Thread: Lesson learned: oil pressure vs flow

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We all have pet peeves, and I probably have more than most.

In one of my past employment eras I worked in the lubrication field, and one of the hardest things to deal with was people's obsession with oil pressure.

Now don't get me wrong. System pressure is important: as a measuring tool for operating conditions within the system, NOT as a sole indicator of whether or not the system is lubricating properly.

Oil FLOW is what lubricates the engine components.

Here is a link to a site where one fellow learned that lesson the hard ($$$$$$$) way, and had the courage to share that lesson with us. http://www.cobralads.com/2006/05/how...gine.html#more


(Edit note) Since this thread was originally started the author of the linked site condensed his story into an overall narrative on his experience with his engines. The upshot is he had his oil lines hooked up wrong which gave him great pressure readings on the gauge but no flow of oil to the bearings.

Originally posted by Bob Parmenter
We all have pet peeves, and I probably have more than most.

In one of my past employment eras I worked in the lubrication field, and one of the hardest things to deal with was people's obsession with oil pressure.

Now don't get me wrong. System pressure is important: as a measuring tool for operating conditions within the system, NOT as a sole indicator of whether or not the system is lubricating properly.

Oil FLOW is what lubricates the engine components.

Here is a link to a site where one fellow learned that lesson the hard ($$$$$$$) way, and had the courage to share that lesson with us. http://www.cobralads.com/butcher55.html

Hey bob That was a great article. Alot of info. Some of the smallest mistakes can have the most dire repercussions.
Rich

Great article Bob. Nothing like some good expert advice in the analysis/troubleshooting field to set ya straight!! It just goes to show ya, that what seems to be the most logical answer to the problem, is not always the right one. It is indeed, a hard lesson The Butcher learned.

Larry M.

The Butcher rules! It could have been me. THANKS!

Ernie

All the oil flow in the world won't keep your bearings alive unless there is a restriction at the end if the system so that pressure is generated. That said, the flow rate has to be sufficient to replace the oil at the bearings before it gets too hot. Pressure measurement is the one most important variable to watch, but the oil pressure gauge takeoff needs to be after the filter, cooler. and whatever else you are cramming your oil through before it hits the bearings. High volume oil pumps will cost you HP, heat up the oil more, and cause more wear on several components. If the standard volume oil pump gets it up to 45 psi you are fine. The fact that you have pressure means that the volume is sufficient. Any more volume after that and all that volume is just getting blown through the bypass in the pump. That bypass route adds heat to the oil.

Another of my peeves are anonymous internet hackers who repeatedly go for the put down while apparently ignoring the intent and content of the original post.

In reality "all the oil flow in the world" WILL lubricate the bearings.......it's called a splash system ,or also, a "once through" system. As an example, Chevrolets through 1954 ran quite successfully (albeit not as efficiently) without a "pressure" system. Lawnmower engines have operated for decades with a splash system. Two cycle engines...........etc. etc. etc. Should we talk about the transmission and differential? HHHMMMMMM!!

I subscribe to a systems approach, ie there are a number of considerations to total system function, and all need to be considered when evaluating the operating conditions.

Think about this when contemplating your oil pressure gauge, it might help with the concept:
"If the oil don't flow, the needle don't go!"

Sorry Brent.

Bob I am certainly no hacker and was not "going for the put down" as you referred to it. I simply disagree as to the mportance of having the oiling system pressurized. Without pressure, the thin film of oil can get squeezed out from between the two surfaces, then your bearing will stick to the crankshaft and get spun. Comparing the lubrication requirements of meshing gears to that of your rod and main bearings is misleading. The high loads subjected to a rod bearing require oil pressure to live very long. What I was interested in sharing with the people here is that if your oil gauge shows pressure (better than 40 psi lets say) than putting on a high volume pump will not help anything, and will increase wear on the dist gear and stretch on the timing chain, without improving oiling at all.

Mr. Fixit,

Thanks for the clarification of the tone of your response. No more needs be said about that as far as I'm concerned.

