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304065 304065 is offline
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Join Date: Dec 2001
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Ultimate Oil Pressure Relief Valve Thread

Many of us have had high, or low, oil pressure issues, so I thought I would combine all the information I could find into a single thread for reference.

First, a big thank-you to Walt Fricke, whose pioneering work in this area is the source of much of what appears below.

First, the factory diagram. Copyright Porsche KG.

Second, a note about the oil bypass modification. The factory started doing this in 1976, it had been discovered much earlier, supposedly by the Kremer Brothers, and was adopted by the factory. This entails drilling a diagonal hole between the oil pressure RELIEF passage and the inlet passage to the pressure side of the pump, then plugging the original outlet of the oil pressure RELIEF passage into the case. This way, when the 76.8 psi limit of the oil pressure RELIEF valve is achieved, excess oil pressure is routed back into the inlet to the pressure side of the pump rather than going into the bottom of the case.

As everyone knows by now, this modification reduced the oil level in the case, reducing losses due to windage, and allowing the scavenge section of the pump to be reduced in size, allowing for a larger pressure section in the same overall package.

Here is the original service bulletin describing this change.

Here is a better diagram from Bruce Anderson.

Here is a photo of the mod performed by Craig Garr:

Here is how the factory did it, this is my own photo of a 964 case:

NOTE: Both bulletins above do not say ANYTHING about changing the springs in connection with the oil bypass mod. Of course the piston is changed-- the old-style piston has holes through the top, and if used in an engine with the oil bypass mod, would bleed air from the engine case back into the pressure pump inlet, and the engine wouldn't make oil pressure.

These are the old and new pistons compared:

The system diagram changed accordingly. (Note that is is a later diagram, showing the external oil cooler and relocation of the pressure transducer to the pulley end of the engine.)

The HORIZONTAL spring is the oil SAFETY valve.

It rides in a bore like this:

The VERTICAL spring is the oil RELIEF valve. See the endoscopic photos below.

Here are some actual photos of the oil bypass passage, these are mine, taken with a USB endoscope. You can see the side passage that is uncovered by the piston when the pressure gets too high. More on that in a second. At the far end (top) of the passage, this is where the oil pressure sender is located on an early car, and the switch on a later one.

Now, well below the piston there is another drilling into the case, this allows oil that gets below the piston to drain into the case, and also allows the air behind the piston to vent, so the piston can move. If this were not present, the piston would probably hydrolock.

'66 911 #304065 Irischgruen
'81 R65
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Ex-'89 944 Turbo S (Sold 8/21/20)

Last edited by 304065; 10-23-2012 at 09:18 AM..
Old 10-22-2012, 11:39 AM
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