When to go up in viscosity because of wear?

[Mitch Alsup]

Originally Posted By: Ken2
You just need enough flow of oil into the bearings to have sufficient oil present for the bearing to create its hydrodynamic wedge of oil, and that can develop a pressure of hundreds of psi inside the bearings.


You need just enough pressure to refill the bearing with the oil that gets flung out. The hydrodynamic wedge developes in any loaded bearing that is not devoid of oil. The oil pressure and the hydrodynamic wedge have nothing in common except for the bearing remaining filled with oil.

 

[mene]

Originally Posted By: CATERHAM
Originally Posted By: JAG
Originally Posted By: TFB1
At the same temp, seems to me 30 psi of a 3.5 cSt oil would flow more quickly than 25 psi of 3.0, especially if the losses are due to clearances inside the pump...

That's what I think too.

I don't agree because the psi figures you're likely referring to is not the oil pump pressure but a measure of oil flow resistance or the back pressure of flow through the engine bearings et al.
Using your example, the 25 psi represents less resistance to oil flow because of the lighter 3.0cSt oil and therefore faster oil flow.

Hi Catherham. Can you ease my mind in an area that keeps me concerned about low viscosity oils?, with lower viscosity more oil flows through the bearings which is good for cooling at a reduced pressure as I understand it, but also that means that less oil is reaching the highest points of the engine to lubricate the camshafts and rockers, moreover the lower vis. oil will have less shear resistance than a higher vis. one, so less oil and less lubricity from it. What am I missing? Thanks in advance!
George.
P.S. We had an 88~89F. day and the pressure drop to within 100 RPM max. pressure so I might have to raise vis. a little in summer will see.

 

[fdcg27]

If the oil pump is a positive displacement design (it almost certainly is) then the volume of flow to the main bearings is a constant at any given number of revs, without regard to oil viscosity.
Now, Caterham has always said that an accurate oil pressure gauge is the best determinant of viscosity requirements.
If adequate oil pressure can be maintained with a thinner oil, then you should use the thinner oil and reap the benefits of better fuel economy and a more reponsive engine, as well as easier cold starts, with maybe less cold start wear.
Now, you've put nearly 300K on a vehicle with 440K total.
You state that the engine still runs well and that consumption remains low, especially for a Mitsu motor.
Why would you change anyhting that you're doing?
Maybe it is you who should be telling us of the virtues of thicker oils.
The longevity of your engine would seem to be a testimonial to their use.
I wouldn't be too concerned with what anyone here says about your choice in either oil brands or viscosity.
You've clearly found the soloution that works for your engine as you use it.

 

[CATERHAM]

Originally Posted By: mene
Originally Posted By: CATERHAM
Originally Posted By: JAG
Originally Posted By: TFB1


Hi Catherham. Can you ease my mind in an area that keeps me concerned about low viscosity oils?, with lower viscosity more oil flows through the bearings which is good for cooling at a reduced pressure as I understand it

That's a common misconception.

The oil pressure provided by the positive displacement oil pump is not really affected that much by an oil's viscosity. When one refers to "oil pressure" in an operating engine it is the oil BACK PRESSURE that is being referred to. That is what an oil pressure gauge actually measures.
So when an engine is first started the "oil pressure" reading on an OP gauge is high because the oil's viscosity is high and the flow through the engine bearings is slow because it is being highly resisted. As the engine warms up and the oil's viscosity becomes lighter the reading on the OP gauge will be lower, not because the oil pressure provided by the oil pump has declined but rather because the resistance or back-pressure to the oil flow through the bearings etc has declined.

The oil pressure provided by an engine's oil pump that is not worn and is operating correctly we have no control over. Oil will get to every part of the engine no matter how light it is so that is not an issue. What is an issue is running an oil that is too light to maintain an adequate oil wedge in an operating engine under maximum load. Obviously you want to avoid bearing wiping at all costs.

How does one know what's the minimum oil viscosity an engine needs? Well a good place to start is the minimum oil pressure spec' provided by the manufacturer. Maintaining this OP will comfortably keep the oil viscosity above the minimum required to satisfy all the viscosity requirements for the engine.
In one sense an OP gauge is better thought of an a viscosity meter since the oil pressure it is displaying is directly related to the oil's viscosity.
The objective for optimizing a chosen oil's viscosity to any particular application is to choose the lightest oil that still maintains the minimum oil pressure spec' when the oil is as hot as it will ever get. Hence the lubrication axiom, "as light as possible, as thick as necessary".

 

[mene]

Hi fdcg27!, what I did was something that sounded reasonable at the time but probably incorrect, I took the FSM and look for the lowest "W" viscosity recommended for all situations and the highest hot vis. recommended for all situations too and looked for the best oil and compatible filter I knew back then that covered the "range" above, all ended being M1 oil and filter, mind you this was by the end of 1995, and knowing that synthetics can last up to twice the recommended OCI of a conventional oil I took the conservative approach of changing it every 5K miles since the manual recommended 3K miles for severe service. Now, although M1 5W50 says 50 it is actually around 40 in an engine asking for 30 and used in a hot climate. I do attribute the longevity of the engine to several factors, engine design, regular oil change with the best I knew, never mistreated or over stressed the engine while we had it and probably the most important factor, "luck". I'm in BITOG and asking questions because I want to know what I did right, what I did wrong and most important if I can do better. I never intended this to be another thick vs. thin debate, each I suspect have it's rightful use, If I were living in AZ and loading the van to it's limit plus pulling a heavy trailer I'll be using RL 20W50, but if I were in Alaska with a 5 miles commuting drive for the classic 9 to 5 job I'll probably be using a light 5W30 or even lighter with this wear and all. Maybe you don't remember but in one of my post I wrote that this engine is constantly turned off and on and many times the oil cools down a bit, so my special needs for this particular engine is a fast flowing oil at start up since as far as I know starting an engine constitute 90% of the overall wear with enough protection to keep it alive as long as I can, looking it from another angle I would like to know How much of the "over the top" metal to metal protection can I trade in favor of getting the oil faster to the parts in need of it and the reduction of work at the oil pump and related components.
George.

 

[mene]

Thanks!
George