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Looking at the oil pumps output volume - 49.7 qts/min = 12.4 GPM - that's A LOT of oil flow. And that's at 43 psi discharge pressure, which means it can put out that much flow before it's pressure relief valve opens at 85 psi.
hmmm..cold flow and 45lb of differential? That would be only possible if less than half of the output (at whatever output the oil pump was producing) reached the engine @ 85lb. Which cannot occur if the pump is not in relief. We'll, for the moment, assume that the media could withstand 45lb PSID and had no bypass.
Call Bernelli.
Not quite what I meant. This is kind of long and drawn out to get the whole point across ... so hang in there.
Let’s look at those oil pump specs again:
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Pump Type: Trochoid type
Performance (oil temp 80 C which is 176 F)
5000rpm: Discharge pressure 43psi; Discharge rate 49.7 US quarts or more per minute
Relief Valve working pressure: 85psi
Imagine that the engine is running at a constant 5000 RPM, and the oil temp is 176 deg F. The oil pump will be putting out 49.7 qts/min (12.4 gpm) at an output pressure of 43 psi. Remember that the engine in this example is always running at 5000 RPM and never changes.
Now imagine that the oil is slowly cooled down … making the oil viscosity correspondingly thicker (higher density). The oil pump will keep putting out 12.4 gpm, but as the oil temp starts to decrease the pump’s output pressure will start to increase in order to keep the 12.4 gpm flowing.
Now imagine that the oil temperature is at yet a some cooler temperature – let’s assume the oil temperature is 100 deg F. The pump output pressure is now 84.9 psi … just at the verge of the pump to start pressure relieving. At this point, the pump is still putting out 12.4 gpm since the pump is still not in relief mode.
So, at this point, if the oil filter develops a PSID greater than its bypass setting, then it will also start to bypass. If the filter's bypass pressure was set to low for this engine, it's entirely possible that it could go into bypass with relatively warm oil since the pump's output is so high volume - much higher than most normal cars IMO.
Now imagine the oil temperature is starting to cool down once again from the 100 deg F point, with the engine still running at 5000 RPM. As the oil becomes cooler and cooler, more and more of the 12.4 gpm output gets spit back into the engine’s sump by the pump's relief vavle, and less flow goes to the filter/engine circuit. But, keep in mind that the viscosity is continually becoming greater as the oil temperature decreases.
Here’s the key … even though the flow rate going through the filter/engine circuit is becoming less and less as the oil cools down, the PSID is also coming down some due to less volume flow, but the PSID is not coming down very fast due to increase in oil viscosity factor … they cancel each other out to some degree.
So … lets assume that when the oil is cold (say 34 deg F) like shown in the chart below, that the oil pump is sending 4 gpm to the filter/engine oil circuit. Also assume that the filter being used has the same flow vs PSID performance curve as the black line (Fram Doubleguard) and has a bypass valve setting of 23 psi for the Subaru.
Well, you can see that in this case, there is a PSID of 23 psi across the filter, so it would be right at the verge of going into bypass mode.
Now assume you put another filter on the Subaru that had a bypass setting of 10 psi. In this case, the filter would have went into bypass at 10 PSID. Looking at the flow graph, this means that ~2.3 gpm will be going through the filter element, and the other 1.7 gpm will be bypassing the filter element to achieve the 4 gpm going to the system.
Obviously Subaru has specified a filter bypass valve setting higher than most engine manufactures because the oil pump is pretty high volume. Any oil pump that puts out 12.4 gpm at 5000 engine RPM at 43 psi discharge pressure also means the oiling system on those engines are not very restrictive, and Subaru wants to feed lots of oil volume to their engines for some reason.
Personally, I would not run any filter that doesn’t have the 23 psi bypass valve on this car … otherwise there could a lot more bypass action while the oil is cold and in the warm-up transition.
Bernoulli not required, and is out riding his Benelli, and unavailable at this time.
QUOTE BY SUPERBUSA.
