Another Oil Pressure data point

#41

True. You cannot focus on only one particular factor and ignore the rest.

At the end of the day, you must have an oil film between the rotating metal parts. Some of the factors that come into that equation include…

Viscosity
Clearance
Surface finish of the metal bits
Cleanliness
Temperature
Pressure
Length/diameter ratio
RPM
Load
Etc.

Obviously many of those things are not in our control unless we are the engine designer with a clean sheet of paper in front of us. Some things we can control though.

“Adequate” pressure is required to make sure oil is getting fed to the bearings faster than it can be flung out through the clearances. How much is “adequate”? There is no one answer. One can either do the hydrodynamic film analysis and CFD work for your particular design or follow the tried-and-true rule of thumb. One is easier to do than the other.

The reason the required pressure goes up with rpm is that the centrifigal forces in the spinning oil galleries of the crank can sometimes work against you and pressure needs to rise to compensate. Also, the reversing loads of say, the con rod bearing, tend to pump oil out of the bearing faster, requiring higher flow rates.

#42

[quote]drumbeater wrote:

What are the factors? I’m just curious, assuming filter’s clean and engine seals/rings/etc are tight.

Viscosity
Covered oil intake/external forces
[/quote]

Our oil pump is designed to provide constant flow, but it’s really only capable of that within relatively narrow operating parameters…that is to say flow restriction. So if the flow restriction is a lot, the pump’s attempt to maintain flow results in OP going up, and if the flow restriction is low, the pump is unable to maintain enough flow and OP goes down.

–The OP’s check valve limits peak pressure and therefore impacts average OP. I don’t recall what it’s set at, but it’s around 65psi. A weak or unusually strong checkvalve spring will change that, and thereby change average pressure values.

–Oil viscosity obviously impacts average pressure. Thick oil increases flow restriction and therefore OP goes up. Note that two different types 10W40 oils will have different viscosity. And last year’s 10W40 might not be the same as this years.

This is one of those times that it’s important to remember that bearing lubrication is about flow, not pressure. More correctly, it’s about the maintenance of the “hydrostatic wedge” of oil inside of the bearing. OP going up because flow restriction reduces flow is not necessarily a good thing.

–Oil temp impacts average pressure. My engine is running cooler now due to better air flow to the radiator, and a bigger oil cooler. Colder oil is more viscious, and therefore increases OP.

–Bearing condition impacts average pressure. The oil pump is trying to pump oil thru the holes (journals?) in the crank to the main bearings and rod bearings. Worn bearings have larger gaps and therefore oil can flow thru the bearing more easily. Oil flowing more easily means the pump might not be able to maintain higher pressures. That’s why 25psi at redline might be an indication of worn bearings.

This is why I suggested the 524TDI pump, an idea that everyone hated. The TDI pump is no more expensive and apparently can maintain normal pressures in the face of 30% less flow restriction.

It is a mistake to see low OP as a cause of problems. More often then not low OP is the result of problems. The exception is when the pickup goes dry which interrupts oil flow. We see the interrupted oil flow as a loss of pressure, which can cause the loss of a little bit of bearing material.

It’s also worth noting that bearings are designed to be sacrificial. That is to say that they are designed to lose material. But at some point they’ve lost enough material that there’s so little flow restriction that our limp dick pump can’t handle the flow needed to maintain pressure. Then your oil analysis comes back with copper in your oil, and your OP are red line is 25psi. TDI pump anyone?

#43

Actually, since oil pumps are driven by the crankshaft, their flow rate is proportional to crank speed (rpm).

If nothing else were done, you might see 100 psi at high revs with thick oil. Your oil filter cannister would blow up.

To avoid this, a pressure relief valve is added. Ranger, if you look at your data, you can clearly see that your pressure is being controlled to 55 psi by that valve. The valve works by dumping off some of the pumps flow to maintain that pre-determined pressure.

When something is going wrong (like trying to pump aerated oil), the valve cannot keep the pressure up because there is just not enough flow.

#44

Ranger wrote:

[quote]drumbeater wrote:

What are the factors? I’m just curious, assuming filter’s clean and engine seals/rings/etc are tight.

Viscosity
Covered oil intake/external forces
[/quote] This is why I suggested the 524TDI pump, an idea that everyone hated. The TDI pump is no more expensive and apparently can maintain normal pressures in the face of 30% less flow restriction.[/quote]

Is that a diesel-engine oil pump? Diesels often cool their pistons with oil jets. This raises the flow requirement, neccessitating a higher-volume oil pump.

I, too, think that’s not a solution because it will still lose pressure when trying to pump aerated oil and you will be consuming extra horsepower to create extra flow that only gets dumped by the releif valve 99% of the time.

#45

Dyno wrote:

[quote]Actually, since oil pumps are driven by the crankshaft, their flow rate is proportional to crank speed (rpm).

If nothing else were done, you might see 100 psi at high revs with thick oil. Your oil filter cannister would blow up.

To avoid this, a pressure relief valve is added. Ranger, if you look at your data, you can clearly see that your pressure is being controlled to 55 psi by that valve. The valve works by dumping off some of the pumps flow to maintain that pre-determined pressure.

When something is going wrong (like trying to pump aerated oil), the valve cannot keep the pressure up because there is just not enough flow.[/quote]

There are constant pressure pumps and constant flow rate pumps. Ours is constant flow rate. It goes without saying that flow rate is prop to crank speed.

Re. 100psi and blowing the oil filter. No one is suggesting removing the check valve.

Re. 55psi. You are misinterpreting the data. The checkvalve operates higher then that. 55psi is what I get at high rpms with that oil at that temp. If you want to see your checkvalve pressure, look at your OP when you first crank a cold motor.

Re. valve can’t keep pressure up because not enough flow. It’s the pump that can’t keep pressure up.

#46

Well, I agree with some of that, disagree with some also.

#47

Well I can say that you guys have absolutely convinced me…

That I will drive my 246k motor until it blows! And not worry about any of this oil stuff, cause you are a bunch of bright, intelligent, engineers and you can’t agree on what to do, so I am going to go out and play around with tire pressure, and just put in alot of laps and work on the nut behind the wheel.

BTW After 4 race weekends and 4 additional track days, my motor has used 0 oil.

I have owned alot of e36 m3s and they all used 1 quart every 1000 miles. This old E30 with 246k on the clock uses none.

The oil in my brain is the fluid I am going to worry about.

Have fun.

#48

If it ain’t broke, don’t fix it!

#49

I just want to know if any of these modified oil pans are for sale yet. Is Paul Poore selling his?

#50

King Tut wrote:

I think he’s selling them. Call him @ 267-374-8365. The guy really knows a lot so if you have other E30 questions, seize the moment.

#51

Are they as nice as his exhaust system? :laugh:

#52

Yes. I spoke to Paul and he is selling them.