Oil Pressure Vs. Oil Flow

[Scud]

I can't see taking a motor apart if it runs fine just because you get oil pressure readings like that. I am pretty sure the Guzzi motor is a plain bearing motor. Plain bearings do not rely on high oil pressure to work, they rely on oil flow. They need oil to work but they do not need high oil pressure to work.

 

 

Here's an update regarding the LeMans oil pressure situation. 

• I've put about 2,500 miles on it in this condition - and I assume it was this way when I got it.
• Pressure readings are consistent with previous posts.
• It doesn't burn oil.
• It still feels weird to see the low oil pressure on the gauge, but there is always some pressure - it never drops to zero.
• I'm glad to have the gauge rather than the light - the light would come on sometimes at idle, but since it only comes on at something like 4 PSI, it only provides a limited amount of information.
• Use has been mostly commuting, but I got about 600 miles of spirited riding in Northern Arizona during the last few days. It's running strong even under hard acceleration and long high-speed sections.

My conclusion - the engine seems to operate well with oil pressures lower than commonly-held beliefs (self included) would suggest. At least for the short-term. Long-term test now in progress...

[docc]

For sure, I've always been in the "30-50 psi" camp.

 

Low pressure at idle could simply mean the idle is too low (common).

[pete roper]

I can't see taking a motor apart if it runs fine just because you get oil pressure readings like that. I am pretty sure the Guzzi motor is a plain bearing motor. Plain bearings do not rely on high oil pressure to work, they rely on oil flow. They need oil to work but they do not need high oil pressure to work.

Do you know anyone else local to you with a V11? You could mount your oil pressure rig on theirs and see what pressures you get, but even if yours is lower I can't see taking it apart if it runs well. Look at the moving parts that you can see without taking it apart. Do they show signs of heavy wear?

Absolutely 100% incorrect. The exact opposite is true. Pressure is the most vital factor in the oil being able to form a wedge. Volume is needed for cooling but pressure is the vital ingredient.

 

If the 25PSI figure quoted by the OP is accurate then one or other of the major bearings is seriously compromised. The reasons for this are well known.

 

Pete

[GuzziMoto]

 

I can't see taking a motor apart if it runs fine just because you get oil pressure readings like that. I am pretty sure the Guzzi motor is a plain bearing motor. Plain bearings do not rely on high oil pressure to work, they rely on oil flow. They need oil to work but they do not need high oil pressure to work.

Do you know anyone else local to you with a V11? You could mount your oil pressure rig on theirs and see what pressures you get, but even if yours is lower I can't see taking it apart if it runs well. Look at the moving parts that you can see without taking it apart. Do they show signs of heavy wear?

Absolutely 100% incorrect. The exact opposite is true. Pressure is the most vital factor in the oil being able to form a wedge. Volume is needed for cooling but pressure is the vital ingredient.

 

If the 25PSI figure quoted by the OP is accurate then one or other of the major bearings is seriously compromised. The reasons for this are well known.

 

Pete

 

Well, then plain bearings can't work in a Guzzi I guess.

They certainly don't have enough oil pressure to over come the forces applied to the bearings. It is the rotation that forms the hydrodynamic wedge allowing a plain bearing to work in a motor.

Assuming hydrodynamic is not just a fancy word but also how plain bearings can work with low oil pressure.

[motoguzznix]

Scud

 

There might be another reason for the low oil pressure when the engine is hot:

 

Behind the oil cooler connections at the oil pan there is an intermittend part to the oil filter/thermostat unit. These two parts have an Oring front and rear. Change these 4 Orings and take care they are a tight fit in the bores. If these Orings do not seal properly, oil can escape thus reducing pressure. There is only flow in that area when the oil warms up and the thermostat starts to open. 

[Scud]

@ Pete and GuzziMoto - thanks for the contrasting views. What do you think of the short discussion at the link I presented earlier in this thread? (quoted below for convenience)

 

Doing a little reading on the topic, I chanced upon a discussion forum for automotive engineers. It seems they don't really concern themselves with pressure very much. One engineer said "...any positive number should be acceptable. Customary minimum numbers are basically safety factors." Another talked about differences in pressure due to different weights of oils - and said the differences don't matter so long as there is enough flow. Another actually talked about how thinner oil and lower pressures can be better... because they flow more volume through the bearings.

 

I'm no engineer, but I found this short discussion very interesting - and counterintuitive. I have always assumed that more pressure is generally better.

 

http://www.eng-tips.com/viewthread.cfm?qid=354922

 

 

I remember that temperature, pressure, and volume are related (from some science classes). I think those formulas mostly applied to gasses, but there's some application to liquids as well. To use Pete's concept from a different thread yesterday...  Volume is how much gas you "fart through silk." Pressure is that uncomfortable feeling in your gut; pressure is how much can continue to "fart through silk." Analogously, oil volume is how much it flows, and oil pressure at the gauge is how much oil is waiting for future flows. There is a third factor - the pressure within the bearing. But this bearing pressure is not measured on the motorcycle. Bearing pressure, according to my understanding, is far higher than gauge pressure; it is generated by the rotation (I don't have a fart analogy for this one, sorry).

