The MGA With An Attitude

This lesson discusses the relationship between oil pressure, flow, and lubrication of the moving engine parts, and also the advantage (or lack thereof) of a high volume oil pump and higher oil pressure.

Now a little more oiling system information.

The stock oil pump in good condition can pump up to 54 quarts per minute at 7000 rpm engine speed (I learned that the hard way). At relief pressure (50-80 psi at road speed), about 12 to 20 quarts of oil per minute (depending on engine condition) goes through the engine, and the excess gets dumped over the pressure relief valve directly back into the sump. An oil pump has to be in excessively bad condition not to be able to pump at least 20 quarts per minute at 3500 rpm, so an engine in decent condition will always be dumping some oil over the pressure relief valve at road speed. The pump is a positive displacement type, meaning that it always moves a certain volume of oil regardless of the pressure (except for a little internal leakage).

Oil pressure is a result of two things, oil flow volume from the pump, and resistance to flow throughout the engine. Smaller oil flow passages provide more resistance to flow, and therefore higher system pressure. Larger passages provide less resistance, resulting in lower system pressure. This is why worn out bearings result in low oil pressure. Maximum system pressure is limited by the pressure relief valve. Minimum system pressure is determined by how slow the engine idles and the condition of the bearings (all of the bearings). Larger clearances result in more (and easier) oil leakage from the bearings. Loose main bearings will dump too much oil, reducing oil flow to the rod and cam bearings. A loose rear cam bearing will further reduce oil flow going to the rocker shaft. With really bad bearings the oil pressure will be quite low and the rocker shaft will get very little oil.

Incidentally, increasing oil viscosity increases oil pressure at low speed, but it also DECREASES flow through the bearings at ALL speeds. It is important to have a reasonably good match between oil viscosity and the size of the internal bearing clearances. Smaller bearing clearance wants thinner (less viscous) oil, while larger bearing clearance wants thicker oil. And if you run 20W50 oil in your MG at cool temperature (below 40dF) you may notice significantly lower oil pressure on startup until the oil warms up some.

A certain amount of power is required to drive the oil pump, and power is energy. None of the oil flow does any useful mechanical work, so all of that energy turns into heat when the oil pressure returns to zero as the oil escapes back into the sump. When oil is passing over the relief valve, all of that oil flow represents heat being generated directly in the oil. The greater the pressure drop, the more heat. The greater the flow, the more heat. If you increase spring force on the relief valve to get higher pressure, you get more heat, but only a little more flow through the engine. If you install a higher volume oil pump, you get more flow over the relief valve, and more heat, and no more flow through the engine (except at low speed).

The only time a high volume oil pump increases flow through the engine is at low speeds and low flow rates when there is no oil going past the relief valve. This can increase pressure at idle but will not affect the pressure at all at higher engine speeds, and is generally not required with an engine in good condition. Higher flow and higher pressure in the system takes more power to run the pump, heats up the oil, and produces more wear on the drive gears.

Heat is also generated in the oil by friction of the moving parts. Oil flow through the bearings is important to carry away the heat generated by friction in the bearings. High volume oil pumps are generally applicable to racing engines where increased bearing clearances are used to increase oil flow in the bearings for improved cooling of the bearings. When oil flow is too little to carry away the heat, the oil will overheat and the oil film will break down allowing metal to metal contact and catastrophic failure of the engine. This is why an engine will give up the ghost very soon after an oil pressure failure or if it runs out of oil.

A large increase in pressure will NOT give you a large increase in flow. Pressure increases exponentially with flow. This means that a large increase in pressure will only get you a little increase in flow through the engine.

One other point. The pumping pressure in the oiling system does not support the load on the bearings, it just drives the flow of the oil. The load on the bearings is carried on the thin film of oil trapped between the bearing surfaces. The momentary working pressure on the connecting rod bearings (just one example) far exceeds the oil pumping pressure. No pumping pressure is required to support the load, only to provide the flow. I hear that certain Rolls Royce engines run nicely on 4 or 5 psi system pressure. Also, many early auto engines had no oil pump at all, and therefore no pumping pressure, where oil is supplied only by splash, spray and gravity. I'm not saying that these engines had no oil pressure, just no pumping pressure; there is always pressure on the oil film from the working load. The oil pump is used to maintain oil flow through the bearings, while the pressure you see on the gauge has very little to do with the load bearing capacity of the bearing.

Barney Gaylord
1958 MGA with an attitude

© 1997, 2009 Barney Gaylord -- Copyright and reprint information