Technical Oil Flow or Oil Pressure

An aquaintense from my neighborhood stopped by my house last evening.
He perceded to tell me how low the oil pressure was on his motor (Ford FE 390 CI) at engine temp idling. I asked him what the pressure was 15psi at idle and about 60 when driving.

I said I would be more concerned about oil flow rather than oil pressure.
He disagreed and said oil pressure was more important than flow.
I have only owned Fords in my life and have read that oil flow is what matters.
Maybe I'm wrong, maybe its both.

 

Oil pressure is the indicator of how much oil is flowing.
So you are both right. We measure flow via pressure.
That is assuming that the tolerances are all correct and the oil pump is the correct application then monitoring oil flow via the oil pressure gauge is how we can monitor lubrication.

 

Simply put: Pressure is resistance to flow...

 

Both are important.

 

The car I'm working on generally has 20 to 25 psi when driving, and around 5 at idle, varies with temp and whether or not it's in gear, etc. I'm seriously considering disconnecting the gauge, and relying on the idiot light....then it'll be fine.

I guess there is enough leakage that it flows right on out of the oil system, as fast as it can.

 

Oil pressure is an indicator of resistance to flow.
Flow is not measured by pressure.
Oil can flow without any pressure, once downstream of a restriction. When you start to restrict that flow , pressure is built up and then can be measured. If that flow starts to fall away as the pressure rises, then the trouble starts, then you measure the flow loss.
Early engines depend more on flow as tolerances are less so it’s more about flooding those critical areas. Later engines with higher tolerances need oil pressure to force oil into the more critical areas and maintain it there.
IMHO.

 

Flow is directly related to the "size" of the passages, the "size" of the pump.

If you don't open the oil passages (galleys), you WILL NOT get more flow or volume.

The old trick of putting a big Chevy oil pump on a small Chevy engine...all that did was to make the engine work harder to turn the larger gears and to "try" to push the larger pump volume of oil into the block. It really did, nothing...good.

Mike

 

depends on the engine, I think. Really early ones didn't have pumps (ever work on a Model T?), and flow to the bearings was done by splash then draining down the inside of the crankcase, for the most part.

Seems to me that worrying about whether flow or pressure is more important, is rather pointless. As long as the moving parts are getting enough oil to stay lubricated, it's not a big deal.

 

On my hot rod Flatmotor I have 60 lbs dead cold medium idle 70 mph is 30-35 idle is 5 should I be worried. NO partial flow filter. This is running straight 30 wt oil. Inviting answers—-needs work or drive it ?

 

I agree Squirrel, and no , only As onwards.
I thought the question was revolving around oil pumps.

 

Interesting thing about the T, the bearing clearances are about the same as modern engines---.001" per inch of bearing diameter.

 

My 394 was like that. Factory spec was 35-40 psi driving, which it had. But warm idle was about 1 psi. I take that as an indicator of an engine that is completely worn out.

I understand the thought process behind the mindset that as long as there is flow and stuff is getting enough oil, the pressure doesn't matter. I disagree to a point. There are significant compressive forces put on bearing surfaces, rod and main bearings especially. Having oil that gets squished out too easily because the bearing clearance is too wide tells me that the engine is going to be susceptible to wear and metal-to-metal contact. I'm not saying you need 40 psi at idle, but you need some pressure.

 

I wouldn’t of picked that to be honest Squirrel, but I suppose if they were any larger clearance they would have knocked badly and beat themselves senseless.
On the A big ends I was surprised by the equivalent pressure they reckon is generated by the dippers verse rpm!

 

I did some digging and found a interesting read on "Synforce Lubicants web site.

Oil pressure, the real story

A customer changes their brand of oil and then complains of low oil pressure ... is this perception or reality and is low oil pressure inherently bad, why don't customers complain about high oil pressure?

Customers who change brand and experience an increase in oil pressure usually show delight, when in fact they should show concern.

The first thing we have to understand is the word 'pressure' ... Pressure is resistance to flow





A pressure sensor is usually placed down stream of the filter to monitor the oil pressure the read out being the oil gauge in your dash.

The most important criterion for good lubrication is OIL FLOW, not oil pressure

** Low oil pressure is not necessarily bad, in fact it can be very beneficial within limits

** High oil pressure is not necessarily good, in fact it can be cause for concern

As pressure is resistance to flow, an engine with wide oil galleries and using a low viscosity oil will have a faster flow rate and lower oil pressure, a very desirable condition for minimising wear.

