911sc oil pressure

[56khrm]

Hello-my tech question concerns oil pressure on my stock 79 911sc. When I bought it a couple years ago the pressure on the gauge was about 3 bar at 3000 rpm. Last week, the pressure gauge started fluttering so I immediately took it to my trusted mechanic, who replaced the pressure sensor (the one you see when you raise the engine deck). Now the gauge is steady, but only reads a little under 2bar at 3000. The mileage showing is 130,000, and this 911 runs great-no unusual noises or smoke or anything. The oil level is correct. The oil light never comes on with the engine running. My mechanic says don't worry about it. The owners manual almost says that also. Doing research on this topic, I am getting conflicting views from different books and web sites-many tell me how many psi is normal, but I am never sure if the writer knows the gauge reads in bar and I should convert to psi or thinks the gauge reads in psi. I guess I have two questions:
1. Why should changing the sensor cause the reading to drop, unless the previous owner had somehow altered the sensor to show a high reading. I would not put that past the guy.
2. What is the normal reading on the pressure gauge for a hot engine? The owners manual says 4 bar at 5500 rpm, but goes on to say that a drop in pressure with hot oil is normal. It does not say how much of a drop. Again, I am reading about 2 bar at 3000 rpm, and the warning light never comes on.

Help! I don't want to drive until I know if something serious is wrong.

[GrantG]

I think there is more than one type of sensor and that could be the reason for your different reading (had the wrong sensor before or now).

Generally, you want to see about 1 Bar per 1,000 rpm with the oil at operating temp (some say 10psi per 1,000rpm is enough - which is what you are seeing now).

It doesn't sound like a mechanical problem to me - more like an electrical issue with the sensor/gauge.

Someone more knowledgeable will hopefully chime in...

[Flieger]

I have about 12psi per 1000 rpm, measured hot, level, and running at 2000 rpm. At higher rpm, I get more like 1 bar per 1000 rpm. My gauge reads in psi. 14.7 psi = 1 BAR
Engine is 2.7 RS spec with front center additional oil cooler.

[James Brown]

Generally, as components age in the engine, oil pressure drops. You can compensate it by adjusting the pressure relief valve. But first, verify the gage is reading correct by using a direct reading gage that is accurate. Once you know what the engine is outputting, compare that to the oil pressure readings in the car. If it is a stock gage in the car, it's in bars. as Flieger stated. Just drive the car and enjoy it! Oh, welcome.

[G450X]

I noticed my '82 SC has gradually (as the temperature has increased) "lost" a bit of oil pressure due to the hotter weather we just received in my area (90+ at times).

In the nice and cool spring the temp gauge rested at the first heavy line on the gauge and the pressure (fully warm) was around 3.5 bar or so at 3k rpm. In recent hot weather at 3k rpm it sees more like 3 bar. The temp gauge has run past the first mark and rests in the middle, but well below the next higher heavy line on the gauge.

I decided to go with a 28 row brass early Carrera oil cooler which I think will be fine for my usually moderate climate (July/Aug can get hot here though) to lower the oil temps a bit.

Do you have the stock trombone, and how hot is the weather where you are at? I'd guess the thinner hot oil could cause some of the difference, but I'm sure the experts will correct me if I'm wrong!

[trond]

I wouldn't be too worried if you see a change on the gauge when you replace the sensor as this would be related to the sensor. Get it measured, or buy one more sensor and see if the new reading is real.

Generally oil pressure does nothing in an engine in itself. Oil flow does. Roller bearings, tappets and cylinders only need oil to be present, they don't care about pressure and very little about volume (think 2-stroke with 2%oil mix). Plain shell bearings need sufficient oil to fill the gap, pressure is meaningless except as an indication of general wear and condition/suitability of the oil. A gradual loss of oil pressure means larger clearances in plain shell bearings from wear, or oil is getting thinner and may be compensated with using slightly thicker oil.

Worse than slightly low pressure is too much oil pressure. If you see full pressure at a too low RPM, the pressure relief valve will be dumping back to the sump and volume flow to the bearings will be compromised at higher rpm which is very bad. Generally oil pressure must increase with approx 10 psi/1000 and peak out at the pressure relief valve setting at approx 6000 rpm. If it peaks out before, the engine needs thinner oil to ensure full oil flow at max rpm.

[johndaroza]

Oil pressure is at just 1 bar and your at 3000rpm's you got a problem!

[James Brown]

3 year old thread