Subaru Oil Filter ByPass Questions & Filter Comparison

Joined
Oct 28, 2017
Messages
700
Location
Wisconsin, USA
I own a 2021 Subaru Forester Limited that I do my own oil changes on. I learned on BITOG that the by pass setting is 27 psi on Subaru branded filters. I use Wix, Fram and Purolator oil filters and found only the Wix has a 27 psi setting. Fram and Purolator spec a lower range as shown in the comparison table below. All of these filters are of good quality and value and because I follow the owners manual to change the oil at 6,000 miles I could use them all. I'm currently using the Purolator One on my Forester.

Questions:

1. Are the Fram and Purolator filters engineered to bypass the filter at the same frequency as the Wix at 27psi? I don't want to be dumping more unfiltered oil into the engine with a lower PSI filter.
2. If the answer is yes, how is it done? Does the bypass PSI get engineered together with the filter media?
3. Why do Purolator and Fram specify a psi range and Wix only specifies one psi number?

Thank you!

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recently MrSubaru.com noted you dont get genuine Subaru-ToykoReki in USA at the dealers etc unlless directly from japan, some good info from him for Subie owners IMO!
 
Purolator boss is 99% at 25micron. Published plain as day on their website.

They likely don't specify the exact bypass PSI because they expect some variation and in normal circumstances/applications its not that critical.
 
recently MrSubaru.com noted you dont get genuine Subaru-ToykoReki in USA at the dealers etc unlless directly from japan, some good info from him for Subie owners IMO!

That has been known for eons, it's not exactly new information.

Bypass pressure is so important to Subaru that none of the new, replacement filters that you currently buy from the dealer have the higher opening pressure. They are AC Delco PF2057's without the sticker on them... Bypass pressure is 13-17 PSI.

My opinion is use something with the "correct" higher bypass if it is readily available but I don't think I would spend double, or go way out of my way to get it either.
 
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That's what the inside of my (formerly my wife's grandmother's) Subaru Outback w/ H6 engine looks like after using Hastings filters with an 8 PSI bypass spec and regular bulk Pennzoil 5w30 for its entire life (~15 years, 96k miles) until I started maintaining it. :eek:

Wife's grandmother had OCD and would start the car from cold, drive it up the driveway to the mailbox, back to the garage, and park it. Then wait a few hours and do it again and again until the mail was delivered. She did this every single day that there was mail service for several years. If any Subaru engine was bypassing the filter, it was this one as the most likely time for filter bypass is when the oil is cold.

Not saying that filter choice doesn't matter, but it probably doesn't matter. I use Fram or those same Hastings filters these days. No, clean engines don't necessarily have good bearings... but there are better things to spend time worrying about.

Before anyone asks, it was apart because the rear main seal and valve cover gaskets were leaking.
 
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My opinion is that it is more important how you operate the thing than what the spec is for the bypass. Park it in a garage that does not go below 20 degrees, and don't put your foot in it before it warms up and the little blue light goes off. I use 0W/30 Mobil 1 AFE. My Outback has 255k on it, and it runs as new.
 
Thank you both for your real world experiences on a couple of high mileage, well maintained vehicles. JRed that engine in the photo is pristine, incredible. I appreciate the driving detail and the filter bypass setting it lived with. 4wheeldog your warmup advice is prudent . . . 255K miles speaks volumes for your care and maintenance also.

Both of your experiences give me the confidence that any of the filters on my table will serve me well. I'm grateful for your detailed feedback.
 
Both of your experiences give me the confidence that any of the filters on my table will serve me well. I'm grateful for your detailed feedback.
I also had an Impreza WRX and used the Purolator PureOne filters(which I guess is now just "One", the same that you're using) and never had any issues, and that engine ate a ton of dust/dirt over several thousand miles at one point because the dealership didn't reinstall the airbox lid correctly.
 
I also had an Impreza WRX and used the Purolator PureOne filters(which I guess is now just "One", the same that you're using) and never had any issues, and that engine ate a ton of dust/dirt over several thousand miles at one point because the dealership didn't reinstall the airbox lid correctly.
Great to know your experience with the Purolator One. When I was done with my comparison table
I concluded that the "One" hits a sweet spot for performance, construction and value and is why I chose it. Your feedback reinforces my decision. Thank you!
 