Conversely, an engine with smaller galleries and a higher viscosity oil will have less oil flow and a higher oil pressure, resulting in less efficient lubrication. In extreme cases, the oil would never get to the critical parts to do it's job of lubrication and reducing wear, and in some instances the oil would 'by pass' through a valve fitted to most engines, that is to allow the oil to return to the sump, reducing the amount of oil getting to the those moving parts.

High oil pressure (to high) can actually cause damage to seals and components, and should be seen as an indication of poor flow.

Cold start, When an engine starts from cold, all of the oil is in the sump.

The oil pump cannot deliver oil pressure until it has sucked up the oil and has begun pushing the oil through the galleries, hence cold start lubrication is best achieved from an oil with good cold flow Properties. This is essential as wear is most experienced at cold start due to the lack of oil available to engine components.

You may have noticed that your oil gauge reads higher at cold start up, this is telling you have more resistance to flow, and probably at a point in time you need faster flow (less pressure).

At this point and until the engine warms up and the oil pressure stabilises, you do not have proper lubrication.

Until stable oil pressure is obtained, wear rates are high due to inadequate lubrication and a sign of a good oil is one that gives stable oil pressure the fastest.

So when you change brands and experience higher cold oil pressure, think of the above.

Normal operation

Under ideal circumstances, oil pressure should be stable within reason, and any large increase or decrease should be investigated for possible mechanical damage. Consult your Vehicle hand book or experienced service centre to learn of the correct oil pressures your Vehicle manufacturer designed the engine to run on, both cold and hot.

In Summary

1. Watch for any large decrease or increase in pressure from normal, this usually suggests that you have a fault and it should be investigated.
2. Remember, Correct flow is more important to lubrication than pressure
3. Choose a good quality oil with a high Viscosity Index to minimise thinning at temperature.
4. Cooling is an important function of any engine oil, and flow rates effect this.

Do not be fooled by those oil companies building a 'thicker' oil under a standard viscosity rating so you will endorse their product over others, based on the false premise of higher oil pressure is better.

Beware of high oil pressure, it may cause damage.

 

Well known engine builder and racer Smokey Yunick gave the opinion that 10 PSI oil pressure per 1,000 RPM is adequate. Good enough for Smokey is good enough for me.

Pressure and flow are related, but quite different. If your bearings and every other part of the engine is tight enough to build reasonable pressure you have adequate flow. Just what "reasonable pressure" is will vary by opinion, but I'll stick with Smokey on that. Like said by a couple others, pressure builds when flow is restricted. Put a high pressure and high volume pump on a engine that is not excessively worn and you'll likely be blowing a lot of oil out of the pressure "pop off" that many (most?) oil pumps have. All you're doing then with the excess flow is wasting HP and likely adding heat to the oil and possibly aerating it in the process. Oil pumps do take a surprising amount of torque to build pressure. Anyone who has primed the oil system in an engine with a electric drill knows it takes a firm grip to hold on to the drill. Why build high oil pressure only to just waste HP pumping it in circles.

Lynn

 

We go by a general rule of 10psi per 1000rpm for your 60's on street/strip stuff, 10 pound at idle is generally fine, if you've got 20 pound at 4000rpm then you have cause for concern.

Edit,
Lake beat me to it

 

BTW........tell your neighbor don't worry "most FoMoCo engines have low oil pressure (compared to a durn Shivel-lay".
I've had Fords blink the oil pressure light off/on at traffic lights and still last for years and thousands of miles.
6sally6

 

I wonder how the oil gets out of the bearings, between the time you shut off the engine and start it again?

I've taken apart a lot of engines, some that have been sitting for decades since last run....and oil comes running out of the bearings, after all that time. Very strange.

there are some neat things going on in engines that we probably don't understand quite as well as we think we do.

 

The GMC 6 engine I built fo my son used as many of the racing parts as possible. This included the stock rods we used racing with .018” side clearance. It idles a 20 psi warm which I expected and runs at 40 @2000 rpm.. I also did not seal the stock rocker arm overflow. One nice thing is the cam gets a lot of oil from the crank when running.

 

I believe side clearance on the rods lets the oil escape. IF the bearings are good. I sure as hell would not want 6 lb of oil pressure at 6,000 rpm. LOL The bigger the bearing clearance the bigger the pressure. Load and pressure go hand in hand. Those bearings are riding on a film of oil. The bigger the force on the bearing the more oil pressure is required to maintain the film between bearing and crank. Or rods. Our A/Fuel hemi's ran 4 thou on the mains and 2 to 3 thou on the rods. Big clearance takes big oil pressure. I believe in more than normal oil pressure. Call it peace of mind. Guys tell me big oil pressure will make the seals leak and gaskets too. I call bullshit. Would you rather have 10 lbs more than you need? Or watch a flickering oil light on your way home on a hot day.

