter, your bearings,
have oil
> supplied to them from the oil gallery at the full gallery pressure. If
the
> pressure drops, you don't get enough oil to the bearings, the oil film
will
> get too thin and your bearing will wear rapidly. This is not good. I
should
> know, I analyse engine bearing perfomance for a living!
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