 

If my understanding is correct, then any pressure at the gauge (and therefore bearing intakes) assures there is adequate volume to be taken in and then re-pressurized. This would explain why my LeMans is still running. Perhaps the motor is worn - maybe more miles than I believed, or maybe it gulped a bunch of air with previous owners (who should have put Roper Plate in to prevent that). But so long as there is some pressure, it means the flow out of the bearings is less than the amount available to flow in - so the bearings are full of oil. In my case, the low pressure indicates that I am getting close to equilibrium and should probably plan on a bottom end rebuild pretty soon.

 

Scud

 

There might be another reason for the low oil pressure when the engine is hot:

 

Behind the oil cooler connections at the oil pan there is an intermittend part to the oil filter/thermostat unit. These two parts have an Oring front and rear. Change these 4 Orings and take care they are a tight fit in the bores. If these Orings do not seal properly, oil can escape thus reducing pressure. There is only flow in that area when the oil warms up and the thermostat starts to open. 

 

Thanks for the idea. I will look into those O-rings. FYI - I noticed on a couple occasions recently that the oil pressure would show a sudden increase, then go back down. Something related to the thermostat could explain that.

 

The LeMans is still running strong. But I'm taking the Scura to work today.

[GuzziMoto]

Pressure is simply when there is more gozinto then there is gozouta.

Flow is how much oil is flowing.

Pressure is how much resistance to that oil flowing.

The link you posted had a fairly interesting discussion on that. 

 

Scuds thought to check those o-rings is a good one. Anything that allows flow to bled off before the bearings means less flow and thus less pressure to the bearings. An oil filter coming loose, those o-rings leaking, or an open pressure relief valve (there is usually one of those in the oil filter and one in the block), can all allow flow to by-pass the bearings meaning there is less resistance to flow. The viscosity of the oil determines its resistance to flow, a thicker oil has more resistance to flow. As your oil breaks down it can get thinner or thicker, changing its resistance to flow. If your oil gets diluted by fuel (I hear this is rampant among people who use a PowerCommander) it will get thinner and thus have less resistance to flow. The clearances in the bearings determine their resistance to flow, too much clearance means they have too little resistance and thus they do not allow any pressure to build up. And, as said earlier in this thread, if your motor is excessively worn you will have less oil pressure. But if it is not making any nasty noises and runs fine I would personally not worry too much down that road.

[pete roper]

The wedge is a rolling wave-form that is created at the point of greatest pressure within the bearing. The smaller the clearance the greater the strength of the wedge. The problem is that friction between the molecules being forced together creates heat, the only way to prevent the heat building up to levels where the oil will break down is to have adequate throughput and it is for that reason that for plain bearings pressure rather than volume per-se is important.

 

Plain bearings are always a compromise and the oil is there to provide three functions, to cool, to lubricate and to clean.

 

Without adequate throughput the oil won't be able to remove enough heat and the oil will break down allowing boundary lubrication. Without adequate pressure the wedge will be inadequate to prevent boundary lubrication. It's always a juggling act and a 'Rule of thumb' WRT bearing clearance is one thou for every inch of journal diameter. Obviously the larger the number and size of journals and bearings to be lubricated the greater the volume of oil required but adequate pressure is vital to the longevity of the bearings and their tolerance to inadequacy will depend greatly on the forces imposed on the oil film and the relative speed of the bearing/journal surfaces. This is the main reason why it is almost always the big ends that will go first in an engine with inadequate oil pressure.

 

The old pushrod engine has six plain bearings, front and rear mains, big ends and cam bearings. The Nuovo Hi-Cam has eight, mains, big ends and two sets of cam bearings but I'd guess that the actual bearing surfaces are much the same size. The main reason for the larger lubrication pump on the 8V though is probably the under piston crown sprays. Crack pressure on the OPRV on the 8V is also slightly higher I believe. The cooling pump on the 8V runs very low pressure but lots of volume in comparison.

 

Pete

[GuzziMoto]

The wedge is a rolling wave-form that is created at the point of greatest pressure within the bearing. The smaller the clearance the greater the strength of the wedge. The problem is that friction between the molecules being forced together creates heat, the only way to prevent the heat building up to levels where the oil will break down is to have adequate throughput and it is for that reason that for plain bearings pressure rather than volume per-se is important.

 

Plain bearings are always a compromise and the oil is there to provide three functions, to cool, to lubricate and to clean.

 

Without adequate throughput the oil won't be able to remove enough heat and the oil will break down allowing boundary lubrication. Without adequate pressure the wedge will be inadequate to prevent boundary lubrication. It's always a juggling act and a 'Rule of thumb' WRT bearing clearance is one thou for every inch of journal diameter. Obviously the larger the number and size of journals and bearings to be lubricated the greater the volume of oil required but adequate pressure is vital to the longevity of the bearings and their tolerance to inadequacy will depend greatly on the forces imposed on the oil film and the relative speed of the bearing/journal surfaces. This is the main reason why it is almost always the big ends that will go first in an engine with inadequate oil pressure.