 

So there is some truth to all of this, and some B.S. too. What they are describing relates very much to viscosity. (technically, a measurement of "Viscosity", is a measurement of an oil's resistance to flow) It sounds to me like this company's product rides at the low side of the viscosity range, and this is their marketing attempt at persuading their customers that lower pressure is good and high pressure is bad. Eh, like I said, some truth to that, and some B.S. A higher viscosity oil will have more resistance to flow, will generally show higher pressure, but it will also have more film strength and provide greater protection against wear, to a point. At some point, increased viscosity will result in more wear due to lack of flow to critical areas in an engine, which is the point made in that article. But they skipped the part about higher viscosity providing increased film strength and reduced wear. It's all about balance, i.e. having high enough viscosity to provide higher film strength, but not so high as to cause a problem with flow. Following manufacturer guidelines is usually the best tactic.

 

Well, not really. You need to read up about hydrodynamic lubrication and how it works. The oil in the bearing becomes under incredible pressure, thousands of PSI, much, much higher than is supplied by the pump. The pressure shown on the gauge just indicates there is more oil flowing to the bearings than can be taken in by them, any excess just causes drag and parasitic loss, as well as generates heat. As far as choosing between having an extra 10 psi or a flickering light, I'll take the extra psi. But that doesn't mean the rest of what you posted is correct.

http://www.marinediesels.info/2_str...lubrication,contact with the bearing material.

https://www.tribonet.org/wiki/hydrodynamic-lubrication/

 

Ya beat me too it! Read that subject many, many years ago and quit worrying too much about pressure.

 

My car is 60 years old and probably has had many different brands weights and viscosity fed through it.

With all the choices of oil in todays market do you still recommend using manufactures recommendations for oil on older car motors. I ask this because alot of the older cars have had motors re-built or highly modified with modern rubber compounds. What are your thoughts on using synthetic or diesel oil in older motors?

 

Doesn't oil flow to the rods and mains when the holes are not under pressure, and stay there while they are?

 

I guess you could mind screw it all day long or make sure your street engine has 30 lbs hot or so and move on to why my wiper blades are black rubber and not white.

 

Well, modern oils are greatly improved over engine oils available 60 years ago. If you could find oil that met those old specs it probably would not be the best thing to use. When I mentioned manufacturers recommendations I was thinking of viscosity. And yeah, I do think following recommendations for viscosity range is still best, for stock & lightly modified engines. For highly modified engines and race engines it's different, manufacturer rec's generally don't consider those applications.

Synthetic oils are good and have a lot of benefits, without any real negative effects, if they're additized correctly. In the early days of synthetics they didn't get that just right, and there are a lot of horror stories about dried/shrunken seals. But I think they've figured that out these days and that shouldn't be a problem. But the reputation of synthetics causing leaks is still out there, a lot of guys don't trust them because of that. Synthetic oil naturally resists oxidation (aging), so they can last a lot longer; and they are more stable under extreme temperatures, so they can add some protection in the event of an overheating condition, or if you get caught in extremely cold weather. In So Cal that isn't a problem I have to deal with.

I love diesel engine oils for older engines, I think they are great. In the old days they were not a great choice, because the additive packages used had to deal with high levels of sulfur in diesel fuel, and it took a high level of ash to neutralize the acids that develop during combustion. And high levels of ash can lead to piston deposits; so it was a great idea to use diesel engine oil in a car engine. But for many years now the sulfur levels in diesel fuel have been reduced to very low levels, and the additive packages have changed, and low ash levels are the norm (there is even a mandated cap on ash at 1%). So the concern about piston deposits is no longer valid. Diesel engine oils are more robust and provide great protection against wear and corrosion, IMO perfect for old flat tappet engines that don't get run every day. And the viscosity grades of 10W-30 and 15W-40 work well in engines designed for SAE 30 & 40 grades. I think they're the best option, and that's what I use.

 

Ya, I could, but I'm just an old dumb guy trying to still learn.

 

Eh, it's good to kick around things like this once in awhile and get the cranial juices flowing. Call it mind screw if you like. What the hell, we got all day, might as well....

 

Since time is all around us... ;
https://540ratblog.wordpress.com/ .
Worth the read & consideration. Probably posted here in the distant past.
Marcus...