OIL PRESSURE RELIEF VS. OIL PRESSURE BYPASS VALVES
12-20-2015, 03:57 AM
#1
Nordschleife Master
Thread Starter
OIL PRESSURE RELIEF VS. OIL PRESSURE BYPASS VALVES
Guys,
Up late as I'm switching from a string of day shifts to covering a night shift tonight, and resetting my internal clock.
I understand the function of the relief valve, but I can't find anything on the function of the bypass valve.
What is its function, and relativity to/and with the relief valve ?
Just an academic inquiry.
Thanks in Advance.
Craig
Up late as I'm switching from a string of day shifts to covering a night shift tonight, and resetting my internal clock.
I understand the function of the relief valve, but I can't find anything on the function of the bypass valve.
What is its function, and relativity to/and with the relief valve ?
Just an academic inquiry.
Thanks in Advance.
Craig
12-20-2015, 04:23 AM
#2
Nordschleife Master
Thread Starter
The flow of oil in the 928 - the oil from the pump is presented with three paths. The first path is a pressure relief that opens at 8 BAR - which is why 928s peg the gauge when cold. The excess oil is routed back to the intake side of the pump. This one is a dead end and no longer is part of the discussion. The second path available is to a bypass valve which limits the amount of oil pressure flowing through the oil cooler so that it never exceeds the design limit of the cooler (This was deleted as of 1991 models) It opens at a pressure differential of.5 to 1 bar. The third path of oil flow for all but 1980-84 USA cars is through the heat exchanger in the radiator or the external air cooler standard as of 1990. There is a thermostat element mounted in the block just above the oil pressure sending unit (Porsche chose this location next to the ports for the oil cooler lines as the best place to monitor oil pressure). When the oil is cold both oil cooler line ports are exposed to full system pressure and being in a state of balance little oil would flow to the cooler but the oil lines and cooler are always completely FULL of pressurized oil. As engine oil temperature increases the thermostat expansion element closes the main flow path and the oil flows out the lower oil line port to the cooler returning to the upper return port. At this point the flow paths converge. From the thermostat / relief housing all the oil flows through the oil filter then into the oil galleys which are made as part of the block /lower cradle assembly at the point where they are bolted together. So what is the point of this description of oil flow ?? No matter where you add an outside source of pressurized oil the oil has the option to flow toward the bearings or to flow back toward the oil pump. The only time the reserve oil can flow is when there is low oil pressure in the system which would mean the pump was trying to pump air. It would make little difference where the oiling "extra oil pressure" is added to the system you must pressurize all the oil from the pump to the bearings including the oil cooler lines, oil filter, heat
exchanger. Unless you have added a one way check valve that all the oil must flow through all the time. (The oil filter mount is a reasonable alternative) And introduce the pressure above the one way valve the oil has the option to flow both ways. Additionally most pressure accumulators have a capacity of 2 or 3 quarts of oil. The accumulator is initially charged with 5-10 lbs. of air pressure on one side of a diaphragm and when the engine is started, oil under pressure displaces the diaphragm (be aware that there are some "accumulators" being sold that are empty cylinders and as such do not "store" any pressure). The discharge of the pressurized oil is a result of the compressed air ( liquids do not compress ). If the accumulator stores 3 quarts @ 5 bar as soon as 1 1/2 quarts have flowed back into the system the oil pressure remaining in the cylinder should be about 2.5 bar. As the last bit of oil flows it is at the 5-10 lbs. of the original air charge to the accumulator. In reality the pressure in the accumulator is what ever the engine oil pressure happens to be every time you get off the gas, brake, clutch and shift. For the accumulator to add oil and pressure there must be a lack of pressure in the oiling system. Those who have lost engines that I have talked with have NOT REPORTED any loss of pressure (Don Hansen as a recent example)( David Lloyd) no red warning, no central warning, no fluctuating oil gauge.... If no loss of
oil pressure the accumulator would not add oil pressure...... It would be a very interesting real world test to remotely mount the air pressure gauge from the accumulator and monitor the pressure or video tape its movements --- if oil is flowing back to the engine the fluctuating air pressure gauge will clearly indicate this..... if not , then it is another problem that kills 2 and 6 rod bearings. Which is the position that we have maintained for many years based upon Mark Anderson's experience using a dry sump oiling system and still having bearing failures of # 2 and 6...until he had the cranks redrilled.
Jim
928 Int'l
Found this on Greg Nichols TIPS site in the Track section. I haven't been to TIPS in years. That site was all I knew for getting 928 info before discovering the Forum. I used that site for 5 years while working on mine before finding this place.
This posting was by the venerable Jim Bailey. Answers my inquiry, and may answer the frequent question of some why when cold, the oil pressure pegs to 5.
exchanger. Unless you have added a one way check valve that all the oil must flow through all the time. (The oil filter mount is a reasonable alternative) And introduce the pressure above the one way valve the oil has the option to flow both ways. Additionally most pressure accumulators have a capacity of 2 or 3 quarts of oil. The accumulator is initially charged with 5-10 lbs. of air pressure on one side of a diaphragm and when the engine is started, oil under pressure displaces the diaphragm (be aware that there are some "accumulators" being sold that are empty cylinders and as such do not "store" any pressure). The discharge of the pressurized oil is a result of the compressed air ( liquids do not compress ). If the accumulator stores 3 quarts @ 5 bar as soon as 1 1/2 quarts have flowed back into the system the oil pressure remaining in the cylinder should be about 2.5 bar. As the last bit of oil flows it is at the 5-10 lbs. of the original air charge to the accumulator. In reality the pressure in the accumulator is what ever the engine oil pressure happens to be every time you get off the gas, brake, clutch and shift. For the accumulator to add oil and pressure there must be a lack of pressure in the oiling system. Those who have lost engines that I have talked with have NOT REPORTED any loss of pressure (Don Hansen as a recent example)( David Lloyd) no red warning, no central warning, no fluctuating oil gauge.... If no loss of
oil pressure the accumulator would not add oil pressure...... It would be a very interesting real world test to remotely mount the air pressure gauge from the accumulator and monitor the pressure or video tape its movements --- if oil is flowing back to the engine the fluctuating air pressure gauge will clearly indicate this..... if not , then it is another problem that kills 2 and 6 rod bearings. Which is the position that we have maintained for many years based upon Mark Anderson's experience using a dry sump oiling system and still having bearing failures of # 2 and 6...until he had the cranks redrilled.
Jim
928 Int'l
Found this on Greg Nichols TIPS site in the Track section. I haven't been to TIPS in years. That site was all I knew for getting 928 info before discovering the Forum. I used that site for 5 years while working on mine before finding this place.
This posting was by the venerable Jim Bailey. Answers my inquiry, and may answer the frequent question of some why when cold, the oil pressure pegs to 5.