Select Page

### Chapter Ten – 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 grade 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)

RPMPressure (PSI)Flow
1,000201
2,000402
4,000804
8,0001608
( 30 grade oil at operating temperature ) (B)
RPMPressure (PSI)Flow
1,000301.5
2,000603
4,0001206
8,00024012
( 30 grade oil at operating temperature )
*Higher output oil pump

If we stick with the same grade 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 grade 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):

RPMPressure (PSI)Flow
1,000301
2,000602
4,0001204
8,0002408
( 40 grade oil at operating temperature )

(D)

RPMPressure (PSI)Flow
1,000451.5
2,000903
4,0001806
8,00036012
( 40 grade oil at operating temperature )
*Higher output oil pump

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 grade oil to a 40 grade oil in your car:

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

RPMPressure (PSI)Flow
1,000201
2,000402
4,000804
8,0001608
( 30 grade oil at operating temperature ) (C) For a 40 grade oil at operating temperature:
The oil is thicker, has more internal resistance and therefore requires more pressure to get the same flow.
RPMPressure (PSI)Flow
1,000301
2,000602
4,0001204
8,0002408
( 40 grade oil at operating temperature )

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 grade 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)

RPMPressure (PSI)Flow
1,000101
2,000202
4,000404
8,000808
( 20 grade oil at operating temperature )

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 grade 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.

The winner: 0W-20 grade oil for my Maranello. I said earlier that I could have used a 10 grade oil. I actually only ran with 185 F oil temperatures around town and the pressures were similar to the 40 grade oil example in (C) above. This is why I also said that in the racetrack condition, with hotter, thinner (0W-20) oil, I may actually get the optimal results as in (D) above.

Now let us go back to the Ferrari recommended parameters in my 575 Maranello manual. It calls for 75 PSI at 6,000 RPM. The pop off pressure has not been reached. As we now increase the RPM we still get an increase in flow rate. This is what we need and this is exactly what they are recommending. We get our maximum flow at the maximum system pressure, at about the maximum engine RPM of 7,700. There is no bypassing of the oil. All oil pumped goes through the system. There is no wasted BHP pumping oil past the bypass valve back to the oil tank. It is the perfect system.

Finally I will compare a single, 30 grade oil, at normal (212°F) and at racetrack (302°F) temperatures:

(A) 30 grade oil at 212°F

RPMPressure (PSI)Flow
1,000201
2,000402
4,000804
8,0001608
( 30 grade oil at operating temperature )

(E) 30 grade oil at elevated 302°F operating temperature. The oil is thinner at 302 F. It requires less pressure to get the same flow:

RPMPressure (PSI)Flow
1,000101
2,000202
4,000404
8,000808
( 30 grade oil at 302°F operating temperature )
The hotter (302°F) 30 grade oil is thinner than the cooler (212°F) 30 grade oil. It has the same flow rate in the constant volume oil pump but at a lower pressure than the oil at normal operating temperature. This allows for a doubling of the flow rate at peak RPM. The thinning of oil at higher temperatures is a benefit. You get more flow, more cooling and more lubrication.

The 30 grade oil at 302°F has the exact same flow rate and pressures as the 20 grade oil at 212°F. See (D) above. Therefore, use the 20 grade for around town driving and the 30 grade on the hot track. You get maximum flow at each situation.

For YOUR engine, substitute the actual flow at 1,000 RPM. If your engine puts out 1.5 liters/min. at 1,000 RPM it would put out 3 liters/min. at 2,000 RPM and 6 liters/min. at 4,000 RPM and so on. The maximum flow in (A) would be 7.5 liters/min. In situations (D) and (E) you would get a maximum of 13.5 liters/min.

Conclusions:
The reason that multi-grade oils were developed in the first place was to address the problem of oil thickening after engine shutdown. Over the years we have been able to reduce the amount of thickening that occurs. Never-the-less there is no oil that does not thicken after you turn your engine off. This is why we have to warm up our engines before revving them up. Engine designers always pick the recommended oil based on a hot engine and hot oil. There is no issue with oil thinning as they are both matched when hot. The problem is oil thickening when the engine cools.

Cold engine showing very high pressures because of the thickened oil at startup:

For a 40 grade oil at 75°F at startup:
The oil is thicker, has more internal resistance and therefore requires more pressure to get the same flow.

RPMPressure (PSI)Flow
1,000601
2,0001202
4,0002404
8,0004808
( 40 grade oil at 75° startup )

At 1,500 RPM you reach the maximum oil flow rate and if you run to 8,000 RPM it is the same rate. The flow cannot increase and it is insufficient. This is why we must wait until our oil temperature comes up to 212°F or higher. The maximum flow rate in this case will then double, up to 3. To get even more flow in our test engine you need to use a lower viscosity grade.

If you have absorbed and digested the information here you should be able to pick out the proper operating oil grade for your car, be it a 30, 40, 50 or even 20 grade oil. I have always used oils that were a grade thinner than recommended even though many use a grade thicker than recommended. I showed evidence that the starting grade should always be 0 or 5 (0W-XX or 5W-XX for thicker oils). If you want the best protection and highest output from your motor use a synthetic based oil. The actual brand is not as critical as the viscosity. The rating must be the SL or SM rating. Change your oil every 3 – 5,000 miles and at least every spring.