Oil Pump PSI relief valve

[Bill_W]

Was thinking about this in regards to oil filter flow rate. I assume the oil pump pressure relief valve is installed before the filter as the oil pump is a positive displacement gear pump. You can see this pressure as the max pressure on your car oil PSI gauge. Seems filter flow rate would change when that pressure relief valve operates. Any thoughts?

Can the pressure relief operation at the engine RPM indicate flow rate of a filter?

 

[WellOiled]

For all practical purposes, the oil pump pressure regulator valve is part of the oil pump. The purpose of the oil pump pressure regulator valve is to limit the oil pressure to a safe limit when the oil viscosity is high. For example at low oil temperatures.

In normal operation, the oil pressure regulator valve is closed and the oil flow rate is proportional to engine RPMs.

If the oil pressure regulator valve is open, part of the oil flow is retuned to the sump and part of the flow goes through the filter and the lube oil circuit.

Sensed oil pressure will depend on the oil pressure regulator valve setting when the valve is open, engine RPMs and oil viscosity when the valve is closed and the location of the oil pressure sensor.

Answer to your question is no because you don't know how much oil is shunted back to the sump.

For more information on oil properties, I refer you to the Oil University.
Oil University

 

[Bill_W]

In normal operation, the oil pressure regulator valve is closed and the oil flow rate is proportional to engine RPMs.

If the oil pressure regulator valve is open, part of the oil flow is retuned to the sump and part of the flow goes through the filter and the lube oil circuit.



Supposed the oil PSI is linear regardless of the oil temperature. Then the oil PSI levels off to a max PSI.

If the new filter installed and you could record that oil temp and RPM, and make a curve with the temp. Increase.

Would you see a different curve with temp/RPM with a dirty filter?

 

[ZeeOSix]

Originally Posted by Bill_W
Was thinking about this in regards to oil filter flow rate. I assume the oil pump pressure relief valve is installed before the filter as the oil pump is a positive displacement gear pump. You can see this pressure as the max pressure on your car oil PSI gauge. Seems filter flow rate would change when that pressure relief valve operates. Any thoughts?

Can the pressure relief operation at the engine RPM indicate flow rate of a filter?


Good explanation by WellOiled above. But to add or say it a different way is that the only time the flow rate going to the filter/engine is reduced is when the pump hits pressure relief and shunts some of the pump output to the sump or back to the pump inlet.

The flow rate going to the engine can be cut back if someone was to stomp on the gas pedal and rev the engine up pretty good with very cold oil. Again, another reason to not get on engines until the oil warms up some - along with doing that will also make the filter's bypass valve open more and longer.

Most cars with stock oil pumps and recommended oil viscosity will have a tough time hitting pump relief even at high RPM if the pump was designed right to match the engine oiling system requirements (ie, not over designed).

 

[WellOiled]

Originally Posted by Bill_W

In normal operation, the oil pressure regulator valve is closed and the oil flow rate is proportional to engine RPMs.

If the oil pressure regulator valve is open, part of the oil flow is retuned to the sump and part of the flow goes through the filter and the lube oil circuit.



Supposed the oil PSI is linear regardless of the oil temperature. Then the oil PSI levels off to a max PSI.

If the new filter installed and you could record that oil temp and RPM, and make a curve with the temp. Increase.

Would you see a different curve with temp/RPM with a dirty filter?

The DP across the oil filter is small compared to the DP across the rest of the lube oil circuit. In other words, the resistance to oil flow is greatest in the lube oil circuit. The viscosity of oil changes radically with temperature. You would need a new curve for each temperature. As the viscosity improvers age, the viscosity vs temperature would also change.

 

[Job]

It's more of an emergency relief valve IMO. It's a big deal to cut oil flow to the engine, so it will be an extreme case of filter restriction combined with a too small bypass valve. Bypass valves come in all different sizes. The ones I like are Fram, Toyota, and Champ Labs blue button. These are all very similar, Fram and blue button are about identical. The oil pump should ideally never go into relief mode IMO. A properly sized bypass valve should prevent this.

 

[WellOiled]

Originally Posted by Farnsworth
It's more of an emergency relief valve IMO. It's a big deal to cut oil flow to the engine, so it will be an extreme case of filter restriction combined with a too small bypass valve. Bypass valves come in all different sizes. The ones I like are Fram, Toyota, and Champ Labs blue button. These are all very similar, Fram and blue button are about identical. The oil pump should ideally never go into relief mode IMO. A properly sized bypass valve should prevent this.

When you start an engine at -30C with 5W30 in the sump, the oil pressure regulator will likely open regardless of the oil filter condition. The viscosity of the oil is on the order of 6000 cP. The resistance of the oil to flow in the lube oil circuit is very high. This resistance dominates the oil filter. Another reason not to get aggressive with the throttle until the oil warms up.