Obviously my perspecitive is different than yours. I look at it from and engineering solutions point of view. From my perspective, the crank and rod bearings are RELATIVELY easy to lubricate as they are what are referred to as "plane" bearings. The bearing and journal surfaces are (ideally) parallel, and the surface area is good sized (assuming proper sizing for anticipated loads). As long as sufficient lubricant is present the mechanical rotation of the mechanizm provides the action necessary to effect sufficient lubrication. Castrol has a commercial out that briefly shows the dynamic that occurs. It is called "hydrodynamic lubrication". What happens is that the rotational action draws the lubricant into the load zone of the bearing forming a wedge of lubricant to support the load (using a phenomenon know as laminar flow). This all assumes that clearances are proper, surface area is sufficient for the application, and a clean, adequate supply of the proper viscosity lubricant is available.

Within the context described, the purpose of the pump is to deliver that adequate supply (flow). The pressure reading is a result of the system restrictions (intended or not, they exist to some level depending on the system). Pressure is secondary to flow, and, as stated in the initial post, can be useful as a "diagnostic" tool to verify system operation. As your first post implied, the pressure sensor needs to be in the correct location to give a valid reading (apparently The Butcher had a problem with that as well). Where you and I agree here is, enough pressure is enough. The "more is better" syndrome has consequences as you indicated. It also can do harm to the lubricant.

When making reference to gear boxes, I had the bearings in mind more than the gears. Gear boxes, as I'm sure you are aware, but others may not be, contain plane bearings (similar to the crank and rod bearings) as well as ball, roller, and needle (very similar to roller in function). Of these, because of surface area, the plane bearing is relatively easy to lubricate. The rolling element bearings have a higher load carrying capacity due to metallurgy, but at any given point have less surface area contact, thus higher loads at point of pressure. As a result, while a plane bearing relies most on viscosity to provide adequate lubricant film, rolling element bearings will usually require the assist of an anti-wear additive (known by a variety of terminologies) particularly at pressure spikes.

Gear teeth are another matter still. Spur gears are the easiest to lubricate and at each tooth interface rely in hydrodynamic films. The most difficult are hypoid gears, (again for those who might read this that are less familiar with this terminology) the typical rear end style of gear (not typically applicable to FWD BTW). The high angle of incidence at the gear teeth interface has almost entirely wiping action rather than the rolling action of a spur gear. This regime requires "extreme pressure" additives to protect the teeth from wear (sulphur phosphorous is most common and imparts that lovely ? smell to EP gear lubes) as viscosity alone won't get the job done.

As with determining the tone of someone's post, writing a meaningful response to a technical issue can be difficult to do, especially briefly, due to the vast number of variable factors. Hope I haven't put everyone to sleep on this one.

Thanks Bob,

Your insight to fluid dynamics is enjoyable to me, as well as others I assume. The most common myths I have heard from other hot rodders reguarding oiling systems in V-8's are Smokey's rule: 10 psi per 1000 rpm recomended oil pressure, and the greatness of high volume pumps. I will stick with smokey's rule, but only up to a point, after 45 psi what's the point of more pressure.(assuming you aren't running nitromethane or something) I also have a strong dislike for high volume oil pumps. One ruined a BBC I used to own. Above 4500 rpm, the pump would suck enough oil to cause a vortex in the oil pan and then it could suck up air. Oil pressure would drop at high rpms, but only noticable after I bought a quick response gauge and plumbed it with -4 line instead of the nylon 1/8" stuff they give you with gauges. Either way, I would rather learn about lubrication by discussion than by replacing parts and trying to figure out why they failed.

This has been very interesting and enlightening. Perhaps you can shed some light on a problem that I'm having.

The engine is a 351 SBF with a Canton Road Race Pan and High Volume Oil Pump and an assortment of other performance parts. On the engine dyno the engine showed a steady 62.5 psi of oil pressure from 3,000 rpm to 6,500 rpm. The oil pressure was taken at the standard location just above the oil filter. A standard oil filter was used.

The engine was then installed in the car using a Canton remote filter adapter which is plumbed to a remote filter and then to an oil cooler using -10 line.

The oil pressure gauge in the car is hooked in the same place as the oil pressure was taken on the engine dyno. A second oil pressure gauge was hooked up on the output side of the remote filter but before the oil cooler.

