moribundman

Do you know it's actually happening, or are are you assuming it might be an issue? While I think it's possible that it can happen, I doubt it usually does happen. Our engine is capable of running oils much more viscous than xW-40 at high oil temperatures, and I would think the choice and design of oil filter for the 12v reflects that.

I'd be more worried about a critical drop of oil pressure when the oil reaches upward of 130 C, which has happened to me (in my Scirocco) after prolonged (>30 miles) high speed (~200 km/h) runs on the Autobahn. Of course, high oil temps may not be an issue with your setup, which may well include a real oil cooler instead of our OEM heat exchanger.

I brought up the issue of the filter going into bypass mode <a href="http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=6;t=003415;p=1#0000 04">here</a>. In case you can't read that thread without being signed in, please let me know.

JWG

Between the upper and lower oil pans, we have what the Bentley manual describes as a "cover" on page D 17-2, which has integrated 5 bar (72 psi) and 11 bar (159 psi) pressure relief valves.

Now, some one explain how that works: One valve triggers at 5 bar, but if the pressure gets to 11 bar despite the 5 bar triggering the second valve triggers at 11 bar?!

If these numbers are correct, the 30+ psi oil filter bypass valve will frequently open on our cars, because we see 1-2.5 bar (15-36 psi at idle and 3-5 bar (44-73 psi) at a mere 3000 rpm.

With an 11 bar relief valve, how much over 11 bar does our oil pump itself put out at 6000 rpm? Yikes!

Additionally, high oil temps are not an issue with my set-up. Both oil filters sit directly in the air stream by the driver's side lower front grill which I drilled out. I rarely see oil temps above 225* (whether high speed driving or idling in traffic on a hot day) with around 212* averages.

GPJ

&gt; If these numbers are correct, the 30+ psi oil
&gt; filter bypass valve will frequently open on
&gt; our cars, because we see 1-2.5 bar (15-36 psi
&gt; at idle and 3-5 bar (44-73 psi) at a mere 3000
&gt; rpm.

The filter bypass valve works with differential pressure across the filter element. This is not the same as 'absolute' oil pressure measured at the oil filter intake.

JWG

As I understand it, there exists a mechanical valve at the base of the filter housing; when the incoming oil pressure becomes great enough, the pressure forces open a spring valve and the incoming oil bypasses the filter and exits without getting filtered.

Do you have a cut-out or diagram as to how the oil filter by-pass valves work based on differential pressure?

I just cannot imagine how a simple spring valve could sense the difference between the pressure on one side of a paper element and the other side of the element and open only when the pressures differ by "x" psi.

Thanks in advance if you can find the answer.

JWG

<center><img src="http://www.transglobal-aerospace.co.uk/Hydraulics/chap2a13.gif"></center><p>So, at system start up, the bypass valve will always open, because the "red" side of the system has no pressure.

However, when enough oil flows into the "red" side of the system, the pressure on both sides will basically become equal and will close the valve.

Thus, the valve will thereafter open only when the pressure on the red side of the system becomes less than that on the green side of the system.

Did I get that right?

ModifiedA4

if the filter becomes clogged. with no bypass valve, the engine would decome starved for oil as the filter gets more and more clogged. i believe this is the main reason for the bypass valve in the oil filter area.

moribundman

I'd say you got that exactly right. The question is, under what actual conditions does the pressure differential exceed 3 bar. For a moment at start-up? For a bit longer when it's cold? But at high RPM driving? Oil pressure may then well exceed 5 bar, but will the pressure differential between oil filter inlet and outlet port exceed 3 bar?

Also, consider that the valve may open only a little.

moribundman

Cold oil may be so viscous that it simply will not flow quickly enough through the filter medium and get in time to the critical bearings. The bypass valve is simply a means to allow improved oil flow should oil pressure fall on the filter outlet port side. As you said, the bypass valve prevents oils starvation.

ModifiedA4

good point!