Question about Subaru filter bypass info ...
- Thread starter papang
- Start date
What are you asking?
It means that good modern filters rarely get plugged up enough to cause the bypass valve to open so the pressure rating of the bypass valve doesn't really matter.
What they are saying is that if the quality of the filter material (media) is very good like a Fram Ultra oil filter or a Mobil 1, then that filter material does not provide significant high restriction and therefore the bypass does not get tripped much, if at all.
In general filter material that does a great job of filtering out small particles has the smallest pores to let the clean oil through, but also has the most restriction which causes the most backpressure that trips the pressure relief valve. It's a design trade off. You chose fantastic filtering - along with that comes high restriction causing high backpressure that trips the bypass valve more often.
Or, you chose low restriction filter material to get low backpressure and not trip the bypass valve often, and the larger pores in that low restrictions filter material also let some small particles through (does not catch them and filter them out).
Ideally you want the best of both. Very good filtering that catches the small particles, AND low restriction of flow so the bypass valve seldom trips.
In engineering this is sometimes called wanting to eat your cake and have it too.
It takes an outstanding filter material to pull off this engineering stunt. And that paragraph is saying that Fram Ultra oil filters, and Mobil 1 oil filters have accomplished that difficult feat. Also, often better performance like this requires the use of better more expensive material going into it during construction, and or is harder to produce, and or takes more time to produce. Hence, cost more.
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That paragraph also is saying that the original equipment oil filters are not of the quality of Fram Ultra or Mobil 1, and therefore are designed to have the bypass trip more often.
In general filter material that does a great job of filtering out small particles has the smallest pores to let the clean oil through, but also has the most restriction which causes the most backpressure that trips the pressure relief valve. It's a design trade off. You chose fantastic filtering - along with that comes high restriction causing high backpressure that trips the bypass valve more often.
Or, you chose low restriction filter material to get low backpressure and not trip the bypass valve often, and the larger pores in that low restrictions filter material also let some small particles through (does not catch them and filter them out).
Ideally you want the best of both. Very good filtering that catches the small particles, AND low restriction of flow so the bypass valve seldom trips.
In engineering this is sometimes called wanting to eat your cake and have it too.
It takes an outstanding filter material to pull off this engineering stunt. And that paragraph is saying that Fram Ultra oil filters, and Mobil 1 oil filters have accomplished that difficult feat. Also, often better performance like this requires the use of better more expensive material going into it during construction, and or is harder to produce, and or takes more time to produce. Hence, cost more.
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That paragraph also is saying that the original equipment oil filters are not of the quality of Fram Ultra or Mobil 1, and therefore are designed to have the bypass trip more often.
Cold [sometimes very cold] starting conditions are also considered, no?
A finer filter would be OK by me if all I had to do was change it more frequently.
A finer filter would be OK by me if all I had to do was change it more frequently.
Sure the cold start-up conditions are considered, but even good flowing filters can go into bypass with cold thick oil if the engine is revved high enough after a cold start. It's always a good idea to stay light on the RPM until the oil warms up, on any vehicle.
The high bypass setting on the OEM Subaru filters is most likely a combination of the media being a bit more restrictive and the fact that those Subaru engine oil pumps flow a ton of oil.
The way to get high filtration with relatively low dP vs flow is to increase the total surface area of the media. That's why the old yellow PureOnes were high efficiency and had very good flow, contrary to the long lasting rumors that they were "too restrictive because they were high efficiency". That was busted when Purolator provided a dP vs flow curve for a medium sized PureOne.
Many filters meet the high efficiency and good flow (relatively low dP vs flow) world - therefore having cake and eating it too. Plus people need to realize that an oiling system uses a PD oil pump for a reason.
Can't really say, since the OEM Subaru (and some aftermarket) filters have a higher bypass valve setting, so they could actually open less often - it depends on each filter's dP vs flow curve at X viscosity. Some controlled testing would have to be done to verify.
My understanding has always been that the higher bypass pressure differential spec for Subies was because the oil is delivered at a much higher than typical flow rates.
That said, I've used a number of filters recommended for these applications by their makers in various Subarus over the years.
The Forester has an M1 on it at the moment and an Ultra is next on deck.
Just do a search of kschachn’s posts of all the posts on this topic. There’s literally hundreds, and it’s been beat to death dozens of times and everyone who thinks it’s an issue has a flippin meltdown but has no data to support their position. Meanwhile, hundreds of millions of filters with “wrong” bypass settings are sold every week for Subarus and we don’t see any crazy rates of engine failures. It’s not a thing. Honest to goodness. Buy a decent filter and go on with your life. THE END!
Yes, that was part of my comment in post 7. Some of those Subie oil pumps put out a lot of flow volume. But if the filter media is somewhat restrictive, than that higher flow rate makes the dP even higher with flow.
OMG point us to just one single Subaru engine failure even just on this message board that can trace to an aftermarket oil filter bypass being insufficient. You can’t. THAT’S why there’s no lawsuits over it. Cut the histrionics. There’d be a dozen class action lawsuits by now if half of what you’re claiming was true, especially in today’s legislatively-happy age.
Yes Subaru oil pumps are out of the ordinary. Yes Subaru filter bypasses are as well. No it doesn’t make it a conspiracy between the two things.
Just agreeing with what you posted.
I've never claimed that oil filter bypassing will cause an engine failure, because that doesn't happen. I only claim that some aftermarket filters will have a tendency to bypass often on some Subarus, because they will.
Nobody's going to sue FRAM because their substandard filter bypassed more often than they said it would and theoretically caused more engine wear. Nobody's suing Purolator for claiming that the BOSS is a 25 micron filter, but that doesn't make their claim true.
God I hate this topic! I’ve owned at least one Subaru consecutively for the past 22 years and have never run an oem filter, with zero engine failures. I’ve put more time than I wish to admit into researching the “Subaru 23psi boogeyman….” Over the years.
As you've shown in other threads, the oil flow rate at hot oil viscosity would have to be near the top end of the pump's output, which means if someone was "racing" their car (lots of high RPM) often then it would be better to go with a filter with the higher bypass valve setting. But the other aspect of the equation at high RPM use is the filter's overall flow restrictiveness which can impact how much the pump cuts back oil flow after it hits pressure relief, regardless of the filter's bypass setting. Ideally, any "race car" kind of driving conditions should have a filter that is going to not bypass much and not make the pump hit pressure relief as much. Subaru owners who just use the car as a normal street car won't really have to worry about that. And of course, any car should be driven gently and keep the RPM down until the oil warms up to help keep the filter out of bypass.
It's possible other filter makers could point it out if they proved it was false advertising ... they might send a friendly legal letter. We know that Purolator's website claim does not match the official Purolator Spec Sheet for the specific filter they reference.
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Yes, as far as I can tell, the bypassing should only happen at higher rpm, only with certain aftermarket filters, and only on the turbo or 6-cylinder Subaru engines with their higher flow oil pumps, at least when the filter is new.
It's important to keep in mind that dP will increase from dirt loading though. A filter's dP will double or triple by the time it reaches its rated dirt holding capacity. If the bypass pressure is only ~20% higher than the initial dP, the filter will start bypassing when it's only loaded to around half of its rated capacity. OEMs seem to spec filters with bypass pressures that are at least twice as high the initial filter dP to reduce the risk of filter clogging.