When the engine is rev'd past 4,500 rpm (standing still or on the road makes no difference) the oil pressure on the primary gauge drops from 45-50 psi to 30-35 psi. The second oil pressure gauge shows a constant 50 psi with no variation.

Any idea of where the oil pressure is going?

I'm thinking that this isn't cavitation of the pump since the pressure after the filter stays constant. It makes me think that there is a restriction limiting flow so that over 4,500 rpm there isn't enough flow to maintain pressure. Possibly crud in the oil cooler.

All ideas are welcome...

Thanks very much....
Herb

I'm working from two beliefs based on what I think I read in your post. One is that the pressure drop occurs on the gauge that's connected to the engine rather than the remote filter mount (trying to keep up with what's first and what's second). Second, though not mentioned, is that when you drop rpm below 4500 again the pressure reading returns to the 45-50 from the 30-35.

Dropping oil pressure would imply a reduction of flow restriction, rather than blockage. Blockage usually increases pressure (as long as sensor is "behind" the blockage). The flow restriction change can result from a couple situations. One is that restrictions are increased in size (or removed depending on your point of view), such as a bypass valve opening. Or, the supply of oil is reduced thus reducing the amount of "backing up" of the oil stream. If you've got a high volume pump and the pan volume and/or rate of drain back of the oil isn't fast enough you could be starving the pump. If your oil viscosity is too high for the operating temps, that could contribut to it not draining back fast enough. Given that you have up to 50# below 4500, assuming at operating temp not "cold", you may be too viscous on your oil. Especially so when I look at the numbers on the dyno.

With the remote system on the in-car installation, you've lengthened the supply path of the oil compared to the on-dyno configuration.

There is no oil pressure at the bearings, the pressure drop or restriction occurs before the load bearing portion of the bearing.

I posted this explination earlier and it seems prevalent so I will post it again:

"Let me start by saying that if you go to a tribology website, there will be no category of bearing wear that has to do with low oil pressure, only lack of lubrication. The oil pressure isn't what supports the crankshaft. When a cylinder fires, you have maybe 600-900 psi acting on piston with a surface area of 13 sq inches and it's being supported by a main bearing with a supported surface area of maybe 3 sq inches? The force on the piston creates a downward force of 8000-12000 lbs while the upwards force 30 psi of oil pressure in the crank journal might be only 90 lbs (it's actually 0 because the oil pressure acts on all sides of the crank, therefore, cancelling itself out). The oil pressure generated by the pump has no contribution to the bearings ability to carry a load, it only ensures that there is more than sufficient oil supply to the bearing. Instead, think of a car hydroplaning. The way a wheel makes contact with the road forms a sort of a wedge infront of the contact patch. If you push that wheel through water at high speed, water gets packed into that wedge, creating tremendous pressure on the wheel. That pressure can actually lift the car off the road and that is when you have lubrication. Inside of your engine, that same wedge forms between the bottom of the crank journal and the bearing shell. As the crank rotates, friction between the crank and the oil pulls the oil down into the wedge much like water is packed infront of the wheel, pressure is generated, the crank is lifted off of the bearing, and lubrication is achieved."

Remember, I am an engineering student and not an engineer. I have taken no tribology classes. This is how I believe it to work and I might be wrong so do discuss what I say and don't take it for granted.

Originally posted by 76GMC1500
Remember, I am an engineering student and not an engineer. I have taken no tribology classes. This is how I believe it to work and I might be wrong so do discuss what I say and don't take it for granted.

Your use of "hydroplaning" as a visual example of hydrodynamic lubrication is very good. The fact that water is a very low viscosity fluid, not under pressure, and with the tire contact patches being relatively small relative to the weight of the car emphasizes the point even moreso. Thank you.

wait wait wait. . . .so I was supposed to put BEARINGS in my
crank journals?

just kidding.

I usually don't read the super long posts, but these were actually very informative and educated.

Hydrodynamic lubrication was studied by Reynolds (the guy that the Reynolds Number is named after)

Here is some poop about hydrodynamic lubrication:

http://www.machinedesign.com/BDE/mec...emech6_16.html