Technical Somebody 'Splain Me Oil Pumps and Oil Pressure

Blues4U, my very first engine, a 430 Mel, blew the filter off, we’ll actually split it op the side. It was due to a sticking relief valve. Put a 100 lbs gauge on it and it pegged it, so I don’t know how much pressure was there. It’s been fifty years, can’t remember was I did to solve it? I think it finally broke loose, as I don’t remember having to pull the pan.
But a car oil pump will split a filter... also swelled it up!




Bones

 

Have you ever primed an engine with a speed handle that had a stuck relief valve I have warm summer day 20w oil started turning the pump and it got hard to turn looked at the oil pressure gauge and it was pegged at 100 lbs . I was likely turning the pump 50 rpm. If the engine was running at 1000 rpm the pump at 500 Rpm 10 times what I was doing where does that oil go? When you speed up an engine and the oil pressure ceases to increase it has to be the relief valve. As for shear wouldn't bearing clearences cause more shear than the relief.

 

I have always considered the 10psi per 1000rpm rule a good rule of thumb, although I like to see 20-25 psi minimum at idle. Other than verifying oil pressure at start-up, I don’t pay much attention to the gauge unless something seems abnormal.

Blues4U I believe the relief flow is commonly directed back to the inlet side of the pump.

 

If the oil pump on an engine is in good condition and the relief valve sticks closed,,,it will bust the filter case,,,or even blow the whole filter off the mount.
Most people don’t realize how much pressure that a stock pump can produce.

And yes,,,,the relief valve operates continuously.
Not so much at low rpm,,,,but as the rpm increases,,,so does the volume produced by the pump. Yes ,,it is a positive displacement pump,,,,so the higher the rpm,,,the more oil that is pumped per minute.
The rod bearings swinging around leak a certain volume per revolution,,,as the rpm increases,,they still leak the same amount.
However,,,the pump is producing much more than it can leak off.
So,,,the valve will increase the leak for it.
The old service manuals even mention that if the valve piston is blue or galled to replace it upon inspection,,so ,,to gall,,it would have to be moving quite a bit?

A higher pressure relief spring will create more pressure,,,,check it out and see!
I have an adjustable relief assembly on my 440.
External oil pump,,,with an adjuster on top,,,easy to see the pressure difference.
Also,,,it changes the pressure at idle,,and upper rpm,,,I have about 40 at hot idle,,1000 rpm,,,,about 80 at top rpm,,,7600.

I don’t have all the answers,,,I can only relate from experience.

Tommy

 

As you note, positive displacement means what it says. If the pump is turning it puts out the exact rate of fluid it is designed to do. Shut the outlet and either the pressure builds until something breaks (blown filter, sheared drive shaft, other?) or the drive doesn't have enough power and stalls. Gotta have a pressure relief on a PD pump. Conversely centrifugal pumps (water pump for example) vary in volume with pressure and can quit producing if outlet pressure is over its capability but keeps on spinning.

 

It looks to me then that all things being equal, it's all about oil flow, not really the pressure per se. The pressure is the only thing we can easily measure. The bearing clearances determine what viscosity is optimal.

A worn high mileage engine with worn mains will benefit from a bit higher viscosity but at some point the pump cannot overcome excessive "leakage".

So a heavier oil like 20w-50 isn't necessarily a good thing in old school engines, particularly in cold temperatures, nor in high temperatures assuming the engine has been rebuilt and has good tighter clearances in the bearings as street driven motors do.

On the other hand the oil viscosity selected should also be decided with an eye on the highest temperatures to be seen and the running temperature. That's why everybody has a hard on for synthetics I suppose. They flow well when cold and provide superior high temperature protection.

It sounds to me though that generally the goal would be to run the lightest viscosity oil that still maintains acceptable pressure when hot, as it will always flow better. Carrying away heat is another critical task that engine oil performs and this is based on good flow.

This particular engine runs 60 psi hot, at 2000 RPM, spec is 35-55, with 10w-40. Allowing for some leeway due to any gauge innacuracy, a 10w-30 or even maybe lighter like 5w-30 should be better? It had new main and rod bearings installed at overhaul though don't know the particulars on clearances. New rocker shafts and rockers etc. Based on the pressure, a lighter weight oil is OK. This goes against accepted wisdom maybe.

 

This will help understand multi weight oil. https://ezinearticles.com/?Understanding-Multi-Viscosity-Motor-Oils&id=6065625