Bypass Valve psi rating : Low vs. High

[CQUG4R]

I have searched the forums, but I did not find an answer to this question. I'm sure everyone will be pasting link’s.

I have a bigger oil filter on my Honda now. It’s a NAPA 1347. It has a psi rating of 8-11 psi. I found another one that might fit. It's a NAPA 1726, but it has a higher psi valve rating of around 20 psi & up.

My question is...what does this valve do at those psi ratings? Let me put it this way.

1. When I'm at a stoplight, just idling, is oil being filtered? 8-11 psi and 20 psi?

2. I'm driving in town 40-45 mph. Is my oil being filtered? 8-11 psi and 20 psi?

3. I'm driving on the highway 70-80 mph. Is my oil being filtered? 8-11 psi and 20 psi?

I know I must sound like a simpleton, but I'm new and need the education. Also, if I choose a filter with no bypass, is that ok or am I making a big mistake.

Thanks, JIM

 

[Winston]

There is some debate on this issue. I would say that in your three scenarios that the oil is being filtered and is not in bypass mode for all three.

There are three scenarios where I think the bypass MIGHT open.

1. Cold startup.
2. High rpm.
3. Filter media loaded.

Choosing a filter with no bypass is a big mistake unless your car has an internal bypass.

 

[ZR2RANDO]

And to add something about the difference in the setting of that bypass…
The bypass is a valve that opens when the pressure difference between the incoming side of the filter and the outgoing side is equal to (or greater than) the setting for that valve. The valve then closes when the pressure difference is lowered. When the valve is open, some oil is still going through the media, it's not an either/or situation.
The 8-11 psi-d (pounds per square inch- differential) valve will open at between 8-11 psi-d. If it is working as it should, that valve will maintain oil flow even though the pressure on the low side is reduced by that valve setting amount. The 20 psi-d bypass valve, by definition would allow a 20 psi-differential to exist before it opened. It forces more oil through the filter, even though the flowrate and the after filter pressure is reduced.
These valves come into play mostly when the oil is too thick/cold to easily pass thru the media, or when the flow rate is so high, that the media cannot meet that flow requirement. That valve has 2 jobs, 1- to keep the filter internals from self-destructing due to too much pressure difference between high and low side, and 2- to avoid total oil starvation in the case of a blocked or insufficiently flowing filter.

 

[CQUG4R]

quote:

---

Originally posted by ZR2RANDO:
The 20 psi-d bypass valve, by definition would allow a 20 psi-differential to exist before it opened. It forces more oil through the filter, even though the flowrate and the after filter pressure is reduced.

---

Forces more oil through the filter. That's got to be good, right?

 

[XS650]

quote:

---

Originally posted by CQUG4R:

quote:

---

Originally posted by ZR2RANDO:
The 20 psi-d bypass valve, by definition would allow a 20 psi-differential to exist before it opened. It forces more oil through the filter, even though the flowrate and the after filter pressure is reduced.

---

Forces more oil through the filter. That's got to be good, right?

---

It is unless the pump doesn't put out enough pressure to do it's job while loosing 20 psi through the filter. It wouldn't be wise to add a bunch of pressure drop to your oil system without verifying that the system would still work properly under all conditions with the extra pressure drop.

 

[ZR2RANDO]

Oil going through the filter is good as far as the oil getting filtered…but could be at the expense of flow. You want as much as possible being filtered, but when the oil is cold/thick, or the filter is clogged or unable to flow sufficient volume…the bypass is the there to maintain at least a minimum flowrate to keep the engine from tanking. It’s just there for those times the media can’t do it all so-to-speak. The best situation overall is to have enough media to only bypass infrequently, and to have enough efficiency to filter the oil to the degree required to reduce wear…it’s a compromise.
There are tons of topics on this BITOG site that go back and forth on this…enjoy
Your hooked now.....happy reading!

XS650- just read yours...XACTLY!, well said.

 

[Virtuoso]

Your oil filter is always filtering. Just how much is squeaking past the bypass valve, is the question.

 

[CQUG4R]

OK ,hold on.

Am I loosing pressure to an oil filter based on what that psi rating is as advertised?

In other words, the psi rating on all oil filters is more or less a loss of preesure to your oil pumping system of your engine? If it's a 20 psi rated filter, I will loose 20 psi of pumping ability to my system and if it is rated 8 psi, I only loose 8 psi of my pumps pressure.

