Oil pressure puzzle

JoeMag · 06-09-2020, 11:56 AM

Gents -- Have an oil pressure situation I'm trying to get my head around... I had my oil pressure sensor where the plug 10 is on the attached pic. Oil pressure was around 80 psi at higher rpm which seems right due to regulation of the spring and piston. When I moved sensor to point 1, it now reads about 115psi. I'm struggling with how a downstream point 1 would read higher. From point 10 to 1 oil is just flowing through the filter, right? gt3 oil pump with case vented to oil tank.

Any ideas?

stownsen914 · 06-09-2020, 01:38 PM

Same sender? (Ruling out a mismatched sender/gauge combo).

Did you make any other changes like removing/replacing the relief piston?
If you move the sender back to the old position, do you get 80 psi again?

115 psi sounds like the blowoff pressure of the safety valve circuit, which could point to a malfunctioning relief valve.

r lane · 06-10-2020, 05:53 AM

The 2 sensor locations are I think connected by a short lateral gallery, so doesn't make since, but fluids and multiple intersecting galleries are confusing. As Townsen mentions, redo it and see if it repeats

JoeMag · 06-10-2020, 06:41 AM

Few comments:

- I actually had the sensor that was in point 10 break and I put my fuel press sensor over there. Read the same. both 0-100 psi sensors.
- Bought a new sensor put it on point 1 and had it fail shortly after i put it in. Put another one in and it was suggested it might be maxing out since it was showing around 96 psi. ...note nothing was changed except moving sensor.
- Put in a 0-150 psi sensor and determined it was around 115 psi.
- Oil pressure regulator piston did not appear to be binding as I just took out the plug (part 18) and spring and piston fell right out.

I think I'll add same sensor I have on point 1 to point 10 so can just move plug connection plug and see if pressure is different.

Theoretically if pressures are as I've seen, oil would not be flowing through oil filter.

r lane · 06-10-2020, 08:29 AM

Which engine. Is the oil filter on the block or in the engine compartment?

JoeMag · 06-11-2020, 08:44 AM

Filter on the block. ...not sure if case is 993 or 964 based. 993 cam towers (solid rockers).

icarp · icarp's Garage

I would check the pressure with a mechanical gauge , just an idea

JoeMag · 02:28 PM

ok gents... Here's data from a lap with a sensor on both ports. I checked the sensor with mechanical gauge on the point 10 and the one on the main galley (point 1) is a an aim gauge and I had two 0-100 psi gauges reading 100 psi so I believe it's over 100 psi at point 1.

Blue - point 10
Orange - point 1

...what are we think'n?

winders · 08-02-2020, 05:54 PM

Isn't point 1 pre-filter and point 10 post filter? There should be a pressure drop, right?

From point 1, my new 964-based race engine with 964 Turbo oil filter console in place of the cooler shows about 80 psi at 6000 RPM.

stownsen914 · 08-02-2020, 06:16 PM

Quote:

Originally Posted by winders

Isn't point 1 pre-filter and point 10 post filter? There should be a pressure drop, right?

Yes, I believe is correct.

Quote:

Originally Posted by winders

From point 1, my new 964-based race engine with 964 Turbo oil filter console in place of the cooler shows about 80 psi at 6000 RPM.

75-80 psi at point 10 is good as far as I know. 110 psi pre-filter still sounds high to me. Can there be a 30+ psi drop for a filter?

winders · 08-02-2020, 06:22 PM

Jerry Woods once told me that 110 psi was okay. He said it might cost you a few horsepower but you know your engine is getting oil where it needs to go.

stownsen914 · 08-03-2020, 03:06 AM

Joe - what was the oil temp when you took the pressure graphs above?

JoeMag · 08-03-2020, 05:21 AM

Yes, it’s my understanding point 10 is before filter and point 1 is after. ...that is what’s really confusing as I would expect pressure drop. Race motor w 70 hrs on it, and I do oil analysis all the time and they’re all good.

Oil temp was 190F at point 1 for this data set. ...about half way through a race.

r lane · 08-03-2020, 06:26 AM

I am permanently confused on the oil flow, esp as the oil filter/cooler change places. It appears in your application that the oil flow, pressure side, comes into the thermostat cavity, #1. Without a thermostat or oil flow diverter, it would seem to me that the oil has pressure access to both in and out ports for the oil filter. That can't work, what am I missing? I am looking at an early oil flow schematic, but I don't think that changed on the later cars.

winders · 08-03-2020, 09:02 AM

Quote:

Originally Posted by JoeMag

Yes, it’s my understanding point 10 is before filter and point 1 is after.

