Really good video from Melling about their LS oil pumps. Explanations, flow curves, pressure curves. Just skip to 4:25.



Selecting bypass spring



How to swap out bypass spring.



Some more explanations about concepts of flow vs. pressure

 

The videos talk about how the oil pump produces flow and not pressure but didn't explain what's really going on. I'm going to take a shot at explaining. Not my area of knowledge to be frank, and I know I have some knowledge gaps so if you know more then speak up!

Pump Makes Flow

Oil is an incompressible fluid, you can't squeeze it down, all you can do is move it around from one place to another. The pump takes a gulp of oil from the inlet port and moves that oil to the outlet port. That's it. End of story. It moves fluid to produce oil flow in the engine. And it will keep doing that as long as the crankshaft rotates the pump. Therefore, the pump performance is described by a flow curve that relates Flow Rate (gallons per minute) to Rotating Speed (RPM of crankshaft). The pump will deliver that flow rate regardless of the pressure head against it, as long as the crankshaft can rotate the pump. In other words, the pump doesn't give a hooey what is the pressure in the engine... It just makes oil flow.

Engine Resistance to Flow Makes Pressure

So what makes the engine oil pressure? It is a result of the engine's "system flow curve". The system flow curve defines what supply pressure is needed to flow a certain amount of oil through the engine. It is a characteristic of all the parts chosen (bearing gaps, lifters, rockers, pushrods, oil squirters, etc.). If the oil pump shoves more oil through the system, then the system pressure goes up so that the amount of oil leaving the system is same. Remember, oil does not compress -- What goes in, must come out. If the pump shoves less oil through the system, then the system pressure goes down so that the amount of oil leaving the system is same. The oil pump is sized to meet the flow requirements of the engine to keep all the lubricated parts happy; and the pressure is an indicator whether the pump is sized right. The same oil pump on two different engines will produce different pressure depending how the engine was built.

Pump Pressure Regulator

Now the pump itself has a pressure regulator. We know the way to change system pressure is to change the pump flow rate, and that's what the regulator does. It bypasses oil away from engine to lower the system flow rate, thereby regulating engine system pressure. The system pressure acts on the piston creating a force that compresses the spring. The plunger moves down the bore and exposes an oil galley that returns oil back to the pump inlet port (bypassing the engine). The pump flow curve is a straight line until the regulator begins to crack open and then the pump flow curve begins to lay over. The further the regulator opens the more flow is bypassed from the engine, and the more the flow curve lays over. The spring is selected so there is at least 10 psi per 1K rpm at max engine speed with engine at full temp. That's not how the pressure curve will look, it's just a spec at max engine speed. If engine goes to 6K rpm then try to hit 60+ psi. If engine goes to 7500 rpm then try to hit 75+ psi.

 

Okay, so what I want to know is why is there a pressure regulator at all? Why does it matter? Is there a rule of thumb for maximum allowable pressure?

• Filter burst pressure?
• Behavior of lifter hydraulics?
• They said the regulator increases pressure at the pump inlet port delaying onset of cavitation and increasing operating range. So if you choose a spring that is too stiff then you won't get enough bypass flow to do that. Porting of the inlet should help that, right?
• What else?

 

You don't want to have too much oil pressure, for a couple of the reasons you've mentioned: filter burst pressure, and lifter hydraulics being two of the reasons.
Other reasons include parasitic losses from turning the pump at excessive pressures, and crank/ rod bearing wear.

I've never seen this first hand, but I've heard from many knowledgeable people (machinists, old-timer mechanics, tech school instructors) that excessive oil pressure (100+ psi, continuously) will eventually wash away bearing material.
I'd assume this mostly applies to lead babbit bearings, and not aluminum bearings as much, since lead is a softer material.

I can't comment on the bypass issue; as I understand it, as long as there are no restrictions on the intake side of the pump, there shouldn't be any cavitation issues.
Yes, porting the inlet helps with this.

 

Keeping this very simple...

-Do to temperature and tolerances, the Pump will produce too much pressure when the engine is started cold.
-The Oil will aerate more easily.
-Yes the Oil-Filter and other Engine Seals can blow-out.
-The more pressure that the Pump makes, the more HP lost.
-Hydraulic Lifters will have Pump-Up and collapsing issues.
-With Hydraulic Lifters, I stay below 80 PSI with the Engine under load (with Solid Lifters 90 or 100 can be okay, but not of any benefit).

The Oil Flow is really more important than the Oil-Pressure.
Some Oil-Pressure is absolutely needed... but the Pressure is only a result of restriction to the Flow of the Oil.

In certain types of Racing, the Crew-Chiefs will tell me to port the Cylinder-Block and joining Oil-Passages to maximize Oil-Flow.
Once a certain flow rate has been reached... I am often told to keep Oil-Pressure to 5 Psi for every 1,000 RPM (partially done to minimize power losses).

 

Unfortunately the Gerotor design of these Oil-Pumps do not perform well past a particular speed.
With the Pump turned directly off of the Crank, at Crank speed...
These pumps suffer past 6,000 RPM.

Throwing more Oil volume at the Pump only masks the issue (and acts as a band-aid to keep enough Oil pressure at 7,000 or 8,000 RPM).
The Engines that I build for 9,000/ 10,000/ 11,000 RPM, all go to an external Oil-Pump and Vacuum-Pump configuration, or a Dry-Sump external Oil-Pump configuration.
As I said before...
The Oil passages restrict the flow of oil.
Try and shove more oil through the same size passage, and pressure increases.

The same thing happens when pushing boost through cylinder heads that can not flow enough air for the size Turbo being used.
Porting the heads will increase air flow... this lowers the boost pressure... but HP increases.

 

The topic of Hydrostatic Bearings, as used in our Engines...
will be getting into more complexities than I care to discuss at this point
(Bearing materials, coatings, orifice size/quantity/ location, Oil composition/ pressures/ temperatures, mating-components, and much, much more).

 

Thanks for sharing, very useful info for me. I was just thinking about buying this oil pump because the old one was out of order due to poor quality oil, so first I recommend using a good oil from reliable manufacturers. I started using 3.6 Pentastar engine oil, this is Mobil 1 advanced full synthetic, this is a famous and high-quality brand of oils for everyone.