I guess what I'm saying is: the psi rating of an oil filter is the amount of psi that will be robbed from your overall oil pumps pressure output?

 

[ZR2RANDO]

That rating is the max oil pressure drop that should be allowed by that filter, but that condition doesn't happen all the time..normally more of a spike unless you are at WOT....

Normal drop in a filter would be low, like maybe 4 or less.
Any restriction to flow causes some pressure loss, a filter is a restriction to flow that increases as it gets dirty/clogged up, but typically is not a very big restriction compared to the rest of the motor..

 

[bulwnkl]

Remember, guys, that oil pumps are positive-displacement units. They do not build a pressure and then the pressure causes flow to happen the way a centrifugal pump does (i.e. irrigation water pump or even a car's water pump). They move a certain amount of oil per revolution no matter what (subject to very slight internal leakage). Pressure is only built by virtue of the resistance to flow in the motor (bearings, etc.).

So, a heavily restricted filter may cause pressure on the inlet side of the filter to be 60psi and the outlet only 40psi (if you had a 20psi bypass in the filter), but the flow would remain the same.

This would argue that no bypass is needed in the oil filter or at least that a higher-pressure bypass is better, but there's another curve: The system's pressure-relief valve (separate from the oil filter bypass valve). If pressure builds too high, that pressure-relief valve will open and oil will start bypassing the whole oil system. The intent of this bypass is primarily to prevent cold, thick oil from blowing seals, bearings, or anything else apart from excessive oil pressure. Many new vehicles will see 40-60psi on the oil pressure gauge no matter what the engine rpm is or how cold the oil is. These systems seem to run that pressure-relief valve partly-open all the time. When that happens, then any increase in filter restriction will cause more oil to bypass the filter and circulate unfiltered.

So, whether the higher filter-bypass pressure is better or not depends on many things.

 

[ZR2RANDO]

Positive displacement pumps are typical pumps used for heavy fluids, the relief is always there to limit the max pressure delivered to the rest of the system, otherwise that positive displacement pump would potentially force so much fluid through the system that any weak point available would become the new relief…

“So, a heavily restricted filter may cause pressure on the inlet side of the filter to be 60psi and the outlet only 40psi (if you had a 20psi bypass in the filter), but the flow would remain the same.”

This would be true if the relief was not available to shunt some of that flow prior to the filter. Once the engine and the filter build up a certain total resistance to that flow, the relief starts shunting that flow…prior to the filter
If that relief was blocked off, then yeah what goes in has to go out, and whatever is in the way better standuptoit…

Lots of discussion on this particular subject on these pages, good stuff to discuss over a few tall ones….

here's a good page on this type pump, kinda cool reading...

http://www.engineeringtoolbox.com/positive-displacement-pumps-34_414.html

 

[Mr Nice]

Filter Guy should know the answer to this question.

 

[Gary Allan]

My view is somewhat different than Zman's. I think it's mainly semantics.

The pump produces a volume of oil. This creates a pressure. Any intermediate restriction elevates upstream pressure. If this pressure exceeds the pump's relief limit, flow will be reduced. Then the filter produces a pressure drop depending on the amount of volume relieved.

But (probably) a 20lb bypass and a 12 lb bypass will appear to have much in common (probably) until the 20lb bypass filter reaches 12 PSID as it's worst case situation.

I haven't tested a 20 or 30 pound bypass filter. So I don't know if it has more inherent resistance ..or just a more substantial and high capacity media.

 

[Eddie]

I'm very leary of using a filter with different specfications than the factory engineers recomend. Why would I use a filter with a 12 psig bypass when the engineer wants a 20 psig bypass?

 

[Gary Allan]

Well, Eddie ..why a 8-12 psi bypass (WIX) ..or a 12-16 bypass (Pure One) ...or any filter with a GM engine (no bypass w/5-7 psi internal bypass in the engine)??

Just what is the factory spec? If it's Wix ..it's different then Purolator.

You have to be leary of quite a bit when you don't know what the factory spec is.

The concensus is that you would be more concerned using a 20 psi bypass filter where a lower setting bypass is usually offered. Again, they probably resemble each other for a great deal of at least one of their lives in service. The one with a higher setting is obviously built with a more substanial media to withstand the additional pressure. So it may be due to some extended drain interval for that vehicle. So far, and I surely could be wrong, the filters that I've seen are on VW and other Euro alloy. Perhaps our teutanic and utilitarian cousins have some plan that they aren't letting on about.