- Joined
- Sep 28, 2002
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Let's see if I agree here (if I do ..that feeling ..and around a buck gets you coffee in some places)
Well, not really. It will assure that differential of 15 PSID isn't exceeded to prevent two things. Protect the engine from oil starvation* (*more later) and to prevent the media from collapsing or being breached. It doesn't equalize the pressure.
Essentially correct. The differential merely expresses the oil's change in velocity. Any resistance when the oil pump is not in relief develops a pressure elevation where the oil accelerated (since the flow is not modified when the pump relief is closed). Normally developing too high a pressure results in the pressure relief valve in the oil pump opening. When you see a filter explode ..or an older style oil pump drive (typically the bottom of the distributor) will round out ...then your oil pump relief valve is stuck.
What my observations have told me is that there's normally very little PSID across your filter. There may be, briefly, if the passages in your engine are empty at startup and you may get some baseline PSID due to loading. Otherwise, the oil flow barely knows that it's there.
Aside from PSID due to loading ..the only time you will see PSID of any appreciable amount is when the oil pump is in relief (up against its limit in pressure). When the relief is closed (and from the view of the filter/pump) you have a series circuit (there's no escape for the fluid flow - NONE) with flow dictated pressure. The engine is WAY ..WAY ..WAY more resistive then the filter is ...WAY (did I say way more?). Now if you know your basic DC series circuit (the two lightbulbs with a dry cell) ..the voltage (a drop in this case - since it's a pressure:resistance dictated flow) will divide between the two lightbulbs. In our case ..the engine is a VERY VERY BIG CHOKE to CURRENT/FLOW ...making the filter ..in its mandated subordinate (other element of the resistive part of the equation) resistance ...VERY VERY SMALL in comparison.
Now when the relief of the pump is open ..then the view changes. There is an "escape" for the fluid flow. Now you have your (normal perception) pressure dictated flow. The bypass setting limits the amount of shunted flow ..or the PSID developed is a function of how much of the full flow is shunted. As the pump output begins to match the flow realized ..that differential across the filter will evaporate.
Now if you've got an oil pump that has virtually no difference between cold pressure and hot pressure ..then you're in relief for a longer period of time. This is where your PSID is going to be effected by viscosity and varied volume put out by the pump.
A bit on loading. I tested a filter with 10k on it. After the relief had closed (below 82 psi) I still had a variable PSID due to viscosity. That was how much the oil had to accelerate through the partially saturated media. My pump relief was closed ..so 100% of the realized flow (minus pump losses) HAD to go through the filter. The PSID was how much energy was dissipated in the acceleration event. As the oil warmed ..this PSID retreated to lower levels.
Well...hmmm... Pete C. added a new dimension to my view when he, in his unique style
You don't want too high a bypass valve setting. If you do, you risk losing too much flow. Keep in mind that the PSID is primarily "enabled" by the oil pump being in relief (and in some cases- at initial startup in some situations) ..shunting flow to the suction side of the pump or to the sump. If you allow too much resistance in the filter (remember - oil has an "escape" now that the pump is in relief) then your realized oil flow will suffer. Take a VW oil filter. They have 30 psi relief valves. I can put one of these on my engine. So I can start up ...get my 82 psi supply ..the relief opens ..and I can have as LITTLE as 52 PSI at startup ..meaning that almost half of my flow is being shunted (if the port can handle that much). I'm @ 5/8ths full flow. Now with a normal 15psi bypass valve ..when in relief, I will have 65 psi minimum ..or 65/82 of the potential flow to the engine. Both the relief valve and the filter/engine combo drop 82lbs gauge in the fluid circuit(s). The relief is transporting that which is not seen by the engine. The differential pressure is a product of the difference of the shunted flow ..and the total flow. Therefore ..the bypass valve setting somewhat LIMITS the shunted flow via the relief.
No, because the pressure behind the filter would also be a bazillion psi. If you were pumping into a 6' conduit that drained onto the ground ..sure. Look at the diagrams that I provided. Filter=choke ENGINE=CHOKE.
Although nooks and crannies do develop pressure elevations ..and are surely taken into account with dry sump systems that operate at mega volumes and whatnot (due to hp taxation) ..if the relief is closed the oil could care less what it has in front of it.
5 gpm flow is 5 gpm flow...whether it's through a 6' pipe ..or a capillary. The pressure developed and the energy spent pumping it will be the only difference (again, assume the relief valve on the oil pump is closed).
This is not an easy concept for many to grasp since is is counter to just about anything you can experience/observe.
Some engines regulate pressure down stream from the filter and pump. Some marine Volvo engines ...and maybe some small diesels do too. They may need to limit pressure to the oil actuated injectors ..and limit "line pressure" for lubrication to some other level. In these cases (none in automotive passenger car use to my knowleged) ..throw most of what I said out the window. There is also an oddity of new development in V8 Titan engines that is a virtual "volume on demand" setup that I haven't a clue in interpreting the pressure alterations that occur ..and how one filter or another can alter them.
Now ..as with everything, conditions and restrictions apply. I've attempted in every way I know how to qualify my assertions. YMMV.
ut:relief" 
