Oil Pressure

[Stallion]

I just have a question about oil pressure. What exactly creates the pressure? And, if too low or too high, what's the impact on the engine?

 

[MaineShark]

The oil pump forces pressurized oil through the oil galleys, which distribute it to the various bearings.

The moving parts of your engine actually ride on a thin film of oil, not direct metal-to-metal contact. If the pressure is too low, that film will not be strong enough to withstand the forces involved inside and engine, and the metal will rub, wearing it down and eventually causing a failure.

If the pressure is too high, the same sort of thing can happen. The oil moves so quickly that it "washes out" the bearings, preventing that film from forming and allowing wear. Additionally, in some cases, having too much oil flow can drain the pan and cause a buildup of oil inside the valve covers and lifter valley, if the drain holes aren't big enough to handle the oil flow.

Oil pressure is related to the internal construction of the pump, the rpm's that the engine is turning, and the viscosity of the oil. For example, if you run heavy oil, and it gets very cold, the pump has to work harder to push the oil, and builds up more pressure, which can do things like blow out the oil filter case or gasket, as well as the rear main crankshaft seal. A lot of people run heavy oil (eg, 20W50) in their engines to reduce leaks, and then they hit a stretch of cold weather like we're having now, and that thick oil becomes like molasses... and ends up all over the ground when the excess pressure causes some seal to fail.

Joe

 

[Ken]

TR, I found some reading for you here: Engine Lubrication Basics……Part 1 from Old Cars Only

One of the subjects that often gets overlooked in an engines performance is the focus on lubricants and how they interact with the everyday life of the engine. Often, the enigma of engine lubrication science and the minutia around it makes it hard to ascertain a "correct or perfect" answer for your particular application. We commonly and sometimes blindly just pour in popular oil, change the filter and go! We just take for granted that everything will be fine and dandy when in fact there is an education to such.

 

[XS650]

The oil pump down in the oil pump creates pumps the oil, the pressure come about because of flow restriction in the oil passages in the engine. No restriction, no pressure.

That's why you have less pressure when you run thinner oil or the oil gets hot. It's thinnner so less restriction.

It's common misperception that oil pressure makes journal (rod and main) bearings work. The pressure gets the oil to the unloaded side of the bearing and a hydrodynamic wedge is fromed as the part rotates. Actual oil pressure on the loaded side of a journal bearing can be a few hundred psi. The oil pump pressure just supplies oil to the jounal bearings, the pressure that makes the bearings work is generated by the bearings turning relative to the shaft.

If pressure is too low, not enough oil is being pumped into the unloaded side of the journal bearings and there is a risk that there won't be enough oil to form the hydrodynamic wedge on the loaded side of the bearings and the shaft will push through the oil film on the loaded side of the bering and do ugly things.

Pressure that is too high wastes power and can blow out seals, split filter cans etc. It won't hurt how the jounal bearings work.

 

[MaineShark]

Pressure that is too high wastes power and can blow out seals, split filter cans etc. It won't hurt how the jounal bearings work.

Excess oil pressure can and does wash out bearings. Increased bearing wear is a common thing on engines built by people who think that more pressure is automatically better. You need the correct amount of pressure at the bearings, not too much or too little.

Joe

 

[Stallion]

Okay, I understand! Thanks, guy!

 

[JohnZ]

An oil pump pumps a specific volume of oil at any given rpm, as it's a positive-displacement pump; pressure in the system it's feeding is a function of flow restriction in that system, which is created primarily by bearing clearances (both journal clearances and rod side clearances). As bearing clearances open up with wear, reducing the relative flow restriction, oil pressure goes down, as the volume being pumped at any given rpm is constant. For a street engine, 10 psi per 1000 rpm is adequate.

 

[Stallion]

Okay, I see, John. If you are running low on pressure, then what is the cause of that usually? Just the pump?

 

[JohnZ]

Oil pumps hardly ever fail, as they're very simple, and are the best-lubricated component in the whole engine. If oil pressure drops to zero immediately, it's usually related to the pump drive shaft from the distributor, or the pump pickup screen has fallen off. If oil pressure very gradually drops over a long period of time, it's usually simple engine (bearing) wear. If it fluctuates, it's most likely low oil level, exposing the pump pickup to air during cornering or braking. With regular oil changes and watching oil level on the dipstick, chances are you'll never have an oil-related problem.

 

[Stallion]

John, what does hte distributor's shaft have to do with oil pressure? How does the oil go over/through it? That would cause a drop in pressure? :confused

 

[JohnZ]

The upper (ignition) part of the distributor is driven by mating gears on the camshaft and the bottom of the distributor shaft - the cam gear drives the distributor gear. The bottom end of the distributor shaft is formed into a flat, like the end of a flat screwdriver, which engages with a slot in the top end of the oil pump drive shaft, which extends straight down and drives the oil pump. So, the cam drives the distributor, and the bottom of the distributor drives the oil pump driveshaft.

Here's a picture of the drive gear at the bottom of the distributor shaft; the gear is hollow, and the flat end of the distributor shaft is about 1/4" up inside the bottom of the gear. When the distributor is fully seated, the drive shaft from the oil pump (with a slot in it) goes up inside the end of the gear and fits over the flat on the end of the distributor shaft. That way both shafts are locked together when the distributor is installed, and the distributor shaft then drives the oil pump as it's turned by the camshaft.

 

[Stallion]

I understand now!

Thanks, John!

 

[UB2 SLOW]

I had a problem with my oil pressure showing low. Turned out to be the oil pressure sending unit.

 

[Stallion]

Oil pressure sending unit? Is that the pump?

 

[MaineShark]

Oil pressure sending unit? Is that the pump?

He's referring to the sensor that reads the pressure, to display in on the dash guage. Sensors in cars are generally called "senders."

Joe

 

[Stallion]

Oh I see, thanks!

 

[UB2 SLOW]

He's referring to the sensor that reads the pressure, to display in on the dash guage. Sensors in cars are generally called "senders."

Joe

Yea what he said. eerrr what I said. eerrrr you get the point.