 

[ZeeOSix]

Originally Posted by Farnsworth
It's more of an emergency relief valve IMO. It's a big deal to cut oil flow to the engine, so it will be an extreme case of filter restriction combined with a too small bypass valve. Bypass valves come in all different sizes. The ones I like are Fram, Toyota, and Champ Labs blue button. These are all very similar, Fram and blue button are about identical. The oil pump should ideally never go into relief mode IMO. A properly sized bypass valve should prevent this.


The oil pump certainly can go into relief in cold weather conditions with some RPM ... so they are meant to do that by design. Engine designers know that there are many conditions that can cause flow restriction, so they expect a lot of pump relief to happen under certain conditions.

Most oil pump pressure relief valves are set to around 80 PSI or more. At a supply pressure of 80+ PSI, the oil flow volume going to the engine isn't going to be anywhere close to a dangerously low amount when the pump hit's relief pressure. The only time it might become an issue is in super cold climates combined with high engine RPM because the pump will bypass very quickly when trying to push even a small volume of super thick oil through a restrictive oiling system. Again, this is why engines should run the correct oil viscosity for super cold climates, and engines shouldn't be revved up much after cold starts until the oil warms up some.

As WellOild pointed out, the oiling system is the big factor here that determines when the oil pump hits relief because the engine is about 15 times more flow restrictive than an oil filter - meaning the oil filter's restriction is only a small factor involved when it comes to pump relief. Cold oil flow and the resulting delta-p across the oil filter is the big factor on the filter bypass valve operation, which is independent of the oil pump relief valve operation.

 

[Bill_W]

I own a 2008 Miata and I took it for a spin. When I got the oil up to temperature I checked what RPM stopped being linear to the PSI gauge and started to fall off. I know my Miata oil pressure relief kicks in at 60 PSI, that is where it no longer goes linear to the RPM. So the RPM at engine temp normal is 3000 when I hit 60 PSI.

I just changed my oil and filter, Ultra and Mobil 1 5w-20. Looks like I have to see what it looks like next year?

 

[ZeeOSix]

Originally Posted by Bill_W
I own a 2008 Miata and I took it for a spin. When I got the oil up to temperature I checked what RPM stopped being linear to the PSI gauge and started to fall off. I know my Miata oil pressure relief kicks in at 60 PSI, that is where it no longer goes linear to the RPM. So the RPM at engine temp normal is 3000 when I hit 60 PSI.


The lower the oil pump's pressure relief setting, the easier it will be to hit pressure relief. If the valve was set to 80 or 90 PSI it might not hit pressure relief until near redline for instance, or not hit pressure relief at all with the oil at full temperature.

Are you running the specified oil viscosity, or something thicker than the manual calls for?

How do you know the oil was at full operating temperature ... have an oil temperature gauge on the car too?

On my Z06 I got the same oil pressure throughout the RPM range running 3 or 4 different brands of oil filters, so none of them were restrictive enough to make the oil pump hit pressure relief any sooner. The Z06 also has an oil temperature gauge, so I always compared oil pressure readings at the same oil temperature.

 

[Bill_W]

Oops I just changed my post to say 5w-20, that is the normal oil. My PSI gauge is a guesstimate OEM needle, but I checked the service manual for the threshold PSI. That OEM needle has never changed since I have owned the car. My thinking is the RPM will be lower when that relief valve kicks in with a dirty filter, but really who knows?

 

[ZeeOSix]

Originally Posted by Bill_W
My thinking is the RPM will be lower when that relief valve kicks in with a dirty filter, but really who knows?


Maybe by a couple of PSI ... but with an insensitive OEM needle gauge, that kind of difference in PSI probably in the noise level and can't be detected very well. The Z06 pressure gauge is digital and reads to 1 PSI, and the oil temp gauge is also digital and reads to 1 deg F. So got some decent resolution on those readings.

 

[Bill_W]

Just for thought... If I had a 6 PSI drop. That would be 10% of the pressure relief valve. Would that valve kick in at 2700 RPM?

 

[WellOiled]

Purchase a one day pass to get access to the TSBs. See what you can find on the oil pump pressure regulator valve and also the oil pressure gauge. The gauge may just be a glorified idiot light in the form of a needle.

 

[ZeeOSix]

Originally Posted by Bill_W
Just for thought... If I had a 6 PSI drop. That would be 10% of the pressure relief valve. Would that valve kick in at 2700 RPM?


Yes, in general terms the oil pump would hit pressure relief at a slightly lower RPM as the filter loads up and adds some flow restriction to the oiling system. The pump would also hit pressure relief at a lower RPM as the oil gets thicker from cold weather ... that's why it's much easier to hit pump relief in very cold weather during a cold start and some RPM with thick cold oil being pushed through the system.

