Can hot weather cause lower oil pressure even when oil temp is the same?
07-09-2009, 07:28 AM
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Re: Can hot weather cause lower oil pressure even when oil temp is the same?
yep, you are right...i got the pictures crossed up today, I figured it out last night and then messed it up this morning.
The opposite of what you are saying should be true....oil pressure will only be constant when the relief valve is lifting. if the relief valve isn't lifting then it'll be at low pressure and the pump will continue to flow a fixed volume (assuming steady RPM)..once the relief valve lifts it'll recirc that oil causing pressure to remain the same (whatever the valve is set at) I'm pretty sure the relief valve is there to regulate pressure since a positive (gear) type pump can't maintain a set pressure on it's own.
The only way this wouldn't be right is if the flow of the pump was so small that it discharges out a hole with no restrictions (blew a hole in your lines or something)
The opposite of what you are saying should be true....oil pressure will only be constant when the relief valve is lifting. if the relief valve isn't lifting then it'll be at low pressure and the pump will continue to flow a fixed volume (assuming steady RPM)..once the relief valve lifts it'll recirc that oil causing pressure to remain the same (whatever the valve is set at) I'm pretty sure the relief valve is there to regulate pressure since a positive (gear) type pump can't maintain a set pressure on it's own.
The only way this wouldn't be right is if the flow of the pump was so small that it discharges out a hole with no restrictions (blew a hole in your lines or something)
07-09-2009, 07:35 AM
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Re: Can hot weather cause lower oil pressure even when oil temp is the same?
In my case the relief cannot handle all of the oil pump flow... so I see 80psi at WOT. All of the LSX pumps are like that... even the stock pump has a relief spring that opens at 32psi, but you'll get mid 50s at WOT.
I'm trying to understand the physics of exactly what's going on and everyone gives a different answer... Here's a long explanation from BITOG.com:
Try as I might... this won't click for me. If my car makes 40 psi at idle (800 rpms) at 210 degree oil temp why wouldn't a more restrictive filter reduce the pressure registered on the gauge?
Because even if you put a more restrictive oil filter on the car the same volume of oil is reaching and going through the engine. NOW ... if the oil pressure sensor was BEFORE the oil filter then you WOULD see a higher oil pressure there if you used a more restrictive filter. When the pump goes into relief mode, the pressure is constant at the max possible that could be supplied to the filter/engine flow path. If you have a fixed pressure and "unlimited" volume, then if you increase the flow resistance (ie, filter) then the pressure on the backside of the filter would have to be less, which means the INLET pressure to the engine would be less since filter outlet P = engine inlet P.
Here's and example.
Filter A is more restrictive than Filter B. Both have bypass valves set to 15 psi (so they will not be in bypass in the example below).
If the oil pump is in relief at say 80 psi with 5w-30 Mobil 1 at 200 deg F with Filter A installed, then engine oil press = 75 psi on the gauge.
If Filter B is installed and the same conditions are achieved as above, then engine oil press = 70 psi on the gauge.
This means Filter A had a 10 PSID under these conditions, and less restrictive Filter B had a 5 PSID. You could only see this difference in filter resistance IF the pump is in pressure relief, because the filter/engine circuit is going to flow less gpm @ 80 psi supply pressure with Filter A than is Filter B was used instead.
Now, let's do an example with the same Filter A and B above when the pump is NOT in relief mode.
When the oil pump is not in relief mode, then ALL the oil volume coming out of the pump MUST go through the filter/engine flow path.
Look at just the point of flow between the filter and engine ... where the engine pressure sensor is located. Again, the conditions are 5w-30 Mobil 1 at 200 deg F. Let’s say that the engine RPM is at 3000 and the oil pump is NOT in relief mode. Also, this particular oil pump puts out say 6 gpm at 3000 RPM.
Assume that Filter A has 8 PSID with 6 gpm of this oil, and Filter B has a 4 PSID. Also, in order to make 6 gpm go down the engine circuit it takes 70 psi – that would be the gauge pressure at the engine inlet (ie, filter outlet).
So, with the engine at 3000 PRM with this oil there is 6 gpm going through the filter & engine, and the pump is NOT in relief mode.
With Filter A the pump output pressure will be 70 + 8 = 78 psi.
With Filter B the pump output pressure will be 70 + 4 = 74 psi.
But the pump’s relief pressure is 80, so it’s not in relief ... BUT, what happened is that the pump’s output pressure increased due to the more restrictive Filter A. And the key here to note is that the engine’s oil pressure stayed the SAME at 70 psi.
The more restrictive filter just caused the pump’s outlet pressure to run closer to the relief pressure setting. If you ran a more restrictive filter you would never know t his unless you had a pressure sensor BEFORE the filter. Most cars have the oil pressure sensor after the filter (before engine), so they will never see the effect of a more restrictive filter unless the pump is in pressure relief mode.
Hope that makes sense. It’s kind of tricky to see, but once you do it’s easy to understand.
