Oil Filter Pressure Relief Valve Question
 

Question on the oil filters pressure relief valve P.S.I. Do I want high P.S.I or Low P.S.I. on
the oil filter pressure relief valve. I have done some research and there are some filters
with 12 to 13 or 14 P.S.I. and some with lower P.S.I. like 7 to 8 pounds, which is better for
the Engine on cold start up high or low? ? ? I know there a lot of filters on the market like
Purolator - Mobil 1 - K&N - Fram - Amsoil - Walmart, even has one that is for Harley's I am
just trying to find out the differences from brand to brand,

my understanding is that the PSI relief in an oil filter is more about a setting that opens the valve when the differential pressure meets the spec. So in an HD filter, if the exit side drops below 10 PSI, the valve opens to ensure there is oil flow. Different than a high pressure by pass senerio. So basically, the lower the number...the sooner the valve opens in a no flow or reduced flow senerio.


K&N KN-171C 14 psi 164” square (73”x2.25”)
Fram PH6022 12 psi 101” square (45”x2.25”)
Amsoil SMF134C 12psi 106” square (47”x2.25”)
A/C Delco PF53 10psi 96” square (55”x1.75”)
H-D 63731-99 10psi 63” square (36”x1.75”)

Hope this response will help you....I specialise in the design of hydraulic systems and filtration is, obviously, a very key aspect of good system design and reliability.

The pressure that is quoted with these filters is the bypass pressure. To be pedantic, that is different to a relief valve. I relief valve will divert oil back to the crank case if a maximum pressure is exceeded, hence starving the key components in the engine the lubrication they need to perform reliably.

A bypass valve essentially allows the lubricant to take an alternative path to the engine instead of passing through the filter.

So, why is a bypass valve fitted?

Firstly, a pressure relief valve is fitted immediately after the pump. The function of the relief valve is to protect the pump in case there is a complete blockage in the oil supply lines somewhere downstream of the pump. In such a case the oil will not make it to the engine and severe damage will occur should this continue for any length of time. Ideally, one would recognise that there is something amiss and stop immediately. The cause of the blockage would be resolved and everything would be back to normal.

The bypass valve is an integral part of the filter and measures the pressure drop across the filter, not the supply pressure from the pump. In the case of a new filter, lets assume for now this is zero, in other words the filter itself is not posing any resistance to flow that passes through it.

As the filter starts to clog up, so resistance goes up and so does the associated pressure drop across the filter element itself. If you continue with a dirty filter and allow to clog up more and more, then ultimately one of 2 scenarios will arise:

The element itself will collapse as it is not designed to withstand such pressure. The result would be all that accumuted dirt PLUS the element material itself hits the key tolerance parts in an engine at the same time which would not be good.

Alterntively, if the element is strong enough, pressure on the outlet of the pump builds up (trying to force fluid through a blocked element) until the relief valve pressure is reached and then flow diverts back to the crankcase. This means the engine is now starved of oil.

To protect against these 2 scenarios the element is fitted with a bypass. This valve would crack open at a pressure lower than the relief valve and lower than the pressure that would bomb the element itself, but...the oil flowing through the bypass valve is not filtered. Call this a lesser evil...

In getting back to your key point, which one should I use - you really need to know what the relief valve pressure is and what pressure is required in the engine itself to function correctly. I dont have my manual here and couldnt find the answer in a quick search.

There is a further complication that you allude to in your OP. when you first start a cold bike the oil viscosity is very high compared to when you have reached running temperature. This results in a much higher pressure drop across the filter as you are trying to pump treacle instead of water through a restriction. This means that there is a risk that you will bypass the filter at start up but then pass through the filter as the oil warms up.

So, in summary, i cannot give you a definitive answer as we don't have all the key info (ie what is relief valve pressure and what is "normal running pressure - whatever that is given the oil temp variance???"). We tend to talk about "system matched filtration", so my advice would be to use the OE element, or in the absence of that make sure you match bypass cracking pressure (I dont think a couple of psi is an issue because I doubt the cracking pressure specified is accurate to within a couple of psi anyway) and micron rating, or in the absence of that do a lot of research before deviating from this spec.

I think you have covered the issue suffiently..and are correct on what you say ..except for one point concerning the oil pump pr valve, while what you say is correct there is another reason for the oil/pump pr valve and that is to bypass excess oil when the pump is pumping more than the engine needs, the pump being a positive displacement type has a fairly linear pumping volume i.e., as the rpms of the pump rise the gpms also rise, at some point the gpms become greater than what the motor needs and the pressure builds to the point where it opens the pr valve bleeding off the excess oil...(that is the short version)

Agree totally. There was a recen thread where I touched on that. Always difficult getting the balance between a solid explanation and over complicating

https://www.hdforums.com/forum/touri...about-oil.html

Looking the filter PSI data outlined above without deviating from the OPs question which I believe has been answered quit well. Which filter rating meets or exceeds the recommended PSI bypass pressure?. Just by looking at the differant ratings where 10psi is the OEM bypass pressure and 14 on the KN, without starting a huge debate and flaming from loyal members to one brand or another, would it not be better to have the bypass engage before the minimum requirements of 10 psi? Forgive me if my diction is off but Im trying to type with a touch pad from my droid and its quit challenging to say the least!

Not sure I understand your question entirely, but...

There is no correlation between the micron rating (filtration rating) and bypass pressure. They are 2 independant variables. Both are important.

If I were on a desert island and had to choose...

I would want a filtration rating finer than specified by the OEM (ie a lower "um" number)

I would want a lower bypass pressure than specified by the OEM

I must stress that this is in the absence of knowing the relief valve setting or the pressure required to keep the engine healthy.

Problem with the 2 above "preferences" is that both tend to reduce the life of the filter.

Yes, we all want better filtration and clean elements all the time, but firstly that does not give you "system matched filtration"

Also, with a very low bypass pressure you run the risk of bypassing too much when the oil is cold, which is also not good.

Personally, I think the right answer is to stick with what the designers specified as they know all the variables. It is too easy to stand on the sideline knocking the designers, in this case I would say "so little gained for such a risk"

Thank guy's, I like it when you talk Technical stuff, lots of good information. SGDiesel
how is the weather In the U.K. ?

The weather in the UK is facilitating me spending hours on forums writing long protracted answers!!

I have a trip planned on the continent end of this week and the long term forecast is more of the same.....CR@P!

Can't remember weather getting me down like this before, tks for asking - I believe your intentions were well meant rather than a wind up ;-)

I asked about the weather because my niece is flying to London Friday on her way home
to South Africa. Thanks for everything OldPhat John