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Old 06-23-2020, 07:33 AM   #1
alanwoodieman
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Default lowest oil pressure for engine to survive

1942 Mercury engine, starts out with good oil pressure 50#, slowly decreases to about 15# as it is being driven, 20w50 oil, added some lucas extender and op went to 18# driving. Worn bearings? bad oil pump? bad relief spring? other wise runs great. no blowby. oil pressure was done with mechanical gauge
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Old 06-23-2020, 07:49 AM   #2
David J
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Default Re: lowest oil pressure for engine to survive

Not really an answer to your question but those readings are quite good for a used 33-34 motor . NO CAM BEARINGS 32-34 and that does not help oil pressure .
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Old 06-23-2020, 07:51 AM   #3
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Default Re: lowest oil pressure for engine to survive

You don't say about the age or condition of the engine, or your intended usage.

If an older higher mileage motor and you're not going coast to coast those figures sound ok to me.

The 59a in my truck and my crusty flatty both have similar or lower numbers than that.

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Old 06-23-2020, 08:10 AM   #4
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Default Re: lowest oil pressure for engine to survive

That sounds about right for 20/50 when it warms
up. You are going to get "stories" of 3 or 4 lbs is
OK but don't believe them. It takes a fair amount
of oil to get a good flow into the bearings. A good
flow also cools the oil when it splashes off the crank
shaft. With an oil temp gauge on an open highway
you can watch the oil temp go up as the engine works
going up hill and cool down as it comes back down hill.
Had fun watching it in my motor home on long trips. G.M.
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Old 06-23-2020, 08:31 AM   #5
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Default Re: lowest oil pressure for engine to survive

I'm fighting sort of the "same" issue.......I'm running straight 40 here in Florida and this is strictly going by the pressure gauge on the car as I have not plumbed in a manual gauge at the block YET!!! SO that would be my first suggestion is to plumb a manual gauge at the block for a more confirmation and possibly accurate information. My next move is on the "next" oil change, drop the pan and maybe work the spring in the oil pump to see IF it will increase the pressure any OR possibly go to a higher pressure pump!!! Mine starts out at around 20lbs but once warm and running I get about 7ish going around 45 -50mph and "0" at idle!!! It runs spectacular!!! We have a "long time" friend who has been an engine mechanic going on 60 years now and as he told us a long time ago...."IF" its running good and has oil pressure when you are "rolling" down the road, run it till you either "have" to pull it OR until you can afford to pull it and rebuild it!!" SO following others comments and or suggestions here!!!
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Old 06-23-2020, 08:36 AM   #6
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Default Re: lowest oil pressure for engine to survive

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I've always heard 10 pounds per 1,000 RPM. Don't know if that's just some old wives tale or not. I do know that I had an 8ba in a '40 big truck and when it got warm it had 10 pounds or less at warm idle. Didn't have it long enough to really know how the low idle pressure affected engine life but it made me nervous looking at the gauge at warm idle.
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Old 06-23-2020, 08:38 AM   #7
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Default Re: lowest oil pressure for engine to survive

are you talking at idle and hot? you should be closer to 60 on start up but its not bad, I think the main thing is when you rev you see the oil pressure go up.
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Old 06-23-2020, 08:41 AM   #8
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Default Re: lowest oil pressure for engine to survive

It could be a combination of all the things you mentioned. How many miles on the motor? If this is an older or un rebuilt engine it could have run a significant number of miles with old parafin based crap oil and just have a lot of wear. If it runs quietly I wouldn't worry.
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Old 06-23-2020, 08:45 AM   #9
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Default Re: lowest oil pressure for engine to survive

i used to work in a Body shop in High School, they has a 1964 Ford F350 Tow Truck with a 292, on a cold start it would have 50 lbs, after it wormed up the gauge would drop to 0 lbs. there was oil flow because it did that for a long time
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Old 06-23-2020, 10:15 AM   #10
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Default Re: lowest oil pressure for engine to survive

Its best to have at least 10 pounds of pressure per 1000 RPM. Theres certainly an issue inside the engine however what on your list is causing the issue is impossible to pick. A low reading like you have will work fine around town since your not really turning very high RPM. Just go easy until the cause is determined which will mean getting inside the engine.
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Old 06-23-2020, 10:29 AM   #11
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Default Re: lowest oil pressure for engine to survive

