Thread: Lesson learned: oil pressure vs flow

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As previously stated correctly the fluid film bearings in a car engine work on the principle of natural hydrodynamic lubrication governed by Reynolds equation. Reynolds equation is non-linear and has no closed form solution so hydrodynamic bearing codes have typically relied on empirical or numerical solutions for their design. Enough academics.

The hydrodynamic wedge action that separates the bearing surfaces also acts to pump oil into and out of the bearing. Thus each bearing requires an amount of oil flow as function of load, speed, oil density, viscosity, and bearing clearances in order to operate successfully. BTW the pressure in the loaded fluid film in a car engine is on the order of hundreds of psi average with spikes exceeding 1000 psi. Just as important though is that there must be enough extra flow to carry frictional heat away from the bearings or the babbit materials will soften and fail.

So the oil pump delivers this flow to the bearings (and all other friction components). The amount of flow the positive displacement gear pump supplies and the restrictions in the supply circuit including the bearings etc determine the pressure the that the system operates at up to the setting of any internal pressure regulating devices that may be present in the system.

Remember the first rule of fluid flow. Flow creates pressure, not vice versa ............. Uh maybe I got off track a little ........

Kitz

Originally Posted by kitz

So the oil pump delivers this flow to the bearings (and all other friction components). The amount of flow the positive displacement gear pump supplies and the restrictions in the supply circuit including the bearings etc determine the pressure the that the system operates at up to the setting of any internal pressure regulating devices that may be present in the system.

Remember the first rule of fluid flow. Flow creates pressure, not vice versa ............. Uh maybe I got off track a little ........
Kitz

As to your comments in the first paragraph I'm quoting, I would only add oil viscosity to your list of factors contributing to pressure readings. I know YOU understand that concept as you referred to it in the previous paragraph (not quoted). Virtually every single inquiry I can recall about oil pressure readings fails to mention two factors; oil viscosity, and observed pressure reading at X rpm. Sometimes it will be prompted out of the questioner, most often not. Well intentioned responders will say something like "Yeah, I get X pounds of pressure at idle"...........totally meaningless as a comparator since it's very incomplete information.

As for the second paragraph; No, not off track at all from the original intent of the thread. However, as someone who has been trying to teach the concept that flow is more important to lubrication than the pressure reading in of itself, I will warn that you're engaging in frustrating endeavor. The concept of oil pressure being the source of lubrication is so deeply ingrained that there is often no room for consideration of it's source or it's significance. In my opening post here I mentioned that pressure, and more importantly a change in pressure when all other factors (viscosity, temperature, engine speed, etc.) is most useful as a diagnositic tool rather than a guarantor of lubrication occuring. The whole point of the story in the link I attached was that the guy destroyed two new engines because he believed that a relatively high pressure reading on his gauge meant he was getting oil to the bearings. He was wrong, and in it's original text he went into great detail as to how he caused his own problems. As you can see, folks have chosen to argue the point even in the face of empirical evidence contrary to popular misconception. When you (me, anybody) appears to be attacking the sacred cow of oil pressure, in the interest of putting it in perspective, all open mindedness seems to flee.

Ok, so here's another question... I use Mobil 1 Synthetic (15W-50) in all my go fast motors, and have never had any bearing problems using stock clearances... With the synthetic oil and a blueprinted pump are the bearing clearances best left to stock or can they be opened up a bit?

I'll just preface my remarks by saying I think you're using too much viscosity, (assuming you're not operating at punishingly high operating temps) not in the sense that it would cause harm to the bearings, but in the sense that it's causing parasitic drag, both in driving the pump, and in the rotational forces of the rotating assembly. Probably only amounts to a very small hp loss, but it's there.

That being said, with that high a viscosity at operating temp you could probably support more bearing clearance, although I'm not sure why you'd want to do that. A less viscous oil would likely accomplish a similar result depending on what your objective is.

One more note about oil pressure. On your small block Chevy, the connecting rod bearings recieve pressuized oil from the crankshaft for only half of every crankshaft rotation.

Absolutely Bob, oil viscosity is critical in the system flow characteristics and thus pressure readings.

Dave I agree totally with Bob on the viscosity. And like he says it does depend on the temperature. Synthetics tend to have a higher viscosity index. The viscosity index gives you a quick reading on how well the oil viscosity holds up versus temperature.

The higher viscosity oils will provide more hydrodynamic action than the lower grades but like Bob said at the price of higher friction and in the end higher temperature rises. There's no free lunch!

BTW Chevy says the warranty is void on my 502 if I use anything but 5W-30! Since the engine has been in my garage for 2 years now that is a mute point . But as you probably know this is an engine designed originally for marine power boat applications cruising at 3500 rpm all day long! I plan on using a synthetic 5W-30 which will actually outperform a fossil based 10W-40. And monitor the oil temperature ..........

In your case I say stick with what you know works!

Regards, Kitz

I pm'd Bob about it. The reason I was wanting to increase the clearance by .001 on the Ranchero engine is because of "sustained hi speed operation", as in down the salt at B-Ville......which would to me justify a looser clearance and higher viscosity. I've used Mobil 1 15W50 in lots of my engines, never had any problems with it..... but I do believe I will go to a lower viscosity on my street stuff...