 

[dnewton3]

To add to Zee and WellOiled, I'll offer this ...
When the pump is in relief, the oil pressure at the downstream engine circuit is not zero. It's not like the oil pressure is 100% dumped at the relief valve when the shunt is open. Think of it merely as a pressure limiting device; it will bleed off excess pressure over it's set-point.

If the relief is set at 80psi, and the pump is giving 70psi, then 100% of the flow will be going to the engine circuit at that 70psi.
If the relief is set at 80psi, and the pump is providing 90psi, then that excess 10psi is going to bleed off at the relief valve, but still assure 80"ish" psi (or close to it) will be going to the engine circuit. In this condition, there will be a reduction of oil volume to the engine circuit, but it won't be anywhere near zero in terms of flow or pressure. AS the pumps typically can provide well more oil than the engine will need for any given rpm, that bleed-off of pressure and volume is a reasonably low risk to the engine.

The relief valve only bleeds off the excess above it's set-point. It will not starve the engine of oil. It's not an all-or-nothing device. Think of it an a hydraulic attenuator that moderates pressure/flow above it's critical level.

At least that's the way I understand it.

 

[4WD]

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[Job]

Originally Posted by dnewton3
To add to Zee and WellOiled, I'll offer this ...
When the pump is in relief, the oil pressure at the downstream engine circuit is not zero. It's not like the oil pressure is 100% dumped at the relief valve when the shunt is open. Think of it merely as a pressure limiting device; it will bleed off excess pressure over it's set-point.

If the relief is set at 80psi, and the pump is giving 70psi, then 100% of the flow will be going to the engine circuit at that 70psi.
If the relief is set at 80psi, and the pump is providing 90psi, then that excess 10psi is going to bleed off at the relief valve, but still assure 80"ish" psi (or close to it) will be going to the engine circuit. In this condition, there will be a reduction of oil volume to the engine circuit, but it won't be anywhere near zero in terms of flow or pressure. AS the pumps typically can provide well more oil than the engine will need for any given rpm, that bleed-off of pressure and volume is a reasonably low risk to the engine.

The relief valve only bleeds off the excess above it's set-point. It will not starve the engine of oil. It's not an all-or-nothing device. Think of it an a hydraulic attenuator that moderates pressure/flow above it's critical level.

At least that's the way I understand it.


That's how I understand it as well. Part of the problem is talking about hot flow, then talking about sub zero maple syrup oil. I learned something about oil pump relief valves. I thought they were much higher psi. In freezing conditions, with a too small bypass valve area, the bypass say is one of those high 30 psi, what happens to the wall of maple syrup hitting the oil filter? It immediately has high back pressure. So if the dp is 30 psi at the filter, it needs more than a 60 psi oil pump valve, or the flow will be kept to the 30 psi. Maybe 30 is OK, but it seems low. The thick oil will take longer to get through the passages. When I had a small motorhome Chev chassis back n the day, I remember hot oil pressure say about 50 psi cruising along at 3000 rpm. Then when a hill is climbed the engine goes much higher probably to 5000 and I remember the oil pressure going considerably higher, maybe 70. Then coming back down after the engine returns to cruise. Many factors affect the oil gauge.

It's all for fun here, some people need to lighten up. If one uses the oil grade recommended in the manual and a name brand filter, preferably OE in my opinion, just to be sure the engineering matches, there should be "no worries." Your car will be long gone before the engine wears itself out. Driving style probably affects engine life more than anything else in equally maintained engines. IMO.

 

[WellOiled]

Originally Posted by dnewton3
To add to Zee and WellOiled, I'll offer this ...
When the pump is in relief, the oil pressure at the downstream engine circuit is not zero. It's not like the oil pressure is 100% dumped at the relief valve when the shunt is open. Think of it merely as a pressure limiting device; it will bleed off excess pressure over it's set-point.

If the relief is set at 80psi, and the pump is giving 70psi, then 100% of the flow will be going to the engine circuit at that 70psi.
If the relief is set at 80psi, and the pump is providing 90psi, then that excess 10psi is going to bleed off at the relief valve, but still assure 80"ish" psi (or close to it) will be going to the engine circuit. In this condition, there will be a reduction of oil volume to the engine circuit, but it won't be anywhere near zero in terms of flow or pressure. AS the pumps typically can provide well more oil than the engine will need for any given rpm, that bleed-off of pressure and volume is a reasonably low risk to the engine.

The relief valve only bleeds off the excess above it's set-point. It will not starve the engine of oil. It's not an all-or-nothing device. Think of it an a hydraulic attenuator that moderates pressure/flow above it's critical level.

At least that's the way I understand it.

Agreed. Thanks for the summary.

 

[WellOiled]

Originally Posted by 4WD
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The chart shows the pressure relief valve is starting to bypass at roughly 2000 RPM. Any idea which engine this is for?