Oil pressure

Some good postings on the List Serve provide information about oil pressure and the lubrication system. I'll also add some specifics regarding the Corvair oil pressure levels, etc., here when I get to that point of my build.

Lubrication System

An oil pressure gauge can give you an excellent indication of the health of various systems in your engine. The key is to establish baseline readings when your engine is healthy, and then be aware of any changes you see over time. In the lubrication system of an engine, the oil pump takes in oil from the sump or OilPan and forces it through the engine under pressure. Since the oil pump is driven indirectly by the crankshaft, oil pressure is to some extent determined by engine RPM. Pressure is kept from rising too high by a relief valve. Typically, this is a spring-loaded ball, where a predetermined amount of pressure lifts the ball off its seat to allow oil to return to the oil pan without circulating through the engine.

The oil is pumped through the block and head, lubricating the bearings and helping cool the engine. After it emerges from the drillings in the crankshaft and other parts, it drains back into the sump to be re-circulated. A filter in the circuit keeps oil clean, separating out dirt and metal filings and preventing damage.

What Determines Oil Pressure?

At lower engine speeds, oil pressure is limited by the clearances between the various bearings and journals. For example, as the space between the crankshaft bearing journals and its bearings increases through wear, oil pressure will be lower because oil can flow out of the space more easily. The same is true for the journals on the big ends of the connecting rods. Thus, everything else being equal, low oil pressure can indicate worn bearings.

There are other factors that affect oil pressure, though. When the oil is colder, it has a higher viscosity (it's thicker), which means it cannot slip through the bearing clearances as easily. You'll notice that oil pressure at idle is quite a bit higher when the engine is first started up. On some cars, low oil pressure at idle when the engine is hot is normal. Oil flow may be perfectly adequate, even though the pressure is low.

Thinner, lower-weight oil will indicate lower oil pressure than a thicker oil, at least at idle and moderate engine speeds. Lower pressure caused by changing to a lower-viscosity oil may not indicate a problem, provided it is not being overheated. If the oil is thinner because it is breaking down, too hot, or diluted with gasoline from an over-rich mixture or worn rings, you should change oil at once and correct the problem.

Potential Causes of Low Oil Pressure

Low Oil Level - You may first notice your oil pressure dip during a steep bank: Add oil to fill level or you can seriously damage your engine!
Diluted or Worn-Out Oil - High Engine temps - What About High Oil Pressure?
High oil pressure is not generally a concern, but if pressure suddenly increases, there may be a problem with the pressure relief valve. Switching to a higher-viscosity oil will also show higher readings. In choosing oil weight, it's best to follow the manufacturer's recommendations for the season

Related terms

OilPressureGauge , OilPressureSender, OilFilter

-- NormanSmit - 13 Jun 2010