Club 80-90 Forums

Out of interest

What is the purpose of the bypass valve and what difference does it make if it's higher or lower?

I have two VW cars and the filters are almost identical but the bypass is different one is 1 bar and the other 2.5 bar and I'm wondering why?

Our Mann 719/12 filters for WBX are 1 bar so I'm wondering if low pressure oil pumps have a lower bypass pressure?

Decent explanation here:

http://www.baldwinfilter.com/TechTips201409.html

Setting is related to where you want to balance the risk of oil starvation versus unfiltered oil

937carrera wrote:Decent explanation here:

http://www.baldwinfilter.com/TechTips201409.html

Setting is related to where you want to balance the risk of oil starvation versus unfiltered oil

oohh thanks, so probably only fit a 2.5 bar if you have a good oil pressure from the pump, hence why a WBX is 1 bar because oil pressure is pretty low by modern standards.

Maybe............ last time we had a fluid pressure discussion BBB popped along with some proper engineering understanding

I'm not so sure oil pressure is so different in modern engines, while at the same time recognising that the bottom end of the type 4 engine is very tolerant of low pressure

My understanding from hydraulics filters is that the bypass valve is there for emergencies only, in case you get a blocked filter, it stops the filter bursting under high hydraulic pressure.
In an engine under lower pressures it keeps the flow up at the risk of dirt getting into the engines bearings.
so as long as you do regular oil and filter changes you don't need to worry about it.
However if you use thicker oil than the recommended one then the pressure difference across the filter may mean your bypass valve opens more often, especially when cold.
I doubt if using 15W50 instead of 15W40 would make enough difference for you to consider a different filter.

On a similar note, my pickup had a heater in the fuel filter that was switched on by a pressure switch across the filter so that in winter when the diesel could be sluggish and waxy,
it would heat the fuel and reduce it's viscosity. Great for running on veg oil.
Don't think we need oil filter heaters in this climate though.

Thanks BBB

in fact I was wrong both my cars are the same filter at 1bar, I was reading the spec wrong and worried I have the wrong filter on.

I was under the impression it would not vent cold with a fresh filter but that article above says different.

http://www.baldwinfilter.com/TechTips201409.html wrote:Engine Starts - When the engine is started and the oil is cold.
Cold oil does not flow through the filter element as freely as
when it is warm. This causes the pressure differential across
the filter element to increase and the by-pass valve to open.
The by-pass valve will close once the oil is warm and the
pressure differential across the filter element drops below the
by-pass valve pressure setting.

I'm glad you confirmed that. I had been trying to figure out why you would have 2.5 bar of pressure difference across a filter before the bypass would open up.

I did see another site that talked about the body of an oil filter physically expanding in high pressure conditions too.

937carrera wrote:I'm glad you confirmed that. I had been trying to figure out why you would have 2.5 bar of pressure difference across a filter before the bypass would open up.:

well I was wrong again both 2.5 bar

My Lupo and Polo both have Mann 712/52 filters specified by Euro car parts and this website says they are 2.5 bar, of course website could be wrong or ECP could be wrong.
https://www.onlinecarparts.co.uk/spares ... =w712%2F52" onclick="window.open(this.href);return false;

What, wrong twice in the same day, cannot be possible.

https://www.bobistheoilguy.com/forums/u ... s/309756/1

All the references in that thread are in the range of a bit above or a bit below 1 bar

I'm doing this with maths:
Pressure difference across the filter - take your 2.5 bar
Generic minimum spec at idle, let's take 0.5 bar as an example

So the oil pump has to be creating a minimum 3 bar at idle in a hot engine for filtered oil to feed the bearings.

Most cars I have seen have about 5 bar max on a cold engine (restricted by the block pressure relief valve), and you would usually see 3-4 bar in normal running, but nothing like that at idle.

Over to others

Mr Mann does say this though, better than resale sites:

https://catalog.mann-filter.com/EU/eng/ ... W%20712~52

2.5 bar for a 1.4 lupo ! but I have never had a pressure gauge on a Lupo / Polo of that vintage

and

https://catalog.mann-filter.com/EU/eng/ ... W%20719~12

1 bar for a T25

Interestingly I just looked up the value for a Saab and that is at 0.8 bar

This next bit is from ETKA, procedures for testing oil pressure on a Lupo, pretty much the same values as a DG

‒ Start engine and increase speed slowly.
At
0.3...0.7 bar
the LED must light up, otherwise renew oil pressure switch.
‒ Increase engine speed further.
At 2000 rpm and an oil temperature of 80 °C the oil pressure should be at least 2.0 bar.
At higher engine speed the oil pressure must not exceed 7.0 bar.

Lots of variables to take into effect.
The volume of oil flow to filter size. Full flow oil filter or partial flow. Oil filter bypass circuit already incorporated in the system. And yes the bypass valve works when oil is cold not just a blocked filter.

Thanks bigherb

I am reminded (yet again) of the relationship between pressure and flow. I really must try and remember that better.

I did understand about the bypass working with cold oil but had not considered whether the oil filter design was full or partial flow.