Oil Pressure/RPM curve
06-24-2003, 02:44 PM
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Oil Pressure/RPM curve
I'm aware of the relationship between oil pressure and RPM--1bar/1K RPM--and my older 911SC (190Kmi)will deliver that pressure, or more, until the engine reaches operating temperature. At that point, with RPM's at about 3500, the pressure plateaus and stays at approx 3.5bar even as the RPM climbs towards 5000. Any ideas if this is mostly related to bad/broken rings. cylinder/piston wear or just heat expansion? I've recently had the thermostat replaced, as we thought it was sticking part way open.
GREGER
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06-24-2003, 09:57 PM
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My 82SC owners handbook says when the oil is at operating temps (194 F) that at 5000 RPM oil pressure should be "about 4,0 bar". I seem to recall my SC, with 120K miles, on the track running between 3,5 and 4,0 bar, maybe closer to 3,5 bar.
06-25-2003, 08:28 AM
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Yep - mine too. I have just discussed this very issue my mechanic (who I rate very highly) and he says that many SC's exhibit this trait. He thinks it is nothing to worry about.
06-26-2003, 09:38 AM
#5
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by greger:
<strong>Any ideas if this is mostly related to bad/broken rings. cylinder/piston wear or just heat expansion? I've recently had the thermostat replaced, as we thought it was sticking part way open.
GREGER</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">
<strong>Any ideas if this is mostly related to bad/broken rings. cylinder/piston wear or just heat expansion? I've recently had the thermostat replaced, as we thought it was sticking part way open.
GREGER</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">
06-26-2003, 09:52 AM
#6
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by greger:
<strong>Any ideas if this is mostly related to bad/broken rings. cylinder/piston wear or just heat expansion? I've recently had the thermostat replaced, as we thought it was sticking part way open.
GREGER</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Pistons/cylinders/rings would have nothing to do with the oil pressure. Oil pressure is produced by the oil pump forcing oil through the various passages and bearing clearances in the engine. Low oil pressure occurs when there is too much clearance, due to excessive wear (probably normal wear on a high-mileage engine) on the crankshaft and, camshaft or rocker arm bearings and/or journals. The pistons, cylinders and rings are lubricated by cranckcase oil splashing on them, and sometimes by jets of oil that spray from small holes in the big ends of the connecing rods.
From your description of the mileage and the problem, I'd say the engine is just starting to wear out. Your oil pressure isn't too terribly low yet, so I'd just drive it and see what it does. I'll bet it will run a long time yet before the pressure really drops. What oil are you using? You might try a higher viscosity oil.
The thermostat was a good idea, because if it was sticking partway open it would cause higher oil temperatures which would thin the oil and drop the pressure.
<strong>Any ideas if this is mostly related to bad/broken rings. cylinder/piston wear or just heat expansion? I've recently had the thermostat replaced, as we thought it was sticking part way open.
GREGER</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Pistons/cylinders/rings would have nothing to do with the oil pressure. Oil pressure is produced by the oil pump forcing oil through the various passages and bearing clearances in the engine. Low oil pressure occurs when there is too much clearance, due to excessive wear (probably normal wear on a high-mileage engine) on the crankshaft and, camshaft or rocker arm bearings and/or journals. The pistons, cylinders and rings are lubricated by cranckcase oil splashing on them, and sometimes by jets of oil that spray from small holes in the big ends of the connecing rods.
From your description of the mileage and the problem, I'd say the engine is just starting to wear out. Your oil pressure isn't too terribly low yet, so I'd just drive it and see what it does. I'll bet it will run a long time yet before the pressure really drops. What oil are you using? You might try a higher viscosity oil.
The thermostat was a good idea, because if it was sticking partway open it would cause higher oil temperatures which would thin the oil and drop the pressure.