That's backwards.....

JoeMag · 08-03-2020, 11:29 AM

...here in lies my confusion. here is the oil diagram i have been working from.

winders · 08-03-2020, 03:06 PM

Hmmm....to be honest, I have never put a sensor at point 10. I always use point 1. If point 1 is post filter and point 10 is pre filter, I would expect the pressure to be higher at point 10.

stownsen914 · 08-04-2020, 07:00 AM

Joe - I think you may have your measurement points mixed up in the different diagrams. Point 1 noted in your first post is the thermostat cavity in the early 911 engines. In later engines and I assume in your motor, you probably have either the cap noted as part #7 in the first post, or a diverter like the old RSRs used. In post #16 you've listed this as Point 10. It's actually the Point 1 from your first post (pre-filter).

Walt Fricke · 08-07-2020, 11:47 PM

I know stock for 3.2 and earlier pretty well, and have an RSR replacement for the engine oil thermostat (and modified an old engine oil thermostat to work the same way) because I don't run the engine oil cooler on my modified motor, using an extra oil filter there. Same flow as the factory RSR. Since the 964 and 993 also dispensed with the oil cooler, there was no need for a thermostat to move the oil one way or the other.

Because the 964/993 have different cam oiling routing, with no external lines, I guess that's why Porsche moved the pressure sensor to the flywheel end of the engine.

On my SC I got tired of the idiot light sensor failing and leaking (JB Weld reinforcement isn't forever, it seems). So I got an appropriate VDO two in one - idiot and full pressure combined. Which looks like just what is going on here, only it is mounted on what is shown in the exploded diagram (as opposed, perhaps, to the schematic).

The old idiot light location is the rearmost vertical drilling on the right case half. If you pull the adapter and sensor out, you are looking right down at the pressure setting valve. If you pull the plug under the engine, and take the spring and valve out, you can look right down through the case and out the plug. That has to be point 1. It is pre-everything.

If my memory and quick review of some photos is right, if you remove the thermostat (or in this case the console with the two fittings on top which replaced the tstat), you are looking down a hole in the center of the bottom of the larger hole the console fits in. That leads to the pressure output of the pump, as well as to the safety valve system.

Looking at the parts diagram, and the schematic, Joe has his points correct on the schematic. The schematic is a bit more schematic for the pressure setting part - in the engine (older ones, anyway, pushing the piston down opens a hole on the side of the drilling, which sends that oil back to the intake (or sump, on the older motors). You can see this sort of operation in the schematic for the safety valve, but it doesn't show up well, if at all, in the schematic.

Here is what I don't understand about the routing: the 935 part diverted all the pressure regulated oil through the filter in drilling (formerly the cooler in port), and sent all the return oil from the filter to the drilling in the side of the big hole which goes to the main oil gallery.

The parts diagram shows the console which goes where the old thermostat went as just being a cap, with no provision to divert anything. Does this mean that the casting and machining of this part of the right case half is significantly different? Pressure regulated oil goes directly to the filter in port, and would do so even with part 7 removed? That would solve what is a mystery for guys like me with no hands on experience with our own 964/993 blocks.

Joe - great visuals and presentation. The schematic also shows the pressure setting piston as keeping max pressure to 80 PSI (5.3bar sounds a lot like 75 psi plus a third of a bar). And that's just what you measured at point 10, and recorded on your data system.

What none of this explains is how it is possible for hydraulic pressure to increase farther down the line at point 1 when it is regulated. When you stick your finger over a lawn watering hose, the pressure there at your "nozzle" increases as the flow decreases, doesn't it? But that pressure doesn't translate back to the house plumbing? It is just P1*V1 = P2*V2? If down in the bearings or squirters you had a big oil leak, the pressure would just measure lower overall, wouldn't it? If you had (heaven forbid) a blocked oil passage to the #8 bearing, the pressure setting valve would maybe depress a bit more, and the resultant 80 psi would remain the same?

Color me baffled as well.

r lane · 08-09-2020, 11:04 AM

This may have already been stated and resolved and I missed it. A 964 had no engine attached cooler nor filter and the thermostat cavity is bare. I have a 965 which has the second oil filter in place of the on board cooler. It is bit of a pain to get to, but during this week I will pull the cap off the thermostat cavity to see what is in the hole. A diverter of sorts has to be down in there. Will report back.