[aehaas]

Motor Oil 201
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...

 

[aehaas]

The winner: 0W-20 weight oil for my Maranello. I said earlier that I could use a 10 weight oil. I actually only run with 185 F oil temperature around town and the pressures are similar to the 40 weight 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 weight oil, at normal (212 F) and at racetrack (302 F) temperatures:

(A) For a 30 wt oil at normal (212 F) 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

(E) For a 30 wt oil at elevated (302 F) operating temperature. The oil is thinner at 302 F. It requires less pressure to get the same flow:
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 because of the oil pop off valve at 90 PSI will be 9

The hotter (302 F) 30 weight oil is thinner than the cooler (212 F) 30 weight 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 weight oil at 302 F has the exact same flow rate and pressures as the 20 weight oil at 212 F. See (D) above. Therefore, use the 20 weight for around town driving and the 30 weight 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 multigrade 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 wt oil at 75 F at startup:
The oil is thicker, has more internal resistance and therefore requires more pressure to get the same flow.
RPM....Pressure..Flow
1,000......60 PSI....1
2,000....120 PSI....2 The maximum flow because of the oil pop off valve at 90 PSI will be 1.5
4,000....240 PSI....4
8,000....480 PSI....8

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 weight for your car, be it a 30, 40, 50 or even 20 weight 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 SL or the upcoming SM rating. Change your oil every 3 - 5,000 miles and at least every spring.

Final examination to follow later.

THE END

 

[mike]

Stunning series of posts. Thank you. As I'm a 'thick' dentist, it's going to take a while for me to process all this information. Many thanks for making this invaluable contribution.

Once again, many thanks for your invaluable information.

 

[nashbap]

Thank you for the very informative series of posts, especially the last one about oil flow characteristics. The question that begs to be asked is why manufacturers in the past recommended oils such as 20-50. Surely the knowledge about flow and pressure characteristics is not new and as you mentioned earlier, a 20-50 mineral based oil starts at 20 with viscosity enhancers added so a thinner oil could just as easily have been made in the past. I realize that today's car manufacturers universally recommend lighter oils like 5-30 or 0-20 but why was this not the case in the past?

If you have the time a section on gearbox oils would be welcome. I have a bunch of questions on that topic.

Regards,

Nash

 

[aehaas]

Good question. A decade or two ago thicker oils were more often spec'd. I really think that they thought it was better and are just now coming around to my way of thinking. I have studied this stuff since high school and could never understand why thick oils were often used. You need to test the oil under your own driving conditions to pick the right grade.

Ferrari used to spec oils thinking that the cars were going to be on the track. They are now specifying several oils based on your use. It is not based on where you live but how you drive.

They used to make patients stay in the hospital for the most simple operations. My father had a cyst removed from his back and was hospitalized. It took years before they figured out what I thought was obvious, that many things can be done as an outpatient.

Also, with mineral oils there was little choice. They could not get the grade variability we have today. The mineral based oil could only be a 20W-50 whereas we have a synthetic 5W-50 now. This is new technology. Before we had a 10W-40 now we have 0 and 5W-40.

Finally the oils of today work better at all the things oils are asked to do, even when thinner to begin with. They do not start with a thicker oil today because they are not worried about it thinning with use, they last longer.

And thicker does not mean less wear. I will reprint this data:I got this from the Amsoil synthetic motor oil web site:

At 150 C, 1,200 RPM 4 ball wear test (not a bearing, just balls rolling in oil from what I can tell)
.....Straight AHR Amsoil 60 weight oil.....wear = 0.39 mm

Same test but with 1,800 RPM instead of 1,200 RPM:
.....20W-50 ARO Amsoil multigrade....wear = 0.39 mm
.....10W-40 XL Amsoil multigrade.......wear = 0.38 mm
.......5W-20 XL Amsoil multigrade.......wear = 0.38 mm

This is a surface lubrication test with rolling balls. All tests were at 150 C. The better results were from the thinner oils at higher RPM. I do not know why. But it does make me think that thicker is not always better. Also, the 60 weight oil was only SJ rated and all others were SL rated.


As far as gear oils I have studied these far less and cannot give you much useful information.

aehaas

 

[raymond]

Your post are all logical, but keep in mind you're talking of modern engines with tight bearings.
Problems are always older engine with more bearing clearance, if you use too thin oil in a old engine your oil pressure will simply escape from the bearing and not grease sufficently although the flow is high.

If the flow would be the only thing that counts why do all manufacturers measure engine wear by the oil pressure?

Again, your postings are logical, for modern engines, but for older ones I'm not that convinced...

I always use the oil recommended by the factory in the specific owners manual, if a 350 GT wants 20W Oil because of the 40 years old bearing clearance I would fill in.

I can remember that Ferrari tried 0W40 Shell in their F355 Challenge cars, after a series of bearing failures they returned to 10W40, then no more engines broke since then.
I got this Info from the double challenge winner Bruno Staub of Switzerland.

 

[aehaas]

Confusion occurs because we over simplify. What was the real story with all the pertinent details? A 0W-40 and a 10W-40 have the same viscosity in a hot engine. That alone was not a cause of failure. If the 0W-40 was a mineral oil and run hot then the VI would deplete and oil thinning would initially occur. Was this compared to a 10W-40 synthetic with no VI added?

Shell does not make a 0W-XX oil in Helix Ultra to my knowledge. They have the 5W-40 and it is recommended now. My 575 Maranello manual says to use a 0W-40 synthetic, or 5W-40 Helix Ultra or in "hot climate race track conditions, a Helix Ultra 10W-60.

Numerous cars in the MB and Porsche family spec the 0W-40 Mobil 1 for all conditions. It is a 40 weight oil.

In older engines with greater wear or clearances the pressures would be too low with thin oils and this would lead you to try a thicker grade. The grade YOU need is based on YOUR engine and driving conditions. Lower pressures is THE indication that you need to use a thicker grade.

If your engine is that old and/or worn then maybe you need a 50 or 60 weight grade. Use the grade your engine needs.

aehaas

 

[superx]

I too would recommend caution when deviating from the factory recommendation, especially if the car is under warrantee (if you care about keep ing it valid).

another anecdote to chain along here.....GM refused warrantees on their diesel engines where 10w40 was used. GM specified 15w40. Apparently there were bearing failures with 10w40. I think the reason is the 10w40 required more VI addatives to achieve the desired qualities. The additives broke down and the bearings failed. This seems consitent with the ferrari CS failures.

Which leads to the question, does synthetic require any VI addatives? If not, how does castrol make a 5w50? this is a huge spread.

Dr Haas, thanks for being so passionate about this topic and sharing your investigation.

 

[aehaas]

Single weight synthetic oils have the same properties of multigrade mineral oils. See www.redlineoil.com. They can give you the oil specs showing that for example a straight 60 weight synthetic has the same properties as a 20W-60 mineral based oil with no VI additives. This oil cannot breakdown and result in failure if this is the grade your engine needs for racing.

I believe that the failures that we have seen are with mineral based oils not synthetic oils.

More and more cars are recommending the use of synthetic oils, some require it. The main advantage is that it thickens less when you turn off your engine. Also, it cannot thin as mineral oil may under extreme conditions (from VI improver use/dilution). Synthetics thicken slower with long term use. Again, under normal use in your Ford I am not sure it makes that much a difference.

Diesel engines have higher heat and much more dirt and blow-by dilution making straight weight oils more useful. Also, vehicles that use diesel engines as trucks are used on a more continuous basis, unlike our cars that are on and off ten times a day. This is especially bad for startup wear.

aehaas