Is higher oil pressure better for the engine?

CJE

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4
Location
Florida
Thread starter
This is a generic question - but the subject vehicle is a 2005 GMC 4.3 engine. Sleeve-type bearings are lubricated because a hydraulic oil ‘wedge' is formed as the shaft rotates relative to the bearing. This is also true of roller type bearings, now that I'm thinking about it, as each roller acts as a rotating shaft - just on a smaller scale. But I am really asking about bearing design / performance in terms of a ‘bushing type' of bearing - such as an auto engine or a large hermetic or open-drive compressor. And in that context: What benefit does oil pressure provide? More / higher oil pressure is not what actually supports the rotating shaft inside the bearing - it is the hydraulic-wedge which is created by the rotation. So: so long as sufficient oil is provided to ensure a full layer of oil on the bearing - isn't any higher oil pressure just a wasted effort? I ask because I recently installed a good-used engine in a 2006 GMC truck. The old engine (essentially a small block chevy ) always ran the 5W30 oil that GM specified. And it always showed about 18-20 lbs oil pressure at hot idle. Maybe 40 lbs. cold at speed - with about 30 lbs. hot at speed. New (er) engine initially idled hot at about 10-12 lbs and ran at about 20-25 lbs. on the road. That troubled me so I asked the shop what oil they had used. They claimed not to know, saying only: "whatever was the right oil for that engine." I use Mobil 1 in everything, and I doubt they used that, so at about 1000 miles or so I changed the oil. Truck calls for 5W30 but it lives in south Florida so I thought: oh I'll just use 10W40 instead - I don't need any 5 weight oil characteristics. Now the cold idle/speed pressures are about 35 & 65 - with the hot idle/speed pressures about 25 & 60 lbs. Which was sort of ‘comforting' to me. All of which has made me ponder the actual benefits of higher oil pressure. I now theorize that the engine shop used 0W20 as that is what many modern Asian vehicles use - and that was why my initial oil pressure was so ‘low'. OK; so now I have oil pressure which seems 'normal' to my virtually antique mechanical mind - but . . . is it actually better for the engine? Thinking about how oil actually acts inside a bearing, and also about how all the newer engines use physically thin oil, am I making a mistake in judgment? Maybe I am just wasting horsepower creating a higher oil pressure while it is actually no actual benefit to the engine? Yes; it does make me Feel better - but is it Actually better? What do you all think?
 
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16,357
Location
NH
Pressure isn't lubrication. A pump deadheaded into plugged pipe has excellent pressure--but zero lubrication for anything no longer downstream. Now pressure is required to push oil into small areas, sure. And when all the clearances open up and the oil pump can't flow enough volume to keep pressure up, then something will start to get starved. And problems may start. But where is that point? That I don't know. Pressure sure looks good on a gauge but above a certain point it's just robbing power--an engine doesn't develop pressure for free. Anyhow. An old school OHV GM 4.3L is probably happy on either xW30 or xW40. I think xW40 may have been spec'd for when towing? The engine you got may have more miles, or, due to production tolerances, may have slightly worse clearances than the motor you pulled. With pressures up where they belong I'd be tempted to just run the 10W40 until something changes. Whether it is better or not, I doubt you are losing mpg (let alone money) using a slightly thicker oil. Sleeping better at night might be a better value. wink
 
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1,326
Location
South Carolina
My Camaro has just 7-8 psi at 200*F oil temp. It runs just fine. You'd be surprised how little oil pressure you actually need. More oil pressure than necessary just stresses parts and increases the risk of aeration.
 
Messages
181
Location
Oklahoma
My 1994 Astro van 4.3 V6 runs about 40 psi hot & idle at about 10-15 psi hot. I use Mobil1 5w30, it has been this way for almost 10 years, so I would say not to worry much about the lower pressure. Cheers2
 
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3,759
Location
down in the park
newer cars have adjustable flow oil pumps so oil pressure can be set at 15 psi regardless of rpm... to reduce fuel consumption. I guess that answers your question? but it's important to always ensure a bit of oil pressure...
 
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783
Location
Austin Texas
The pressure the oil pump creates just cause oil to flow through the engine. As long as the bearings do not get all of their oil thrown out and become dry, they remain lubricated. Lubrication has to do with oil viscosity not oil pressure.
 
Messages
293
Location
MN
In hydraulics (including engine oil), pumps do not create pressure - they create flow against pressure. The difference is important, and taught in Fluid Mechanics. Pressure is resistance to flow. Pressure gauges tell us nothing except if there's flow or not. We do not know what adequate flow is in the design of an engine unless we have access to the lubrication design criteria - from there, adequate pressure to produce that flow rate can be calculated and observed. Therefore, pressure is an indirect measurement of flow. supton alludes to this above. The lubrication design engineer needs to establish how much flow is required to lubricate all frictional machine elements. From there, he/she calculates the size of the supply feed hole for the restriction it produces and the stock oil pump's flowrate vs. pressure. - Retired Lubrication, Mechanical, & Hydralics Engineer
 
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1,885
Location
WY
I remember oil pressure gauges back in the day that had a red band below 5psi and above 80 psi and iirc modern vehicles have their check engine lights/oil pressure lights come on below 5 psi without an upper alarm. That being said....see above about pressure versus flow.
 
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933
Location
Arizona
Having good pressure while running indicates the flow is high enough to build that pressure across the series/paralell oil passages/bearings. Keeping in mind that not all the bearings have identical clearances. So having higher pressure is better because it is more likely that a "Loose" bearing isnt/wont drop too much pressure and cause a problem.
 
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Messages
1,223
Location
Indiana
The old standard for race engines was 10psi per 1000 rpm based on the rod bearings as the critical item. I think that number is outdated now, but for the street half of that is plenty.
 
Messages
46
Location
Gulf Coast
Oil pressure is not the surrogate for lubrication but rather a measure of getting the lubricant to where it's needed. IIRC my ISC8.3 had the low pressure light set at 15psi not for the rods and mains but for the valve train.
 
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