Questions:

1. Are the Fram and Purolator filters engineered to bypass the filter at the same frequency as the Wix at 27psi? I don't want to be dumping more unfiltered oil into the engine with a lower PSI filter.
2. If the answer is yes, how is it done? Does the bypass PSI get engineered together with the filter media?
3. Why do Purolator and Fram specify a psi range and Wix only specifies one psi number?

Q1 & Q2: Yes, the bypass setting of an oil filter is both based on the filter delta-p vs flow performance of the media (which includes the total media area factor), and the worse case max expected oil pump flow rate on any engine that the filter will be specified for. Another thing that could be involved with a high bypass setting is the anticipation of very cold start-ups and they want to help reduce filter bypassing on very cold winter starts, especially if the car is driven off and revved up pretty good before the oil warms up. As others have mentioned above, keep the revs relatively low until the oil get warmed up pretty good.

Here's a delta-p vs flow curve for a medium sized PureOne (back when it was yellow) that shows it only had ~5 PSI of delta-p across it with 5W-30 oil at 200 F (full temperature) at a flow rate of 12 GPM ... way up there where the high volume pump output is on some turboed Subarus. What's the rated max pump flow GPM on your 2021 Subaru Forester Limited? Of course the graph below is on a brand new filter with no captured debris, so if the filter loaded up from a long OCI the delta-p could be a bit more. The bottom line is if Purolator is using a 20-30 PSI bypass (as you table shows) on their Subaru speced filters, then when the oil is hot there's really noting to worry about IMO - the Frams would actually work too, but would have less headroom before the bypass valve might open.

Pretty much every engine should be driven mellow with low revs until the oil warms up, regardless of the filter on it just to ensure the filter isn't bypassing more than it should.

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Q3: Hard to say for sure, but the best theory is that Purolator and Fram are showing the range of PSI of the spring loaded bypass valve from just opening to being fully opened. The WIX bypass spec might be just start of opening pressure, or it could be the average of the range ... only WIX could say for sure.
 
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Purolator boss is 99% at 25micron. Published plain as day on their website.

They likely don't specify the exact bypass PSI because they expect some variation and in normal circumstances/applications its not that critical.
Thank you ZeeOSix for answering my three questions. I appreciate your graph and engineering example. Good to read that the bypass PSI is engineered in sync with the delta-p of the media. I also appreciate how basic good driving technique, what you call “driven mellow with low revs until the oil warms up” reduces the chance of the by-pass needing to even open up. Similar advice as 4wheeldog.

I agree with your theory on Q3 about the psi range and I’m tempted to call WIX!

It would be interesting if Fram, Purolator or Wix could rig up a sample size of oil filters with a device that measures every time the by-pass opens and keeps track of it. The filters would be given to drivers in different parts of the US for the length of an oil change interval, say 6,000 miles to actually see how often these valves open. My theory is because the oil pressure is regulated by the engine, they rarely open to make an impact on engine wear.

Again my thanks for your engineering answers!
 
^^^ Anytime, hope the info helps out.

My theory is because the oil pressure is regulated by the engine, they rarely open to make an impact on engine wear.
There have been a couple of BITOG members who rigged up a delta-p measurement across their engine oil filter and drove around. You can search the oil filter forum for member Jim Allen posts - here's one thread - LINK . Some of the graphs are missing in his write-up posts. The main take away is don't go rev crazy until the oil warms up pretty good. It was possible to make the delta-p across the filter higher than the bypass setting before the pump hit pressure relief. Note that a filter with a higher bypass setting would take more revs to hit bypass delta-p.

The max oil pressure of the oil pump output in most cases won't happen until after the filter hits bypass. The pump pressure relief and the filter bypass valve are totally separate functions, and not dependent on each other. The filter bypass opening is dependent only on the delta-p across the filter which is solely dependent on pump flow output (viscosity held constant), not the pump pressure. One pump could put out 8 GPM max at 85 PSI, and another pump could put out 12 GPM at 85 PSI - also depends on the pump matching to the engine oiling system. Un this example, the filter will have more delta-p at 12 GPM than at 8 GPM. The filter bypass only sees and reacts to the delta-p across the media, regardless of what the pump pressure makes.
 
