At 11:55 PM 2/23/2000 -0600, Todd Abernathy <t_abernathy7@juno.com> wrote:
>I like the idea of using a bigger filter, but the more I think about it
>the more I realize how little I know about oil pressure. So, I'm
>wondering if using a bigger filter could adversely effect the oil
>pressure, since you are now creating additional area in the system
>compared to what space the oil was designed to run through ? If the
>answer is to run the engine after you have placed the bigger oil filter
>on, stop the engine, let the oil settle, then check the oil level and
>fill to the proper level to compensate for the bigger filter, can that
>create a problem if the system is only designed to move a certain volume
>of oil through the engine ? Is there such a slight difference in the
>volume difference between the regular size filter and the larger one that
>such questions really don't matter ? As long as there is sufficient oil
>in the engine does oil pressure even matter ?
Well Todd, let's dissect the modern engine oil system. I'll start basic,
then get to the meat and potatoes. Oil is pumped by the oil pump, then
gets sent to center hole in the oil filter, then out the outside ring of
the filter, then to the engine main bearings (crankshaft main journals),
then to the rod bearings (crankshaft rod journals). After that is splashes
off the crank and rods as it spins giving the cam a bath and then falling
back to the pan. The oil gallery splits at the main bearings and goes up
to the cam bearings, then the top end of the engine (under the valve
covers). The top end is last to get oil. The oil is piped to all
bearings, then flows through and ends up back in the oil pan by
gravity. The oil clearances of typical main and rod bearings is 1/2 to 1
1/2 thousandths of an inch (.0005 to .0015") A sheet of regular notebook
paper is three thousandths of an inch (.003"). So divide the paper
thickness by three (.001") and that is the space the oil flows through
. Pretty small ehh?
The oil pump has a relief valve in it. It is usually regulated to 80 psi
for a high volume/high pressure pump. Most oil filters today have a bypass
valve rated between 60 and 90 psi. When oil is cold, it is thick, and
little gets to flow through the engine bearings. It's too thick. When
that happens, the relief spring in the oil pump bypasses the oil and sends
it back to the pan. So it's pumping oil from the pan and back to the pan,
when the pressure is above the set relief (80-90psi). As the oil warms up,
it gets thinner, and more can flow. At anything above 2000 RPMs, oil is
getting bypassed no matter how thin the oil is. The supply is greater than
what can be forced over the bearings, so pressure builds and the pump
bypasses and stays at the relief pressure. When hot, and your idling, the
pump is pumping all the oil it can at that RPM over the bearings, none is
bypassed (since oil pressure is below it's preset bypass).
Pressure and volume are both essential for proper engine lubrication. You
can have all the pressure you want, but if little oil is getting through
the bearings, failure can happen as more heat is produced than the oil an
carry away, so using a thicker than needed oil is harmful, especially when
cold. Common rule of thumb with oil pressure is 10 psi per 1000 RPM. So
if your engine is gonna spin 7000, you need 70 psi., 1000 - 10 psi. Volume
is what you want. The crank rides on the oil film between the crank and
bearing. Same will the rods and cam. Most can argue that, but it's
true. It's been discussed in extreme detail on other lists, specifically
the Fordnatics list several years ago. Bearings are very soft and can be
damaged easily. Bearings are soft so hard particles, like metal
dust/shavings, are forced into the bearing itself to avoid damaging the
riding journals whether crank, rods, or cam. Some aftermarket bearings are
coated with a white material which is very soft and can be scratched with
your fingernail. This prevents contaminants from scoring the
journals. Remember, the clearance is 1/3rd the thickness of a regular
sheet of notebook paper.
So back to your original question, will a larger filter affect
pressure. No. Once the filter is filled with oil no change in pressure or
volume would occur. A liquid is not compressable. So whatever comes in,
the same amount goes out, once the filter is full. An oil pump can fill a
quart filter in a couple seconds.
Have any questions, ask. Maybe some will think twice the next time their
picking out an oil for the next oil change.
Mike
__________________________
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