This matter of the oil pressure releif valve

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supercity

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This has been on my mind since the last club Dyno run I attended. I witnessed many cars, not properly up to temperature, some dead cold even, and I know running very thick oil were pushed as hard as they could go on a Dyno. The result was, as expected, all vehicles drove away none the wiser.

So, as it is continually pointed out on here, cold (thick) oil and oil "thicker than necessary" will open the pressure relief valve. I think we can all accept it is not ideal, but really, is it that bad? Does it instantly equate to a loss of hydrodynamic lubrication?

Now my thoughts, It's more likely to happen at higher RPM, with higher oil flow rates, and when the oil is thicker (than necessary). This would be an operational window where there would be well in excess of what is required to maintain full film thickness, would it not? So is a little less flow, or maybe a lot less flow, (20%, 30%?) going to have that much of a drastic effect? I understand it is going to depend on specific examples to get an answer but can any typical example be given? Thoughts?
 
Are you talking in the oil pump or filter? Obviously in the filter means you are getting unfiltered oil through the system, probably not bad for a minute or two.

The oil relief valve in the oil pump dumps excess oil back into the pan to control excessive oil pressure.

I think the relief valves are not the issue. The important point is the oil will do its best job of lubricating when its at operating temp.
 
36.gif
 
Originally Posted By: supercity
This has been on my mind since the last club Dyno run I attended. I witnessed many cars, not properly up to temperature, some dead cold even, and I know running very thick oil were pushed as hard as they could go on a Dyno. The result was, as expected, all vehicles drove away none the wiser.

So, as it is continually pointed out on here, cold (thick) oil and oil "thicker than necessary" will open the pressure relief valve. I think we can all accept it is not ideal, but really, is it that bad? Does it instantly equate to a loss of hydrodynamic lubrication?

Now my thoughts, It's more likely to happen at higher RPM, with higher oil flow rates, and when the oil is thicker (than necessary). This would be an operational window where there would be well in excess of what is required to maintain full film thickness, would it not? So is a little less flow, or maybe a lot less flow, (20%, 30%?) going to have that much of a drastic effect? I understand it is going to depend on specific examples to get an answer but can any typical example be given? Thoughts?




You are speaking as if you think the flow actually decreases when the pressure relief valve opens. That is not true- it remains basically constant. Before the valve opens, flow (and pressure) rise along with rising RPM. After the valve opens, the volume supplied into the valve keeps increasing as RPM goes up, but the pressure and flow out of the valve remains constant.

As I've mentioned on other threads, some manufacturers actually design for the pressure relief valve to open at relatively low RPM so that the engine has a pretty constant oil flow at all RPMs. This is somewhat wasteful of energy as the oil pump is doing more work than strictly speaking is needed, but these manufacturers seem to feel that its worth it for better lubrication. Also, some manufacturers have gotten very sophisticated with variable-displacement oil pumps so that the pressure can be varied by load and temperature as well as RPM, and the variation with RPM doesn't have to be linear.

The problems with revving far past the relief valve point when the oil is stone-cold are a) the engine metal parts aren't properly expanded to their intended working dimensions, and b) when the oil is thick, the pressure/volume relationship is different, and while the valve opens at the same pressure, the volume flow through it required to generate that pressure is lower so there is less volume flowing throughout the engine.
 
Originally Posted By: supercity
So is a little less flow, or maybe a lot less flow, (20%, 30%?) going to have that much of a drastic effect? I understand it is going to depend on specific examples to get an answer but can any typical example be given? Thoughts?



I doubt it. The main benefit to high flow is the removal of heat. On cold engine/oil, heat isn't an issue. In fact, more intramolecular friction in the more viscous oil will rob mechanical energy, and thus liberate heat (e the oil itself heats up via internal friction, not external conduction). So viscous or low VI oil, tends to heat up faster, but it costs you some mechanical energy doing so.

