Motorcraft FL1 HP/Fram HP1

[Bob In Ct]

Hey Dan:

It is possible for the oil pressure to increase as the oil heats up. It is also possible to get higher pressure by switching from 20W-50 to 5W-30! Here is the explanation.

Most oil pumps have pressure relief valves, which do not allow the oil pressure to exceed a preset pressure. For the sake of this example let’s say it’s 60 PSI. The 60 PSI is at the pump. Most engines have their pressure sensor located downstream from the pump at the engine filter housing. Oil flowing from the pump has to go through a remote filter, an oil cooler and lots of plumbing before it gets to the pressure sensor. The pressure is reduced by the filter, the cooler and by every inch of pipe along the way. The heavier the oil the greater the loss of pressure. The 60 PSI pressure at the pump may become 35 at the sensor.

Three things affect the amount of pressure loss: the length of the plumbing, the size of the pipes (or tubing and cooler and filter) and the viscosity of the fluid. Viscous fluids, narrow pipes and long distances of flow all lower the pressure. If you don’t believe me just go outside and hookup 10 lengths of garden hose. Turn the faucet on to full and watch how slowly the water comes out the sprinkler. Imagine if the hose was full of cold 20W-50 instead of that nice thin water, it would hardly flow at all. I won’t get into Reynolds number and laminar versus turbulent flow but believe me it matters (ask any Chemical Engineer you know).

Adding a high volume pump will not help. It will just send more oil through the relief valve. Going to a high pressure may increase the pressure a little but at the expense of gear life.

Remember, the pressure is normally measured just before the oil heads back into the engine, not at the pump The best way to get pressure (and volume) where you need it (at the bearings, the lifters, the cam etc.) is to allow the oil to more easily flow through all the obstructions. Use the largest tubing you can, use the least restrictive cooler you can, use the least amount of tubing you can, use the least restrictive filter (dual filters if possible) you can, and use the thinnest oil you can.

I learned all of this first hand when I replaced the 20W-50 Castrol my builder had installed with 5W-30 synthetic, the pressure went up 20 PSI. Warmer oil would probably produce the same result.

Bob