Since you get a higher reading at start relief spring is probably ok...what you lack is flow.
So either worn so flow needed is higher then pump can deliver when hot...or pump worn.
A new pump may take you up to a level you can push on for a few more years...
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Old 06-23-2020, 11:19 AM   #12
alanwoodieman
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Default Re: lowest oil pressure for engine to survive

there is a mechanical gauge on engine and the old electric gauge ran about the same pressure at the same time frame. think we need to pull oil pan and inspect the pump for wear and plastigauge the mains. the engine is of unknown mileage/lineage, runs smooth, does not over heat more than normal, good compression, good vacuum reading. relief spring is under intake.
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Old 06-23-2020, 12:24 PM   #13
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Default Re: lowest oil pressure for engine to survive

Quote:
Originally Posted by rockfla View Post
I'm fighting sort of the "same" issue.......I'm running straight 40 here in Florida and this is strictly going by the pressure gauge on the car as I have not plumbed in a manual gauge at the block YET!!! SO that would be my first suggestion is to plumb a manual gauge at the block for a more confirmation and possibly accurate information. My next move is on the "next" oil change, drop the pan and maybe work the spring in the oil pump to see IF it will increase the pressure any OR possibly go to a higher pressure pump!!! Mine starts out at around 20lbs but once warm and running I get about 7ish going around 45 -50mph and "0" at idle!!! It runs spectacular!!! We have a "long time" friend who has been an engine mechanic going on 60 years now and as he told us a long time ago...."IF" its running good and has oil pressure when you are "rolling" down the road, run it till you either "have" to pull it OR until you can afford to pull it and rebuild it!!" SO following others comments and or suggestions here!!!
Look up a couple posts I did a few years ago that describe how to
build a little test fixture and using a 1/2 inch electric drill and a
mechanical gauge test the oil pump and relief valve to see where
it dumps. Some will slowly drop and not come back up until the
engine is increased. This way when you install the pump you know
it works. I wouldn't put any pump in without testing first. G.M.
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Old 06-23-2020, 12:54 PM   #14
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Default Re: lowest oil pressure for engine to survive

While those numbers aren't very high, my '47 has similar numbers and has been that way for years. Startup I get close to 70# but after warmed up going down the road I might get 20lbs, it drops down to about 5 at a slow idle warmed up, that's with 15w40 oil. I've driven the car from NJ to MD three times, I wouldn't hesitate to drive it anywhere. That said, at some point I plan to go through this engine.
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Old 06-23-2020, 01:08 PM   #15
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Default Re: lowest oil pressure for engine to survive

Quote:
Originally Posted by G.M. View Post
Look up a couple posts I did a few years ago that describe how to
build a little test fixture and using a 1/2 inch electric drill and a
mechanical gauge test the oil pump and relief valve to see where
it dumps. Some will slowly drop and not come back up until the
engine is increased. This way when you install the pump you know
it works. I wouldn't put any pump in without testing first. G.M.
Would that be the one you did with the "plastic tube" and "100psi Gauge"???
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Old 06-23-2020, 04:31 PM   #16
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Default Re: lowest oil pressure for engine to survive

GM, you are right about the oil temp, for a long time I hade one on the 33,it would get to over 100deg C on long uphill pulls , ran around 75-80C on the flat in summer here.
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Old 06-23-2020, 05:05 PM   #17
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Default Re: lowest oil pressure for engine to survive

Hey, old Chevies of that era didn't even have any oil pressure. Strictly splash!
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Old 06-23-2020, 05:09 PM   #18
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Default Re: lowest oil pressure for engine to survive

This, from Ron Bishop's book "Rebuilding The Famous Ford Flathead" - Page 51 - "Ford used 40 non-detergent oil (Summer) in all of the flathead motors. The flathead oil system ran between 10-15 pounds of pressure, but is not considered a pressurized system by today's standards."
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Old 06-23-2020, 08:05 PM   #19
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Default Re: lowest oil pressure for engine to survive

Quote:
Originally Posted by petehoovie View Post
This, from Ron Bishop's book "Rebuilding The Famous Ford Flathead" - Page 51 - "Ford used 40 non-detergent oil (Summer) in all of the flathead motors. The flathead oil system ran between 10-15 pounds of pressure, but is not considered a pressurized system by today's standards."
I find this to be an amazing statement and quite hard to take seriously. I think I have Bishop's book (I bought every flathead book I could ever find) and have not looked at it in years. I guess this helps explain why.

My rebuilt 258" engine with a stock Ford pump runs 55 psi at anything over 2000 RPM. My 1967 L79 Corvette 327 runs at it's factory specification as well. That specification is 45 psi.
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Old 06-26-2020, 05:13 AM   #20
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Default Re: lowest oil pressure for engine to survive

Quote:
Originally Posted by philipswanson View Post
Hey, old Chevies of that era didn't even have any oil pressure. Strictly splash!
The ol' 'Lucky Dip'.
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