Cold engine operation with higher VI/lower vis oils can improve consumption by taking less mechanical energy, and despite slightly more time to reach NOT via less intramolecular friction, FE gains can offset this since the oil warms up by conduction mostly, not intramolecular friction as these oils tend to be highly FMed. This makes sense as we're not consuming energy (kinetic) to more expediently arrive at NOT, at which point any further heat created is just waste. So it's sort of a waste-heat deferral system, slightly longer time to warm up, yet more mechanical energy reserved for actual work

Then the other issue of excess viscosity (ie SAE50 @0C) that, may not cause more wear (increased delivery time notwithstanding), but can put internal parts under tremendous hydraulic pressure. Imagine starting this SAE50 filled engine at 0C, and immediately revving it to high RPM. The rings have to drag molasses along at very high speeds, putting tremendous pressure on them, the oil pump will see very high internal pressures and all of this hydraulic force can break stuff or stretch clearances at the very least. Of course that's in a case of an inappropriate grade under abusive start up conditions- a worst case scenario. The SAE50 engine carefully warmed up at idle, may survive well-enough unscathed.
 
Originally Posted By: supercity
So, as it is continually pointed out on here, cold (thick) oil and oil "thicker than necessary" will open the pressure relief valve. I think we can all accept it is not ideal, but really, is it that bad? Does it instantly equate to a loss of hydrodynamic lubrication?

Now my thoughts, It's more likely to happen at higher RPM, with higher oil flow rates, and when the oil is thicker (than necessary). This would be an operational window where there would be well in excess of what is required to maintain full film thickness, would it not? So is a little less flow, or maybe a lot less flow, (20%, 30%?) going to have that much of a drastic effect? I understand it is going to depend on specific examples to get an answer but can any typical example be given? Thoughts?


As long as the engine had been run at least a few minutes before doing the dyno pull, and it maintained oil pressure throughout, I would say that all the important components had sufficient lubrication for the 30-40 seconds of hard running.

Were the cars at least warmed up so that the thermostats were open before doing the pull? For very high-powered engines, I might be more worried about scuffing a piston due to tight bore clearance than I would be about an oiling system failure.
 
Originally Posted By: 440Magnum

As I've mentioned on other threads, some manufacturers actually design for the pressure relief valve to open at relatively low RPM so that the engine has a pretty constant oil flow at all RPMs. This is somewhat wasteful of energy as the oil pump is doing more work than strictly speaking is needed, but these manufacturers seem to feel that its worth it for better lubrication. Also, some manufacturers have gotten very sophisticated with variable-displacement oil pumps so that the pressure can be varied by load and temperature as well as RPM, and the variation with RPM doesn't have to be linear.

Yup
Variable displacement pumps are the wave of the future. It also facilitates the ability for a modern engine to run on myriad of international viscosity grades without dragging too much
 
You're right, the key is not to run an engine at maximum load in by-pass mode.
An oil's viscosity for any engine should be light enough at normal operating temp's so that the engine is running below the by-pass point at maximum rev's. If you follow the engine manufacturer's recommended OP test spec's with the lightest recommended oil for summer time use you won't be in by-pass mode at maximum rev's.

If you choose to run a heavier oil and can't use maximum rev's when the oil is as hot as it ever gets without entering by-pass mode, you are running an oil that is clearly too heavy to the point of being counter-productive as you are restricting maximum oil flow through the engine.
How much the oil flow is being diverted can be approximately determined by how high you can rev' the engine before hitting by-pass. While oil pressure in most engines will plateau's well before red-line, it usually still does continue to climb as red line in approached. So the closer to red-line you can rev' the engine without reaching max oil (back) pressure, the less into by pass you will be.
Nevertheless, the objective is to be out of by-pass mode when the oil is fully hot.

Running an oil that isn't too thick at normal operating temp's will of course be way thicker than necessary when cold especially with low VI oils. Even running a high VI 0W-20 in my Vauxhall/Cosworth engined Caterham I still need oil temp's of 70C to be just out of by-pass at maximum rev's as I mentioned in the following thread about restrictive oil filters:
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=3166711&page=1

Having you oil temp's up reasonable close to normal even though you are still in by-pass mode shouldn't be problem.
I know Doug Hillary has mentioned with some vintage race cars which are filled with low VI 20W-50 mineral oil that 80C oil temp's much be attained by the driver before WOT is used to minimize the risk of engine damage.

One of the reasons I run the lightest, highest VI oil possible that still allows me to maintain the minimum desired safe operational viscosity when the oil is fully hot, is because I am not always patient during warm-up before using WOT particularly in my sportscars. 10 minutes of running time is no where near enough but the oil has at least started to thin out and I usually wait that long before letting the fun begin.
 
Originally Posted By: supercity
So, as it is continually pointed out on here, cold (thick) oil and oil "thicker than necessary" will open the pressure relief valve. I think we can all accept it is not ideal, but really, is it that bad? Does it instantly equate to a loss of hydrodynamic lubrication?

