Some Thoughts on Oil Pressure, Oil Pumps and Oil Viscosity

[Bob In Ct]

A few thoughts on oil pump, oil pressure, oil viscosity and oil filters.

Oil pumps are not rated for pressure, they are rated for flow. If you have a certain model pump it will flow a certain amount of oil at a given RPM. The flow will be X gallons of oil per minute at that RPM regardless of the oil viscosity. Since the flow is predetermined by the pump manufacturer, the pressure is determined by the tightness of the engine and the viscosity of the oil at any given RPM. The tighter the engine the greater the pressure. The thicker the oil the greater the pressure.

Most oil pumps are equipped with a pressure relief valve. This is a device that will only allow the pressure to get so high. Once that pressure is achieved, the valve opens and a percentage of the pumped oil is allowed to return to the crankcase. The remaining oil is pumped through the engine. This is why many of us reach a certain oil pressure, and no higher, regardless of the RPM or oil viscosity. That being said consider the following two identical cars –

Car “A” is using 5W-30 oil. At 5,000 RPM the oil pump is pushing X gallons of oil per hour. The oil pressure gauge shows 60 PSI. Car “B” is using 20W-50 oil. At 5,000 RPM the oil pump is pushing the same X gallons of oil per hour. The oil pressure gauge shows the same 60 PSI. In both cases the pressure relief valve is limiting the pressure. Because the 20W-50 oil does not flow as easily, more of it is being allow to return to the crankcase without flowing past the bearings and past the cam and past the rockerarms. Although both engines show 60 PSI the engine with the lower viscosity oil is flowing more oil through the engine (assuming the pressure relief valve is open).

More food for thought. Joe’s oil pressure sensor is between his oil pump and his oil filter. Bill’s oil pressure sensor is after the oil filter. Both Joe and Bill decide to try the new WIX oil filter. After installing them Joe has lower pressure while Bill has higher pressure. How can this be? The answer is simple. Joe’s old filter was restrictive and produced high pressure at his sensor since it was located downstream of the sensor. Bill’s filter, is located in front of the sensor and is now producing higher readings since the oil is better able to flow through it. If Joe were to switch to heavier oil he would show higher pressure. If Bill were to switch to heavier oil he would see a reduction in pressure.

Conclusions -

Do not attempt to compare your oil pressure to your buddy’s since he (or she) probably has a different oil pump, a different filter, and higher/lower viscosity oil.

Assuming the same pressure and RPM, thinner oil will flow more volume through the engine than will thicker oil.

Higher pressure is not always better. Too much pressure is tough on more than just oil filters.


Bob