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Old 06-17-2021, 09:24 AM   #1
51woodie
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Default Viscosity VS Pressure

I was running Rotella 15-40 in my stock 59AB, and was getting 20 PSI at idle and 60 PSI at cruise speed. Some posts I read, considered that oil pressure to be too high, and could possibly cause damage to the bearings. So I decided to try Castrol 10-30. I went for a drive after the oil and filter change, and now have 18 PSI at idle and still have 60 PSI at cruise. I would have thought the pressures would be somewhat lower, but no joy. I'm sure the engine was rebuilt at some time before I got the car, so is it possible that a high volume pump was installed? Is 60 PSI really too much?
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Old 06-17-2021, 09:29 AM   #2
19Fordy
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Default Re: Viscosity VS Pressure

I wonder how accurate stock gauges really are? It would be interesting to warm a cup of each oil up to 200 degrees on the stove and then compare the viscosity, if possible.

Last edited by 19Fordy; 06-17-2021 at 09:43 AM.
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Old 06-17-2021, 09:44 AM   #3
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Default Re: Viscosity VS Pressure

It looks to me like the relief spring is a little too stiff. Your bearing clearances and oil pump are good enough to produce over 60 lbs pressure at cruise speed with either viscosity, so the relief valve opens at 60 psi, giving the same pressure at speed. The slightly lower pressure at idle is probably a result of the lesser viscosity of the 30 weight oil. Back in the day, people used to stretch or shim the spring to raise the maximum oil pressure. I'm an "8BA guy", but I think accessing that spring is not difficult on the earlier engines, so you may want to take the intake off and see if it is shimmed or maybe even replace it. That being said, 60 lbs is not high enough to worry about. When I had the engine in my Corvette rebuilt, they put in a hi volume/hi pressure pump, and that engine pegged the 60 lb gauge at hot idle. Now, that is excessive pressure!

I would be happy with the oil pressure you are showing.
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Old 06-17-2021, 10:13 AM   #4
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Default Re: Viscosity VS Pressure

Most people would probably be thrilled to have 60 pounds of oil pressure. Just keep the oil changed and happy motoring.
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Old 06-17-2021, 12:37 PM   #5
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Default Re: Viscosity VS Pressure

Thinner oil lower pressure. The thinner oil leaks out between the
clearance between the crankshaft and the innseart bearing. G.M.
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Old 06-17-2021, 02:22 PM   #6
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Default Re: Viscosity VS Pressure

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Quote:
Originally Posted by 19Fordy View Post
I wonder how accurate stock gauges really are? It would be interesting to warm a cup of each oil up to 200 degrees on the stove and then compare the viscosity, if possible.
"Project farm" on youtube does some testing like this with different engine oils. Interesting to see the different results and oil brand comparisons.
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Old 06-17-2021, 02:36 PM   #7
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Default Re: Viscosity VS Pressure

Quote:
Originally Posted by automotive stud View Post
most people would probably be thrilled to have 60 pounds of oil pressure. Just keep the oil changed and happy motoring.

Yep!!
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Old 06-17-2021, 02:41 PM   #8
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Default Re: Viscosity VS Pressure

Your pressure readings are fine. DONT even consider changing any oil pressure relief valve spring. Continue to enjoy your car and know your readings are fine.
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Old 06-17-2021, 02:44 PM   #9
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Default Re: Viscosity VS Pressure

Quote:
Originally Posted by Will D View Post
"Project farm" on youtube does some testing like this with different engine oils. Interesting to see the different results and oil brand comparisons.
Thanks for the viewing tip. Very informative.
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Old 06-17-2021, 03:08 PM   #10
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Default Re: Viscosity VS Pressure

If my Flathead motor's didn't have 60 or more pounds of oil pressure at cruise speed I
would replace the oil pump (80 psi pump) or shim the bi-pass valve in the pump.
I have owned and re-built dozens of flathead motors. Raced them back in the 60's.
Never spun a bearing due to oil pressure.
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Old 06-17-2021, 05:17 PM   #11
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Default Re: Viscosity VS Pressure

Quote:
Originally Posted by 51woodie View Post
I was running Rotella 15-40 in my stock 59AB, and was getting 20 PSI at idle and 60 PSI at cruise speed. Some posts I read, considered that oil pressure to be too high, and could possibly cause damage to the bearings. So I decided to try Castrol 10-30. I went for a drive after the oil and filter change, and now have 18 PSI at idle and still have 60 PSI at cruise. I would have thought the pressures would be somewhat lower, but no joy. I'm sure the engine was rebuilt at some time before I got the car, so is it possible that a high volume pump was installed? Is 60 PSI really too much?

Of course 60 psi at cruise is not too much. No harm will befall your bearings or any other engine part due to that pressure. The fact that you have 18-20 at hot idle says your bearing clearances are in good order as well...have fun driving it!


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Old 06-17-2021, 07:56 PM   #12
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Default Re: Viscosity VS Pressure

Since I had little to do this afternoon, I went through all of my documentation to see what the factory maximum oil pressure should be. I found only one source that had it, and that was a 1960 "Motor's" automobile repair manual that had entries for the '53 Ford. It lists the maximum oil pressure for a 1953 flathead was 57 psi. Interestingly, this was higher that listed for all of the later OHV engines.

It looks like 60 psi is right in the ballpark, given the possible variance in the gauge.
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Old 06-18-2021, 05:29 AM   #13
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Default Re: Viscosity VS Pressure

I agree with Tubman. I have no clue who started the myth of that kind of oil pressure
causes damage, seems to be a modern myth. When flatheads of all makes ruled the
road that 60# pressure said I'm healthy. Going to overheads back in my bus days all
V6 GMC's ran at 60# all day. A typical Detroit also runs at 60# at speed and about 10# at idle. The I HAVE in ink the straight 8 Buick Bus engine 60# at cruse 35 to 40# idle ::: when at cruse if its a weak 35# and say 10# idle >its time for rebuild,
thats right in the book. So my 59AB used every day; hot 60# all day and 40# idle.
I now have 9500 or so on this stock rebuild which was born on 20-50 racing oil with
zinc and all this is with the original oil pump. Shame on me, yesterday oil and grease its been a year but didn't use a drop of oil all year and no oil leaks anywhere.
Remember cam bearings will effect pressure.......sam
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Old 06-18-2021, 06:52 AM   #14
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Default Re: Viscosity VS Pressure

The higher pressure does not cause damage but it heats the oil and one of the functions of motor oil is to keep the bearing cool. If you have adequate pressure then your engine is getting lubricated. Like others said, you're okay.
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Old 06-18-2021, 08:36 AM   #15
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Default Re: Viscosity VS Pressure

Viscosity generally doesn't have much affect on the kind of pressure a pump can put out into the lube system but the clearances on bearings can show a difference with a different viscosity of oil. The more leakage around a set of bearings the less pressure can be built up. The differences aren't than much though as oil temperature increases.

These old engines need a lot less pressure at low rpm and under less load. They need the pressure under load and at higher rpm so I'd worry more about that than pressure at idle or low rpm. As long as it has some pressure it should be OK.

I use a calibrated direct reading gauge with a T fitting plumbed in to check my instrument readings.
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