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Thank you ZeeOSix for the link to the testing Jim Allen did 10 years ago along with your collaboration with him and others. A great body of work.

I also contacted Wix if their by-pass PSI of 27 has a range similar to Fram's 9-15 psi and Purolator's 20-30 psi for the equivalent filter for my 2021 Subaru Forester. This is the answer I got:

"A bypass valve in a spin-on oil filter is a spring, All springs have a range. It all depends on how plugged the filter is at a given time as to how open the bypass spring valve will be. It also depends on how cold or hot the engine oil is at this time. Cold oil is thicker and hot oil is thinner."

I knew this information already and was looking for actual numbers so I sense they either do not want to share them or did not want to pursue the information with their engineers.

I found in doing my product comparisons between Fram, Purolator and Wix, Fram and Purolator are the easiest and most transparent about filter construction and filter ratings.

To get the filtration ratings for the Wix 57055 I had to go to the NAPA site. Their NAPA Gold is the exact filter. To get the rating for the 57055XP I had to go to the O'Reilly's site for that filters rating.

All three companies are producing a very good product and offer excellent choices. For the person changing their oil at the manufacturer's specified interval, Fram and Purolator offer very good value.
 
To get the filtration ratings for the Wix 57055 I had to go to the NAPA site.
What does the NAPA sight show?

Most normal WIX filers (not the XP) use to be rated at 95% @ 20 microns. For the 57055 all WIX shows not is "Nominal Micron Rating" is 15. Nominal efficiency is 50%, so that's 50% @ 15 microns. I don't think that filter would make it to 95% @ 20 microns by the way oil filter efficiency vs micron size curves typically behave.

WIX 57055:
 
Here is the text for the NAPA GOLD 57055. The 5th bullet denotes the filtration.
  • Important Information: Manufacturer OE Recommended Oil Change Intervals
  • NAPA Gold Oil Filters Provide Excellent Engine Protection Using Conventional, Synthetic, or Synthetic-blended Motor Oils, As Recommended By The OE Vehicle Manufacturer
  • Use For Normal And Severe Driving Conditions Such As Stop And Go Traffic, Dirt Roads, Construction Sites, Short Trips And Interstate Travel
  • When Applicable, NAPA Flexible Silicone Anti-drainback Valve Protects Against Dry Engine Starts Past The Traditional Filter Change Interval
  • Laboratory Test Performance per ISO 454812: 18 Grams Dirt (NAPA Gold # 1515), 99% Efficient At 23 Microns (Based On NAPA Gold # 1515, 1356, 7060)
  • Use Gold For Manufacturer OE Recommended Oil Change Intervals
 
Here is the text for the NAPA GOLD 57055. The 5th bullet denotes the filtration.
  • Laboratory Test Performance per ISO 454812: 18 Grams Dirt (NAPA Gold # 1515), 99% Efficient At 23 Microns (Based On NAPA Gold # 1515, 1356, 7060)
Looking at the WIX 51515, it shows a beta ratio (efficiency) of 95% @ 20 microns, 50% @ 6 microns, and a "Nominal Micron Rating" of 21 microns ... so apparently WIX doesn't mean 50% efficient when they say "Nominal Micron Rating". You would have to call them to get their definition of that. WIX always has used their own terminology and beta ratio format that differs from the filter industry standards.

WIX 51515 Specs
 
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One can only use email to communicate with Wix and with the word salad answer I got from them on my question about by pass PSI I won't be contacting them again.
Both Fram and Purolator use the the ISO 4548-12 test standard to communicate their products media performance. They do this because it is very easy to understand. I've concluded that NAPA and O'Reilly's realize this also and made Wix declare it for their websites so the consumer can accurately comparison shop with confidence. In addition Wix states their filter mileage rating of only 3,750-7,500 miles only in their video based on OEM owners manuals. Therefore at face value Wix does not support or commit to longer oil change intervals for their oil filters.



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