I'm trying to understand the physics of exactly what's going on and everyone gives a different answer... Here's a long explanation from BITOG.com:
Originally Posted by SuperBusa
Originally Posted by Brian_Bowers
Originally Posted by SuperBusa
If you put a more restrictive oil filter on the car, and if anytime the pump is not in relief mode, then the output pressure of the pump will be slightly higher to overcome the added resistance of the more resistive filter – but the same positive displacement of oil volume is still going through the filter/engine because the relief valve is still closed. Even though the pump is producing more pressure to move the same volume of oil through a more restrictive path, if the pump’s output pressure is still below the relief setting then you will never see any difference in engine oil pressure – ie, the pressure between the filter and engine where most oil pressure gauge sensors are located.
Here's and example.
Filter A is more restrictive than Filter B. Both have bypass valves set to 15 psi (so they will not be in bypass in the example below).
If the oil pump is in relief at say 80 psi with 5w-30 Mobil 1 at 200 deg F with Filter A installed, then engine oil press = 75 psi on the gauge.
If Filter B is installed and the same conditions are achieved as above, then engine oil press = 70 psi on the gauge.
This means Filter A had a 10 PSID under these conditions, and less restrictive Filter B had a 5 PSID. You could only see this difference in filter resistance IF the pump is in pressure relief, because the filter/engine circuit is going to flow less gpm @ 80 psi supply pressure with Filter A than is Filter B was used instead.
Now, let's do an example with the same Filter A and B above when the pump is NOT in relief mode.
When the oil pump is not in relief mode, then ALL the oil volume coming out of the pump MUST go through the filter/engine flow path.
Look at just the point of flow between the filter and engine ... where the engine pressure sensor is located. Again, the conditions are 5w-30 Mobil 1 at 200 deg F. Let’s say that the engine RPM is at 3000 and the oil pump is NOT in relief mode. Also, this particular oil pump puts out say 6 gpm at 3000 RPM.
Assume that Filter A has 8 PSID with 6 gpm of this oil, and Filter B has a 4 PSID. Also, in order to make 6 gpm go down the engine circuit it takes 70 psi – that would be the gauge pressure at the engine inlet (ie, filter outlet).
So, with the engine at 3000 PRM with this oil there is 6 gpm going through the filter & engine, and the pump is NOT in relief mode.
With Filter A the pump output pressure will be 70 + 8 = 78 psi.
With Filter B the pump output pressure will be 70 + 4 = 74 psi.
But the pump’s relief pressure is 80, so it’s not in relief ... BUT, what happened is that the pump’s output pressure increased due to the more restrictive Filter A. And the key here to note is that the engine’s oil pressure stayed the SAME at 70 psi.
The more restrictive filter just caused the pump’s outlet pressure to run closer to the relief pressure setting. If you ran a more restrictive filter you would never know t his unless you had a pressure sensor BEFORE the filter. Most cars have the oil pressure sensor after the filter (before engine), so they will never see the effect of a more restrictive filter unless the pump is in pressure relief mode.
Hope that makes sense. It’s kind of tricky to see, but once you do it’s easy to understand.
07-09-2009, 10:14 PM
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Re: Can hot weather cause lower oil pressure even when oil temp is the same?
I understand what those guys at BITOG are saying, but I think there are some things missing. I agree that there will definitely be a difference in pressure between the two filters when the oil pump is at the relief pressure. That pressure difference is completely dependent on the flow requirements of the engine though, so you can't just say that one filter has a pressure drop of 'X' psi. When the flow changes, the pressure drop will change.
This is like I was telling Brian this morning (and didn't get a chance to finish); this is equivalent to an electrical circuit. The oil filter media is like a resistor, the oil pressure is like voltage, and the oil flow (volume) is like current. The oil system behaves just like electricity, where the voltage drop (pressure drop) across the resistor is equal to the current (oil volume) times the resistance (the filter media). That's Ohm's law BTW if it makes it more clear.
Anyway, the oil volume through the engine depends on the oil pressure. Increase the pressure and the flow will increase, decrease the pressure, you get the picture.
If that doesn't make sense, think about a garden hose with a fitting that isn't very tight. The fitting might leak a tiny bit when the pressure in the hose is low, for example. Turn the pressure up, and all of a sudden there is a gusher coming out where there was little or no leakage before. The difference in pressure changed the flow obviously.
So basically you've got two restrictions to flow (the engine and the oil filter) and the sending unit is between them. If the pressure at the pump changes, the pressure between the filter and engine changes. If you change one of the restrictions (the oil filter) the pressure between the filter and engine changes.
The only way you would NOT notice a pressure differences between two different types of filters is if the oil flow through the engine was really low. Then the flow through the filter would be really low, making the pressure drop across the filter very small (small enough to appear the same for either filter).
I'm doing a really horrible job at explaining something that is crystal clear in my own head right now. LOL.. Sorry about that.