Now my thoughts, It's more likely to happen at higher RPM, with higher oil flow rates, and when the oil is thicker (than necessary). This would be an operational window where there would be well in excess of what is required to maintain full film thickness, would it not? So is a little less flow, or maybe a lot less flow, (20%, 30%?) going to have that much of a drastic effect? I understand it is going to depend on specific examples to get an answer but can any typical example be given? Thoughts?


Flow isn't, and never is, lubrication...in spite of the belief systems that some hold. The flow TO the bearings is the flow that is required to make up the side leakage from the bearings. Pressure is the means of getting it there.

In the case that you mention, the viscosity will be high, so the parts are even further separated than they would be at operating temperature.

Bearings are "self adjusting" for want of a better word.

With more viscous oil, they have less side leakage, and thus less flow is made up to them. What this does is means that the oil within the bearing, stays in the longer, and does more laps of the hydrodynamic circuit within the bearing.

This means that the "recycled" oil within the bearing is a greater percentage, and the make-up oil a smaller percentage. That which has been in the bearing longer is warmer, as it is having work done to it by the engine, and is, in reality, much less viscous than the make-up supplied by the pump.

The work done to the oil in the bearing will result in more frictional drag (note friction <>wear, the parts are separated, and moreso than at operating temperature), and less power...could easily waste whole numbers of horsepower running cold oil on a dyno.

The power that is wasted goes into heating the oil up. There are some that don't believe in a temperature rise across the bearing, but the majority of the heat in the oil (that is registered by the temperature gauge) is frictional, due to the oil doing it's job of hydrodynamic lubrication.

As oil temperature rises, viscosity drops, side leakage increases, and the bearing takes more oil.

Edit...ll of the above goes out the window if the suction pipe to the pump can't feed the pump...then things get nasty...
 
Last edited:
Originally Posted By: A_Harman


Were the cars at least warmed up so that the thermostats were open before doing the pull? For very high-powered engines, I might be more worried about scuffing a piston due to tight bore clearance than I would be about an oiling system failure.


Yes most of them would have been. I think a lot of people get the coolant temp up to normal but shut them down to avoid heat soak into the engine bay, inlet manifolds etc. There is no way these cars would have oil at full operating temp. I did see one car, after sitting for 2-3 hours just get started up and driven onto the Dyno, left to idle while it was strapped down and then run. Neither coolant or oil temps were even off minimum on their respective factory gauges. Thats > 50C. The owner was not concerned.
 
Aside from not being a particularly bright thing to do for the engine, you're also not going to get maximum torque and hp readings unless you're going to bring the coolant and oil up to temp' on the dyno.
 
Originally Posted By: Shannow

With more viscous oil, they have less side leakage, and thus less flow is made up to them. What this does is means that the oil within the bearing, stays in the longer, and does more laps of the hydrodynamic circuit within the bearing.



Is this scenario only a result of the pressure relief valve being open? Because below that point, pressure will be higher for a given higher viscosity, hence pushing more fluid through the bearing. Maybe this effect is disproportionate though.
 
Originally Posted By: supercity
Originally Posted By: Shannow

With more viscous oil, they have less side leakage, and thus less flow is made up to them. What this does is means that the oil within the bearing, stays in the longer, and does more laps of the hydrodynamic circuit within the bearing.



Is this scenario only a result of the pressure relief valve being open? Because below that point, pressure will be higher for a given higher viscosity, hence pushing more fluid through the bearing. Maybe this effect is disproportionate though.


CATERHAM is correct in his assertion that pressure is resistance to oil flow through the engine...therefore, when the bearing side leakage is down, the bearing will only take so much oil, and resist the rest, which has to go through the relief.

In some of the bearings I work on, the hydrodynamics is such that they suck their own oil into themselves, and we have to control that flow with an orifice to control the amount of new oil versus recirc, and thus the operational viscosity and oil outlet temperature....the oil pump only supplies oil to the header for the bearings to draw off.
 
Originally Posted By: Shannow
Originally Posted By: supercity
So, as it is continually pointed out on here, cold (thick) oil and oil "thicker than necessary" will open the pressure relief valve. I think we can all accept it is not ideal, but really, is it that bad? Does it instantly equate to a loss of hydrodynamic lubrication?

