Detailed Oil Pressure vs. Volume Question

[PeterC]

[FONT=&quot]Apologies if someone has covered this basic question before .. reference the flathead oil pressure threaded orifice in the back of block on the drivers side (59AB block) where the sender unit is sensing oil pressure and sending to the gauge (in my case a mechanical unit).

My understanding is that the oil pump does not make pressure - it makes volume flow - like an open garden hose flowing water - when you put your thumb over the end of the open hose - it creates pressure ... so the pump is moving volume - and the bearing clearances at the cam, rods and mains are creating pressure - like you thumb on the hose end.... so assuming I have the above right … the question is.... does the sender unit read pressure at the beginning of the oil's journey before it hits all the bearing clearances - or is it reading pressure after the ride through the bearings – said another way – how do I know what the pressure reading is in the #1 rod bearing – from the gauge?[/FONT]

 

[cmbrucew]

PeterC
The pressure will be the same anywhere in the system.
Bruce

 

[Binx]

Think of it as the pressure gauge on a well pump. It is reading the back pressure due to resistance of the system downstream.

Lonnie

 

[Ol' Ron]

Years ago I installed a Gauge in the front galley oil line. I drilled a small hole into the line above the timing cover. Then a 1/8NPT fitting and walla, pressure. You are right in your thinking. A gear pump is a positive displacement unit that produces the same volume of oil per revolution. The faster it spins, the more oil you get. Not theoretically, the pressure should be the same in the entire system. Unfortunately this wasn't the case. At Idle it was close, 20lbs at the rear and about 18 at the front. At 2000 rpm 45 at the rear and 37 at the front. Allot less than I thought, used several gauges. This was with hot oil. That was along time ago and someone out ther has that engine.

 

[JSeery]

The pressure tap at the rear of the engine is only telling you the pressure at that point. It should be somewhat constant throughout the system, but there can always be local variation. 50 psi at the rear tap does not guarantee pressure elsewhere because it is possible to have blockage within the oiling system that lowers the pressure in one area as compared to another area. All the oil pressure gauge is telling you is that there is pressure coming out of the pump and that it is being regulated to a value as it starts it's journey through the system. It should show you approximate average pressure within the system however.

 

[Old Henry]

[FONT=&quot].... does the sender unit read pressure at the beginning of the oil's journey before it hits all the bearing clearances - or is it reading pressure after the ride through the bearings [/FONT]

The gauge reads the pressure in the pressurized part of the lubrication system, i.e. before the bearings, etc. After the oil passes through all restrictions and heads back to the oil pan there is no pressure. It's just draining down.

 

[ford38v8]

As Ol' Ron proved, pressure is relative to flow, the flow itself releasing pressure along the route to the front pressure gauge in Ron's engine. Had there been no release points, the pressure would be exactly equal. An engine with sloppy bearings would have plenty of flow, but less restriction to build up the pressure.

Sorry if I sound like an expert which I'm not, it's just the only way I can explain the way I think I understand it. ??? Wat?

 

[36coupe]

Yes I agree

I think Ron's experiment shows that with an increase of pressure and volume there is an increase of friction which is seen in a pressure drop the farther you go from the source.
And I'm no expert either.

As Ol' Ron proved, pressure is relative to flow, the flow itself releasing pressure along the route to the front pressure gauge in Ron's engine. Had there been no release points, the pressure would be exactly equal. An engine with sloppy bearings would have plenty of flow, but less restriction to build up the pressure.

Sorry if I sound like an expert which I'm not, it's just the only way I can explain the way I think I understand it. ??? Wat?

 

[Ol' Ron]

I like the definition of "EXPERT". X= x husband, ball player, etc

Spert is just a drip under pressure.

SO, if your an expert, j your just a drop. Me too.

 

[Ronnie]

http://www.youtube.com/watch?v=7IpJlYssvkM

R

 

[G.M.]

Most low oil pressure problems are from a worn or weak spring in the forward relief valve or the adjustable valve on later pumps. First of all I DON"T trust any pump unless I test if, running it with a 1/2" electric drill and a test sleeve with a gauge and output hose over the open pump body. Now I know the source of volume and pressure work, the bypass valve on the later pumps is also set at the max pressure you want, I set them at 55 lbs. A pump may have warn gears or excessive clearance between the end of the gears and end plate, this will show as pumping a good flow until a restriction is put on the pump output. As soon as a slight restriction is applied the pump pressure drops to almost zero. On one engine as it heated the oil pressure dropped, I even dropped the pan and forced oil in though the gauge port looking for a massive leak from rods, cam bushing or other worn parts but just got small normal leaks from these areas. That's when I pulled the oil pump and built a tester and discovered in a few minutes with a restriction on the pump output the oil pressure slowly dropped to 10 lbs and even down to 5 to 8 lbs. Cleaned the bypass valve and installed a spacer on the spring and the oil pressure went up over 70 lbs, adjusted the thickness of the spacer to get 55 lbs with a restricted output of oil. Car was driven on number of long tours and never had low oil pressure again. This adjustment applies to the front valley valve also. To adjust the front valve remove the spark plugs, drop a little oil or MMO in all 8 cylinders, put a rag over the plug holes and with a mechanical oil pressure gauge installed spin the engine with 12, YES 12 VOLTS. The oil pressure you see will be the pressure the engine will have at driving speeds. The front valve is adjusted the same as the pump valve. If the engine has both valves the front one should be shimmed a little more so the pump valve controls the pressure. The front valve has a little flat spot on the valve so it always lets a little oil drop on the timing gears. This test can be done in the car or with the engine in a stand or sitting on the floor. G.M.

 

[PeterC]

GM - you said "front valve is adjusted the same as the pump valve" -where is the "front" valve on a 59a block?

 

[BillM]

Inside the valley, at the front end of the oil tube passage: Bolt 6666, spring 6654, and valve 6663.

 

[Ol' Ron]

I wonder if "George" the Mellings Tech has fixed the junk they sold me when I was building engines. That was 20 years ago and All I got from them was, "Return it to yor distributor and we;ll replace it". Well I had to replace three of them as I remember and others did as well. Since then, I've used the original pumps and with out the rebuild kits the Mellings sells, they're just part of the junk. IMHO

 

[G.M.]

I wonder if "George" the Mellings Tech has fixed the junk they sold me when I was building engines. That was 20 years ago and All I got from them was, "Return it to yor distributor and we;ll replace it". Well I had to replace three of them as I remember and others did as well. Since then, I've used the original pumps and with out the rebuild kits the Mellings sells, they're just part of the junk. IMHO

Ron, that's why I have been "preaching" for a number years to test ALL oil pumps, new or used before installing in the engine. When Skip rebuilt the "Polish tank" engine (37 engine) for his 34 P/U he grabbed a used old style pump, put the pick up in a pan of oil and turned it by hand. After 2 turns of the drive gear it put out a good flow of oil, he said that looks like a good one. This was shortly after I made the pump test sleeve. I said let's test it anyway. We ran it with a half inch drill and as soon as we put a slight restriction on the output hose it dropped to zero pressure. We took it apart and it had a large clearance between the bottom cover plate and the bottom of the gears. We corrected the problem and the pump put out 90 lbs, the engine oil pressure was adjusted to 55 lbs with the front valley valve. G.M.