 

The old pushrod engine has six plain bearings, front and rear mains, big ends and cam bearings. The Nuovo Hi-Cam has eight, mains, big ends and two sets of cam bearings but I'd guess that the actual bearing surfaces are much the same size. The main reason for the larger lubrication pump on the 8V though is probably the under piston crown sprays. Crack pressure on the OPRV on the 8V is also slightly higher I believe. The cooling pump on the 8V runs very low pressure but lots of volume in comparison.

 

Pete

Well, I do agree with much of what you said. But perhaps I can use your own words to help you see my point. I see you used the term "adequate throughput" several times in your explanation. And, as I understand your explanation, you could have used the term "adequate flow" in its place. Perhaps you realized that, perhaps you didn't. Either way, here is what you said with little more than replacing the one term with the other.

//The wedge is a rolling wave-form that is created at the point of greatest pressure within the bearing. The smaller the clearance the greater the strength of the wedge. The problem is that friction between the molecules being forced together creates heat, the only way to prevent the heat building up to levels where the oil will break down is to have adequate flow and it is for that reason that for plain bearings volume per-se is important.

 

Plain bearings are always a compromise and the oil is there to provide three functions, to cool, to lubricate and to clean.

 

Without adequate flow the oil won't be able to remove enough heat and the oil will break down allowing boundary lubrication. Without adequate pressure pressure.//

 

Now, clearly oil pressure is needed in an internal combustion engine. But oil pressure is not what creates the hydrodynamic wedge. Nor is it required to create a hydrodynamic wedge. You can create that simply with oil and spinning. I know there is a lot of technical aspects in there, including a bunch of physics and what not. But what is not required to create it is oil pressure itself. The pressures involved in the hydrodynamic wedge far exceed the kinds of pressures the oil pump can create. You could just run the plain bearing in a bath of oil and it would likely work.

Oil pressure means the pump is trying to pump more oil at a faster rate than the system wants to flow, i.e., the oil pump is trying to flow more oil than the system wants to flow.

Now, a reduction in oil pressure can mean something is wrong. It can mean the bearing tolerances are opening up. It can mean the oil has thinned out. It can mean there is a leak in the pressure side of the system. We have pretty much covered all that already.

He is another way to look at the relationship between pressure and flow. If you take an oil pump that is flowing its rated capacity and increase the pressure in the system by tightening up the clearances in the bearings the actual amount of oil the pump will be flowing will go down, unless the pump was not already at its rated capacity in the first place. All else being equal, for pressure to go up flow goes down and vice versa. More flow can be a good thing, as you kinda mentioned it keeps temps lower and reduces stress on the oil. It also makes sure there is plenty of oil to do its job.

The issue in this case could be the clearances are opening up. It could also be a leak in the pressure side of the system. And it could even be a worn oil pump or a blockage on the suction side of the system. As the second and third sets of issues don't require full dis-assembly to check I think those would be good options to explore. But as the motor does not seem to have any horrid noises or running issues that would usually be from worn out bearings I think I would wait on the full dis-assembly myself. But since you could likely do it in your sleep I would not be surprised if your mentality was different on that.

Sorry if all I did was piss you off.

[pete roper]

No, I don't get pissed off, don't get me wrong, I'll happily argue a point with someone who is intelligent. It's fucktards who know nothing that wind me up!

 

You do make valid points about pressure per se, if a bearing couldn't wedge without pressure all those horrid stationary motors that use a spoon on the big end rod eye to lubricate the big end journal wouldn't work but as long as the oil is pressurised the higher the pressure the greater the strength of the wedge, it works as a multiplier.

 

Pete

[Scud]

This has been a really useful and informative thread for me. Great writing, reading, wrenching, riding... and relationships. I like this place.

 

I'll probably get into the oil pump, pressure valve, o-rings, etc. in the next month or so.

[Scud]

 

I'll probably get into the oil pump, pressure valve, o-rings, etc. in the next month or so.

 

...and then real-life got in the way... But today (finally) the pressure puzzle is solved.

 

A piece of the upper sump gasket was missing. It was one of the two tabs that goes between the housing for the filter and the block - it was metal to metal contact at that point (where the oil was supposed to pass through under pressure). The lack of gasket created a substantial, pressure-robbing, internal leak. After finding that problem and installing a new gasket, I have much higher oil pressure - about 40 PSI at 2,000RPM and holding steady at about 60 PSI under normal riding conditions. 

 

Thanks again to everyone in this thread.

[docc]

40-60 psi! Now, that's more like it!

 

Well done, sir ! !

[Scud]

Pull the sump and please check your oil filter.

Check the gasket, look for particles in the sump.

Something is wrong, check the big easy things first.

May be just that simple, oil filter loose, cross threaded two or no gasket.

I do hose clamp my filter, found it loose once.

 

Thumper - you were the closest to identifying the problem. But after verifying the simple things (such as existence of gaskets), I never imagined that an upper gasket would be installed with a small part missing.