High Mileage Subaru EJ251 Oil Filters

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Originally Posted by Hangman
I have received SO MUCH more information and SO MUCH more valuable input than I could have EVER imagined.

Thank you ALL for your answers.

So, in essence, the bypass acts just like a turbo's wastegate; sees too much pressure, begins to open. Am I understanding that correctly?



The oil-filter bypass valve responds to differential pressure across the filter media. The valve is plumbed in parallel with the media such that it senses the pressure differential. If the pressure differential across the media exceeds the setting of the bypass valve, the valve opens, allowing unfiltered oil into the engine. Situations where the bypass will open include a restricted filter media, too-high oil viscosity, cold oil [which is more viscous] and high RPM. The bypass valve is different from the oil-pressure-relief valve, which open when engine-oil pressure exceeds the setting of the valve. When the relief valve opens, it dumps oil back into the oil sump, thereby limiting the maximum oil pressure applied to the oil filter and downstream components.

As an example of bypass-valve operation, let's say that the inlet pressure to the oil filter is 50 psi. With Subaru's 23 psi bypass-valve setting, the bypass valve will open when the pressure downstream of the oil filter decreases to 50-23 = 27 psi. With a 50 psi inlet pressure, the bypass valve will remain closed if the downstream pressure is greater than 27 psi. Most auto manufacturers specify bypass-valve settings of less than 23 psi. Some aftermarket oil filters that are listed as fitting Subaru engines have bypass-valve settings significantly lower than 23 psi.

HTH
 
Originally Posted by krismoriah72


I have given up on driving it and need to sell it.. I do wish that i could somehow buy one brand new and start over with zero miles...



Wanna sell it? Where are you located? What kind of condition is it in?
 
Interesting about Subaru's higher bypass. Lots of discussion all the time about it. They might have a reason relating to the oil pump. One thing should be clear, and I became aware of on this board, is the flow stays the same unless the oil pump relief valve opens or the oil pump is variable output. The 27psi example is at the inside center of the oil filter only. From there the pressure increases/decreases at the galleries and main bearings. Some people reading might think 27 psi means lower oil flow in the engine and therefore oil pressure is not as good with the OE filter. It's the same flow as with any filter. The Fram Ultra will get restricted in time and then it has only the 12 psi bypass. Especially when people are extending filter intervals to many oil changes.The filter is just a restriction like any other restriction in the oil system..
 
The filter by-pass valve seems like it exists primarily to keep the oil filter intact. The positive displacement type oil pump will move the oil up to a preset pressure, period. If there is too much restriction thru the filter, that oil will still go SOMEWHERE, somewhere ideally being thru the by-pass valve, or, not ideally, tearing thru the filter media, or rupturing the gasket or can itself ... although an oil pump with a properly functioning pressure relief valve likely won't burst the can.
 
Most Subarus have a very high volume output oil pump, that's one (and probably the maun) reason they spec the OEM filter with a high bypass valve setting.

Another reason along with the high output oil pump is their OEM filter may be some what flow restrictive (ie, higher delta-p across the filter). Combine that with the high fllow oil pump (which results in even more delta-p), and the expected debris loading, then all that combined drives the bypass valve setting requirement to increases accordingly.
 
Originally Posted by geeman789
The filter by-pass valve seems like it exists primarily to keep the oil filter intact. The positive displacement type oil pump will move the oil up to a preset pressure, period. If there is too much restriction thru the filter, that oil will still go SOMEWHERE, somewhere ideally being thru the by-pass valve, or, not ideally, tearing thru the filter media, or rupturing the gasket or can itself ... although an oil pump with a properly functioning pressure relief valve likely won't burst the can.



I think it is interesting that Subaru chose Fram to produce their OEM oil filter in light of fact that the common knock on Fram is that their cans are weak and internal construction could be better (ie fiber end caps vs. metal end caps). So apparently a Fram Extra Guard in a blue can with a 23+ psi bypass is more that adequate to meet Subaru's requirements.
 
Originally Posted by Ignatius


I think it is interesting that Subaru chose Fram to produce their OEM oil filter in light of fact that the common knock on Fram is that their cans are weak and internal construction could be better (ie fiber end caps vs. metal end caps). So apparently a Fram Extra Guard in a blue can with a 23+ psi bypass is more that adequate to meet Subaru's requirements.



Most oil filters have a burst strength well above 250 psi. A Subaru oil pump has a relief valve set at 85 psi.

If the oil can't go thru the filter by-pass valve (unlikely ... but possible ), it will tear a hole somewhere in the filter media, and go thru there. If the filter media / centre tube collapses and completely blocks all oil flow thru the filter, AND the oil pump relief valve is not working, THEN there is the risk of gasket failure, or a burst filter. But at that point, your engine isn't getting any oil, so a burst filter is the least of your worries ...

Most filter issues are invisible to the average owner. A spike in oil pressure rips a hole in the filter media, some oil always passes thru unfiltered, but nobody knows anything is wrong unless the filter is cut open on removal.
 
We also have an 87 grand national in our fleet that runs 9.30's, makes a little over 800lb-ft; after seeing it implode a Fram filter, I'll never touch one again. I'm sure it could've been a total fluke, but that was the first time using a fram on there car and will be the last.

Thankfully when it imploded, we smelled the burning oil and shut the car off immediately, as it drained every bit of oil out of the car in seconds.
 
Honda and Subaru specify their design, Fram makes it. These are huge contracts. Motorking said the media was special for Honda, and others have measured Honda Fram can thickness and found it thicker. Subaru surely engineers the filter the same way and has Fram make it. It's not a Fram filter.

Fram has a test video showing the filters bursting. It looks like at the gasket at about 300 or whatever psi. very high, not the can. Then if someone is actually racing with maybe modifications to the oil system they make a racing filter for that purpose.

Fram cans are standard thickness like many others, not premium thickness. That's one reason an Ultra can be $8 while a Boss or other is more. There is nothing wrong with standard thickness, it isn't they are thinner than standard. For normal use standard thickness is more than adequate as their video test proves..
 
Originally Posted by ZeeOSix


Most Subarus have a very high volume output oil pump, that's one (and probably the main) reason they spec the OEM filter with a high bypass valve setting.

Another reason along with the high output oil pump is their OEM filter may be some what flow restrictive (ie, higher delta-p across the filter). Combine that with the high flow oil pump (which results in even more delta-p), and the expected debris loading, then all that combined drives the bypass valve setting requirement to increases accordingly.



And the stock filter for the older models was TINY. As for flow, speculation is that it used the same filter media as Honda, which in the ONE whole data point available, indicated it wasn't very efficient filtration wise, so it SHOULD flow " better ... " . Maybe. Possibly. Sort of ...
 
Originally Posted by geeman789
Originally Posted by ZeeOSix


Most Subarus have a very high volume output oil pump, that's one (and probably the main) reason they spec the OEM filter with a high bypass valve setting.

Another reason along with the high output oil pump is their OEM filter may be some what flow restrictive (ie, higher delta-p across the filter). Combine that with the high flow oil pump (which results in even more delta-p), and the expected debris loading, then all that combined drives the bypass valve setting requirement to increases accordingly.



And the stock filter for the older models was TINY. As for flow, speculation is that it used the same filter media as Honda, which in the ONE whole data point available, indicated it wasn't very efficient filtration wise, so it SHOULD flow " better ... " . Maybe. Possibly. Sort of ...


A tiny filter with less surface area can also be a factor in increased delta-p and the need for a higher bypass valve setting.
 
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