Originally Posted By: Gary Allan
Quote:
As pump flow increases, so does the PSID across the element.
No, it doesn't
Not in anywhere near the frequency of instances where it does not.
Well, let's qualify that. It does ..but you would be very hard pressed to detect it unless the engine routinely operated cold at near peak volume output ...which would, in most cases in the vast majority of engines, also cause the oil pump to be in relief.
Hmmm ... isn't that basically what I said above?
And it is certainly true that as you increase the flow of any liquid through a fixed resistance, the volume of the flow will increase, the velocity of the flow will increase and so will the pressure drop across the "resistor". In the case of an oil filter, it too is a fixed resistor to flow (assume it stays clean and doesn't choke from loading).
So as the oil pump's volume output is increasing with the engine's RPM, then you will see a higher PSID across the filter - up to the point where the oil pump goes into relief mode. If the filter PSID raises to the level of the bypass valve pressure setting, then the filter will bypass some of the flow around the element. Yes, there is no doubt that in order for this to happen, the oil would have to be on the viscous side (ie, cold oil) and the volume flow high (ie, high engine RPM). This is what I said already in a previous post.
Of course, as the oil heats up, the PSID across the filter (and also the engine's oiling circuit) drops ... this is why an engine's oil pressure gauge always registers much lower with hot oil then with cold oil at the same engine RPM ... no surprises there.
Originally Posted By: Gary Allan
The PSID is manipulated (artifically = for lack of a better term) by the confines of the ratio it has to maintain with the total pressure elevation due to flow at visc through the engine.
If you ran two graphs of total PSI and PSID ..the ratio would be (something like) 20:1 25:1, 30:1, 40:1, 60:1 70:1 between total to filter. It's a YMMV situation since much depends on starting visc and volume ..etc..etc ..but you're not going to see true flow effects through a filter until some higher velocity through it.
Yes, we have established that the engine's oil circuit resistance is going to huge compared to the filter's resistance. As a result, the engine's oil circuit resistance will determine the pump's flow rate through the whole oiling system ... including the filter.
But again, and you've eluded to this again above like me, the situation that is concerning is the cold start, high RPM conditions. This is the condition that could cause a filter with more flow resistance and a lower bypass setting to go into bypass and stay there longer than a different filter designed for the car. This is the exact issue that LoneRanger is concerned about with his Subaru, that has an OEM filter that have the bypass valve set at 23 psi. Subaru must have a reason for that high setting. I'm thinking it's probably because that Subaru's oil pump puts out some pretty good volume, and/or maybe they think their owner's rev the [censored] out of the engines when they are stone cold.
There's no doubt that an engine manufacturer specs out the filter's bypass setting to match the pump output and oiling circuit resistance parameters. Likewise, if a filter manufacture does a good design job for a specific vehicle application, they too try to match the filter bypass setting to the engine's oiling system parameters (ie, pump pressure, pump output, circuit resistance) to help ensure minimal bypass events.
My car has the filter bypass built into the engine block, so it has no idea what filter I'm running, and as a result if I used one that was much more resistive to flow it would cut the headroom the filter has before it goes into bypass mode; depending on the built into block bypass valve setting (which BTW I can not find the psi spec for). I have to be a little more careful which filter I run on this car so I don't decrease the bypass pressure headroom by running a more restrictive filter.
Quote:
As pump flow increases, so does the PSID across the element.
No, it doesn't
Well, let's qualify that. It does ..but you would be very hard pressed to detect it unless the engine routinely operated cold at near peak volume output ...which would, in most cases in the vast majority of engines, also cause the oil pump to be in relief.
Hmmm ... isn't that basically what I said above?
So as the oil pump's volume output is increasing with the engine's RPM, then you will see a higher PSID across the filter - up to the point where the oil pump goes into relief mode. If the filter PSID raises to the level of the bypass valve pressure setting, then the filter will bypass some of the flow around the element. Yes, there is no doubt that in order for this to happen, the oil would have to be on the viscous side (ie, cold oil) and the volume flow high (ie, high engine RPM). This is what I said already in a previous post.
Of course, as the oil heats up, the PSID across the filter (and also the engine's oiling circuit) drops ... this is why an engine's oil pressure gauge always registers much lower with hot oil then with cold oil at the same engine RPM ... no surprises there.
Originally Posted By: Gary Allan
The PSID is manipulated (artifically = for lack of a better term) by the confines of the ratio it has to maintain with the total pressure elevation due to flow at visc through the engine.
If you ran two graphs of total PSI and PSID ..the ratio would be (something like) 20:1 25:1, 30:1, 40:1, 60:1 70:1 between total to filter. It's a YMMV situation since much depends on starting visc and volume ..etc..etc ..but you're not going to see true flow effects through a filter until some higher velocity through it.
Yes, we have established that the engine's oil circuit resistance is going to huge compared to the filter's resistance. As a result, the engine's oil circuit resistance will determine the pump's flow rate through the whole oiling system ... including the filter.
But again, and you've eluded to this again above like me, the situation that is concerning is the cold start, high RPM conditions. This is the condition that could cause a filter with more flow resistance and a lower bypass setting to go into bypass and stay there longer than a different filter designed for the car. This is the exact issue that LoneRanger is concerned about with his Subaru, that has an OEM filter that have the bypass valve set at 23 psi. Subaru must have a reason for that high setting. I'm thinking it's probably because that Subaru's oil pump puts out some pretty good volume, and/or maybe they think their owner's rev the [censored] out of the engines when they are stone cold.
There's no doubt that an engine manufacturer specs out the filter's bypass setting to match the pump output and oiling circuit resistance parameters. Likewise, if a filter manufacture does a good design job for a specific vehicle application, they too try to match the filter bypass setting to the engine's oiling system parameters (ie, pump pressure, pump output, circuit resistance) to help ensure minimal bypass events.
My car has the filter bypass built into the engine block, so it has no idea what filter I'm running, and as a result if I used one that was much more resistive to flow it would cut the headroom the filter has before it goes into bypass mode; depending on the built into block bypass valve setting (which BTW I can not find the psi spec for). I have to be a little more careful which filter I run on this car so I don't decrease the bypass pressure headroom by running a more restrictive filter.
