- -----Original Message-----
From: Donald W. Miller [mailto:millerdw]
Sent: Friday, April 30, 1999 4:00 AM
To: gmcmotorhome
Subject: RE: GMC: Time to "Slay the myths again" What is the NORMAL oil
pressure
Don Wrote...
>Isn't most of the protection provided by the oil film itself ?
The protection comes from the film strenght of the oil under pressure.
Think of what would happen if you did a belly flop into water from a height
of 150 feet. It would almost certainly kill you. The oil in the bearing at
pressure can overcome the downward force of the piston when it fires and
completely isolate the journal from the bearing inserts
(this is why start up is bad, oil can't completely isolate the parts).
Since there is clearance in the bearing, a certain amount of oil will leak
out. The more clearance, the more leakage until the oil pump is unable to
keep up, causing lower pressure, and a vicious cycle until failure. Higher
viscosity will overcome excess clearance to a degree, but its not a good
fix. There can be other causes for low pressure, some of which were
mentioned, but oil pumps? I'm an optimist, but when it comes to worn
bearings in a 12,000 vehicle vs. an oil pump full of oil, its usually excess
bearing clearance.
As for Mondello data on oil pressure, alot of hot rod folk go to incresased
bearing clearance and compensate with high viscosity oil. Maybe this is
what Mondello data speaks to. But getting to your question, no the oil by
itself is not able to keep the journal and the bearing from meeting at zero
pressure and it would quickly disintegrate.
> I thought the advantages of a pressure oiling system is more oil moves
through the
>bearing surfaces for improved cooling and there is more even oil
>distribution.
Cooling is a function. There is an most efficient volume. Too little or too
much and it is not as effective.
>Am I overlooking another 455 killer lurking in this low oil pressure arena
>other than metal to metal contact ?
Metal 2 Metal is a result of the low/inadequate pressure to keep a
sufficient oil film in place. You want to have enough oil pressure to
overcome any potential loading of the bearing to keep metal to metal fromm
ever happening. In Heinz's case, he should keep a watchful eye on the
pressure to make sure things don't take a turn for the worse. Othrwise, he
is doing what I would do.
>Low oil pressure often indicates a high time engine nearing overhaul but
>that is a different matter altogether.
Maybe not. Assuming something is not wrong like the oil gallery plug, it
sounds like something has caused the bearing clearance to increase. Oil
pumps (at least in my experience) are rarely a culprit. Coolant leaks (into
the crankcase) are a big cause of engine failure through excess bearing
clearance. The antifreeze acidifies the oil and eats the bearings.
Hopefully, its just a bad gauge.
>Lower oil pressure has some advantages as long as there is enough pressure
>to move oil to all parts of the engine where it is needed.
I don't think so. You need to keep the parts from every touching each other
(bearings) and still get to all other moving parts, valve lifters, etc. The
pressure required is a function of load. I don't know what is safe for the
455 in a GMC but if oil pressure is lower than that rule of thumb several
have quoted, I would be concerned. I'm currently rebuilding a Ford engine
that we watched the oil presure at idle fall below that rule of thumb. At
about 7 lbs idle (the thresehold for the idiot light to come on) the real
culprit reared its ugly head, excessive rod bearing clearance. At first at
about 2,000 rpm under load, then later, all the time. The oil presure at
time of tear down dropped down to 0-2 lbs at idle and never got above 10 lbs
at higher speed. Needless to say the rest of the bearings failed also.
walter bright, 76GB, Delanco, NJ
From: Donald W. Miller [mailto:millerdw]
Sent: Friday, April 30, 1999 4:00 AM
To: gmcmotorhome
Subject: RE: GMC: Time to "Slay the myths again" What is the NORMAL oil
pressure
Don Wrote...
>Isn't most of the protection provided by the oil film itself ?
The protection comes from the film strenght of the oil under pressure.
Think of what would happen if you did a belly flop into water from a height
of 150 feet. It would almost certainly kill you. The oil in the bearing at
pressure can overcome the downward force of the piston when it fires and
completely isolate the journal from the bearing inserts
(this is why start up is bad, oil can't completely isolate the parts).
Since there is clearance in the bearing, a certain amount of oil will leak
out. The more clearance, the more leakage until the oil pump is unable to
keep up, causing lower pressure, and a vicious cycle until failure. Higher
viscosity will overcome excess clearance to a degree, but its not a good
fix. There can be other causes for low pressure, some of which were
mentioned, but oil pumps? I'm an optimist, but when it comes to worn
bearings in a 12,000 vehicle vs. an oil pump full of oil, its usually excess
bearing clearance.
As for Mondello data on oil pressure, alot of hot rod folk go to incresased
bearing clearance and compensate with high viscosity oil. Maybe this is
what Mondello data speaks to. But getting to your question, no the oil by
itself is not able to keep the journal and the bearing from meeting at zero
pressure and it would quickly disintegrate.
> I thought the advantages of a pressure oiling system is more oil moves
through the
>bearing surfaces for improved cooling and there is more even oil
>distribution.
Cooling is a function. There is an most efficient volume. Too little or too
much and it is not as effective.
>Am I overlooking another 455 killer lurking in this low oil pressure arena
>other than metal to metal contact ?
Metal 2 Metal is a result of the low/inadequate pressure to keep a
sufficient oil film in place. You want to have enough oil pressure to
overcome any potential loading of the bearing to keep metal to metal fromm
ever happening. In Heinz's case, he should keep a watchful eye on the
pressure to make sure things don't take a turn for the worse. Othrwise, he
is doing what I would do.
>Low oil pressure often indicates a high time engine nearing overhaul but
>that is a different matter altogether.
Maybe not. Assuming something is not wrong like the oil gallery plug, it
sounds like something has caused the bearing clearance to increase. Oil
pumps (at least in my experience) are rarely a culprit. Coolant leaks (into
the crankcase) are a big cause of engine failure through excess bearing
clearance. The antifreeze acidifies the oil and eats the bearings.
Hopefully, its just a bad gauge.
>Lower oil pressure has some advantages as long as there is enough pressure
>to move oil to all parts of the engine where it is needed.
I don't think so. You need to keep the parts from every touching each other
(bearings) and still get to all other moving parts, valve lifters, etc. The
pressure required is a function of load. I don't know what is safe for the
455 in a GMC but if oil pressure is lower than that rule of thumb several
have quoted, I would be concerned. I'm currently rebuilding a Ford engine
that we watched the oil presure at idle fall below that rule of thumb. At
about 7 lbs idle (the thresehold for the idiot light to come on) the real
culprit reared its ugly head, excessive rod bearing clearance. At first at
about 2,000 rpm under load, then later, all the time. The oil presure at
time of tear down dropped down to 0-2 lbs at idle and never got above 10 lbs
at higher speed. Needless to say the rest of the bearings failed also.
walter bright, 76GB, Delanco, NJ