This is like I was telling Brian this morning (and didn't get a chance to finish); this is equivalent to an electrical circuit. The oil filter media is like a resistor, the oil pressure is like voltage, and the oil flow (volume) is like current. The oil system behaves just like electricity, where the voltage drop (pressure drop) across the resistor is equal to the current (oil volume) times the resistance (the filter media). That's Ohm's law BTW if it makes it more clear.
Anyway, the oil volume through the engine depends on the oil pressure. Increase the pressure and the flow will increase, decrease the pressure, you get the picture.
If that doesn't make sense, think about a garden hose with a fitting that isn't very tight. The fitting might leak a tiny bit when the pressure in the hose is low, for example. Turn the pressure up, and all of a sudden there is a gusher coming out where there was little or no leakage before. The difference in pressure changed the flow obviously.
So basically you've got two restrictions to flow (the engine and the oil filter) and the sending unit is between them. If the pressure at the pump changes, the pressure between the filter and engine changes. If you change one of the restrictions (the oil filter) the pressure between the filter and engine changes.
The only way you would NOT notice a pressure differences between two different types of filters is if the oil flow through the engine was really low. Then the flow through the filter would be really low, making the pressure drop across the filter very small (small enough to appear the same for either filter).
I'm doing a really horrible job at explaining something that is crystal clear in my own head right now. LOL.. Sorry about that.
Last edited by Fabrik8; 07-09-2009 at 10:23 PM.
07-10-2009, 05:08 AM
#34
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Re: Can hot weather cause lower oil pressure even when oil temp is the same?
My one hangup is they're still saying that if an oil pump is not in relief then no change in pressure will be noted. However... are they assuming the pressure gauge is right at pump outlet and before the filter?
07-10-2009, 07:08 AM
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Re: Can hot weather cause lower oil pressure even when oil temp is the same?
just so you know i looked at my gauge today when driving.
At idle with 220 oil temp...it was 25psi. 5w30 and ac delco filter(if i remmeber correctly)
I use to read a little higher before my old sender went bad. Also lookin at your tach, your car idles higher then mine. Still on the cam tune?
At idle with 220 oil temp...it was 25psi. 5w30 and ac delco filter(if i remmeber correctly)
I use to read a little higher before my old sender went bad. Also lookin at your tach, your car idles higher then mine. Still on the cam tune?
07-10-2009, 07:41 AM
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Re: Can hot weather cause lower oil pressure even when oil temp is the same?
Fabrik8...I know what you mean, it's hard to put it down into words. You do the opposite of me though, whenever I see an electrical problem I convert everything into mechanical terms. I need to read what the other forum said slower, they keep tossing in terms and measurements that don't mean anything to you as the guy in the car reading your gage.
The problem with the scenario is that we are dealing with a positive displacement pump which means constant flow at whatever RPM no matter what downstream pressure is, it'll simply meet that pressure to move the fluid.
I've been out of a power plant for 2 years now and haven't been teaching so my explanations have gone down the drain
The problem with the scenario is that we are dealing with a positive displacement pump which means constant flow at whatever RPM no matter what downstream pressure is, it'll simply meet that pressure to move the fluid.
I've been out of a power plant for 2 years now and haven't been teaching so my explanations have gone down the drain
07-10-2009, 03:29 PM
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Re: Can hot weather cause lower oil pressure even when oil temp is the same?
So would you agree that with a constant flow at a given RPM, if you have two restrictions in series (filter and engine) the pressure between the two restrictions will change if either of the two restrictions changes?
07-10-2009, 08:29 PM
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Re: Can hot weather cause lower oil pressure even when oil temp is the same?
yep, I think the pressure downstream would still change...it's just more complicated with a gear pump to work out exactly what to expect. A regular pump is easier because as discharge pressure goes up it moves less volume while a gear pump moves the same volume no matter what.
Clearly brian needs to make another thread where he installs 5 or 6 more gages for us to play around with his engine
I question the value of a single oil pressure gage in the first place, especially if it's not in the worst flow location or after the bearings....you could have perfectly fine pressure on the gage and still eat up a bearing. I guess it let's you know if your oil pump failed, but from my experience in my duster I didn't have enough time to get off the road before I blew up anyways...I didn't have the benefit of flashing CEL or anything though either, just a small pressure gage that I wasn't paying attention to.
oh yeah, screw this thread it's been bugging me for days now
it has me staring at my pressure gage more than the road
Clearly brian needs to make another thread where he installs 5 or 6 more gages for us to play around with his engine
I question the value of a single oil pressure gage in the first place, especially if it's not in the worst flow location or after the bearings....you could have perfectly fine pressure on the gage and still eat up a bearing. I guess it let's you know if your oil pump failed, but from my experience in my duster I didn't have enough time to get off the road before I blew up anyways...I didn't have the benefit of flashing CEL or anything though either, just a small pressure gage that I wasn't paying attention to.
oh yeah, screw this thread it's been bugging me for days now
it has me staring at my pressure gage more than the road
Last edited by Digger70chall; 07-10-2009 at 08:37 PM.
07-11-2009, 09:27 AM
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