Now my thoughts, It's more likely to happen at higher RPM, with higher oil flow rates, and when the oil is thicker (than necessary). This would be an operational window where there would be well in excess of what is required to maintain full film thickness, would it not? So is a little less flow, or maybe a lot less flow, (20%, 30%?) going to have that much of a drastic effect? I understand it is going to depend on specific examples to get an answer but can any typical example be given? Thoughts?


Flow isn't, and never is, lubrication...in spite of the belief systems that some hold. The flow TO the bearings is the flow that is required to make up the side leakage from the bearings. Pressure is the means of getting it there.

In the case that you mention, the viscosity will be high, so the parts are even further separated than they would be at operating temperature.

Bearings are "self adjusting" for want of a better word.

With more viscous oil, they have less side leakage, and thus less flow is made up to them. What this does is means that the oil within the bearing, stays in the longer, and does more laps of the hydrodynamic circuit within the bearing.

This means that the "recycled" oil within the bearing is a greater percentage, and the make-up oil a smaller percentage. That which has been in the bearing longer is warmer, as it is having work done to it by the engine, and is, in reality, much less viscous than the make-up supplied by the pump.

The work done to the oil in the bearing will result in more frictional drag (note friction <>wear, the parts are separated, and moreso than at operating temperature), and less power...could easily waste whole numbers of horsepower running cold oil on a dyno.

The power that is wasted goes into heating the oil up. There are some that don't believe in a temperature rise across the bearing, but the majority of the heat in the oil (that is registered by the temperature gauge) is frictional, due to the oil doing it's job of hydrodynamic lubrication.

As oil temperature rises, viscosity drops, side leakage increases, and the bearing takes more oil.

Edit...ll of the above goes out the window if the suction pipe to the pump can't feed the pump...then things get nasty...



Thank you for this post. Very well said and easily understood!

10.gif
 
Originally Posted By: Shannow


Bearings are "self adjusting" for want of a better word.


So in your opinion, can any halm come from using a lubricant that is thicker than necessary?
 
Originally Posted By: supercity
Originally Posted By: Shannow


Bearings are "self adjusting" for want of a better word.


So in your opinion, can any halm come from using a lubricant that is thicker than necessary?


You're using a oil far thicker than necessary every time you start a cold engine...

Using Shannow's theory and logic, apparently a open bypass valve isn't the sin we've been lead to believe...
 
Originally Posted By: supercity
Originally Posted By: Shannow


Bearings are "self adjusting" for want of a better word.


So in your opinion, can any halm come from using a lubricant that is thicker than necessary?


Not stupidly thick...I've run 25W-70 shell in -7 winter to demonstrate the lack of engine explosions that occurRed (straight 30 the next summer to show the same).

25w-70 cost me a lot of fuel compared to sensible oil choices...and when you are running on bypass, pump effort is only going into oil heating, so it's wasted energy there too.

If the pump starves because the oil is stupidly thick when cold, you will break stuff. A guy I knew who liked impromptu burnouts with some Isuzu twin cammer in his Gemini panel van waste two engines because it kept too much oil up top. Another guy doing the same with a 351 clevo didn't have the same problem...they are different designs with different foibles.

Holden grey motors could shear the oil pump drive pin with too much oil pressure.

Thick oil, unless starved won't underlubricate a bearing...and the bypass isn't doing the starving.
 
Originally Posted By: TFB1

Using Shannow's theory and logic, apparently a open bypass valve isn't the sin we've been lead to believe...



STOP confusing a bypass valve with a relief valve!

Bypass valves open in response to excessive pressure differential across the filter. RELIEF valves open in response to excessive pressure at the input to the oiling system from the oil pump. Relief valves are usually upstream from the filter and therefore upstream from the bypass valve. TOTALLY different function, totally different behavior in the real world. In most engines opening of the relief valve is an everyday occurrence, but opening of the bypass valve is a relatively rare event.

There is absolutely NO harm when the relief valve opens unless its because you're using grease instead of oil. Period, end of story.

Whenever the bypass valve opens, there is a (slight) risk of larger grit or other contamination getting into the pressurized part of the oiling system and ultimately into a bearing. But modern oiling systems are usually quite clean- and "missing a pass through the filter" on one circuit of the engine is highly unlikely to introduce any grit big enough to harm a bearing, so even though there's a non-zero risk when the bypass valve opens, its still not a horror.
 
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