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Chapter 10, The graduate.

I am going to bring up the constant flow pump concept. First, it goes back to the principal that doubling the pressure of the same weight oil does not exactly double the flow but it is close. Also doubling the RPM for the same reason does not exactly double the flow but again it is close.

This shows the problem best:

(A) For a 30 wt oil at operating temperature:

RPM....Pressure..Flow

1,000......20 PSI....1

2,000......40 PSI....2

4,000......80 PSI....4

8,000... 160 PSI....8 The maximum flow because of the oil pop off valve at 90 PSI will be 5

(B) For a 30 wt oil at operating temperature

and a higher output oil pump:

RPM....Pressure..Flow

1,000......30 PSI....1.5

2,000......60 PSI....3

4,000....120 PSI....6 The maximum flow because of the oil pop off valve at 90 PSI will be 5

8,000... 240 PSI....12

If we stick with the same weight oil and increase the oil pump output we will increase the pressure and the oil flow too. If we double the oil pump output we will double the pressure and we will double the oil flow.

(C) For a 40 wt oil at operating temperature:

The oil is thicker, has more internal resistance and therefore requires more pressure to get the same flow. Compare this with (A):

RPM....Pressure..Flow

1,000......30 PSI....1

2,000......60 PSI....2

4,000....120 PSI....4 The maximum flow because of the oil pop off valve at 90 PSI will be 3

8,000....240 PSI....8

(D) For a 40 wt oil at operating temperature

and a higher output oil pump:

RPM....Pressure..Flow

1,000......45 PSI....1.5

2,000......90 PSI....3 The maximum flow because of the oil pop off valve at 90 PSI will be 3

4,000....180 PSI....6

8,000... 360 PSI....12

The situations (A) and (C) are close to real life, assuming no loss in the system. This is what happens when you change the 30 weight oil to a 40 weight oil in your car:

(A) For a 30 wt oil at operating temperature:

RPM....Pressure..Flow

1,000......20 PSI....1

2,000......40 PSI....2

4,000......80 PSI....4

8,000... 160 PSI....8 The maximum flow because of the oil pop off valve at 90 PSI will be 5

(C) For a 40 wt oil at operating temperature:

The oil is thicker, has more internal resistance and therefore requires more pressure to get the same flow.

RPM....Pressure..Flow

1,000......30 PSI....1

2,000......60 PSI....2

4,000....120 PSI....4 The maximum flow because of the oil pop off valve at 90 PSI will be 3

8,000....240 PSI....8

At 6,000 RPM the maximum rate of flow has been reached with the thinner oil (A). When you go to 7, 8 or 9,000 RPM you do not get any more flow. You only get a maximum rate of 5. The internal forces on the bearings increase but there is no additional flow of oil.

With the thicker oil you reach maximum flow at 3,000 RPM (C). Worse yet is that the maximum flow is now only 3. As we increase RPM to 4, 5, 6, 7, 8, 9,000 RPM we get no additional pressure and no additional flow, no increase in lubrication.

Next let us look at a 20 weight oil at operating temperature. We get the same flow out of our constant volume pump but the thinner oil requires less pressure to move through the system. This even goes along with the rule that we should use an oil that gives us 10 PSI per 1,000 RPM:

(D) RPM....Pressure..Flow

1,000......10 PSI....1

2,000......20 PSI....2

4,000......40 PSI....4

8,000.. ...80 PSI....8

The maximum flow rate has not been reached. If the engine went to 9,000 RPM then the flow would be 9 at 90 PSI, our maximum pressure at pop off. The engine now has 3 times the flow rate as with the 40 weight oil at full RPM. The nozzles at the bottom of each cylinder are spraying 3 times the amount of oil lubricating and cooling this section. Everything runs cooler and the separation forces in the bearings are 3 times higher.

For engines that redline at 5,000 RPM they usually pop off the oil pressure at 50 to 60 PSI. For engines that go to 8-9,000 RPM the pressures max out at 90-100 PSI. You can now see that you can only get the maximum flow rate if you follow the 10 PSI / 1,000 RPM rule.

Continued...