Thin vs Thick Discussion, Chapter 5

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Motor Oil 105
Part Five. Let’s use top gear:

Let’s go racing. I will discuss driving in traffic jams in the Florida summer as well as racing in Sebring though there is little to no commonality. People lump these two driving situations together but there is little overlap.

On the race track one usually uses all the BHP their engine can give them. You briefly step on the brakes for the corner then put the pedal to the metal the rest of the time. Your oil will get up to 302 F, but your cooling system is around 212 F. The engine produces tremendous heat but can only pass it off so fast to the cooling system. There is a lot of air moving past the cooling radiator so the antifreeze / coolant is able to get rid of the extra heat from this part of the system with relative ease. Much of the engine cooling is performed by the oil system.

The temperature of oil on your gauge is not actually as hot as it really gets. This temperature is an average with oil from different parts of the motor. Some parts are hotter than others. It is said that some of the oil gets as hot as 400 or 500 F in these racing situations.

In an earlier section I said that thicker oils are usually needed in racing situations but not necessarily. Remember that a major function of oil is to cool the inside of your engine. In ASTM D 4485 3.1.4: “Terminology: Engine oil- a liquid that reduces friction and wear between moving parts within an engine, and also serves as a coolant.” Since the oil with a viscosity of 10 cS at 212 F thins to a viscosity of 3 cS at 302 F we will get some more flow. The pressure will go down some as well. This is OK as long as we have a minimum of pressure to move the oil.

This increased flow will result in increased cooling by the oil. This is a good thing. You would probably want more oil flow in these situations and you get it. The hotter oil thins and this increases flow. This is because the pump does not reach its pop off relief pressure. I will show you how this works later. The higher flow works harder to separate the engine parts that are under very high stress. It all works out for the better. Higher revving engines need thinner oils. You do not necessarily need to go to a thicker oil while racing. Only experimentation will tell.

The best way to figure out what viscosity of oil you need is to drive the car in the conditions you will use. Then use the oil viscosity that gives you 10 PSI per 1,000 RPM under those circumstances. For some reason very few people are not able to get this simple principal correct.

These same rules apply to engines of any age, loose or tight. Just because your engine is old does not mean it needs a thicker oil. It will need a thicker oil only if it is overly worn, whether new or old. Yet the same principals of 10 PSI per 1,000 RPM still apply. In all cases you need to try different grade oils and see what happens. Then choose the correct viscosity.

I used 0W-20 in my Ferrari 575 Maranello. It had over 5,000 miles on the clock. There will be a day (my estimate is 100,000 miles) when one will have to go to a 0W-30. In the future one will have to increase the viscosity to a 0W-40, then a 0W-50, maybe. You should use whatever it takes to get 75 PSI at 6,000 RPM during the lifetime of the engine. This formula works in all situations.

Some people have tried this and occasionally get a somewhat low oil pressure while at idle. This is fine. There is no stress on parts at idle, the smallest oil flow will do the trick. It is at higher RPM where more BHP is produced. This is where we need the flow. Remember that Ferrari uses 75 PSI at 6,000 RPM as the place to test your oil viscosity needs. If your oil gives this value under your driving conditions then your lubrication system has been optimized.

Do not go 5,000 miles with the same oil if you are racing your car. You should change the oil every 1 or 2,000 miles. If you drive your car only around town then you need to change the oil for that situation. Use racing oil on the track and urban oil around town. The best situation as described by Ferrari is to use the 0W-40 around town and the 10W-60 “racing oil” on the track. It has to be that “hot” track though. A compromise situation would be to use the 5W-40 for both but this may not be optimal. Certainly, if you are just an urban driver as me use the 0W-40 or even a thinner oil as I do in my Maranello. Again, I used the 0W-20 grade.

FYI. The Formula 1 cars that run at 15,000 RPM and higher use straight 5 and 10 and 20 grade oils.

Now let me discuss what people think is a similar situation to racing. That is hot summer traffic jam driving. Your car should be able to handle this. If you have problems then you have a problem with your car, most likely in need of a cooling system overhaul.

When you drive that car down the road mid-winter in upstate New York or mid-summer in Florida the engine and oil temperatures will be around 212 F. But your Florida vacation is suddenly altered by a hurricane. You have to get out of Tampa, but so do a million other people. It is now 95 F and you are in a snarl. Everyone thinks they need a thicker oil for this situation. This is false.

Your engine is not producing much heat at low RPM and low BHP output. The production of heat is relatively slow. It can easily be transmitted to your cooling system. The problem is that your cooling system has trouble getting rid of the heat. The oil and the coolant will slowly rise in temperature. They both rise together. The increase is no big deal for your oil. It goes to 220, then 230 F. The problem is that the cooling system can only handle heat up to 230 F. After that you overheat the cooling system and the car must be shut off. The oil never got that hot, It was just that the water got a little hotter than its system design.

You now see that overheating in traffic is a cooling system problem and not an oil system problem. Do not change to a thicker oil based on your traffic situation.

AEHaas
 

4WD

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Flow does not increase because oil thins … pressure can decrease slightly while flow is fixed to the pump RPM and what is regulated by the PRV.
Pressure is not flow … most modern vehicles have variable output ECM controlled pump “systems“ …
In the table of truths of what parameters impact circulating pressure - viscosity is at the bottom …
Exotic cars are interesting … but I doubt 5% of the members here have one.
 
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Interesting. The specified oil pressure requirements for the two cycle Detroit Diesels was 4psi at idle (500rpm) and 30psi at high rpm (2250). Most heathy engines I tested ran 10psi at idle and 40-50psi at high rpm. The Detroit Diesel oil pump, and oiling system, was designed for flow and they determined these pressures would deliver the required flow.
 

BeerCan

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The best way to figure out what viscosity of oil you need is to drive the car in the conditions you will use. Then use the oil viscosity that gives you 10 PSI per 1,000 RPM under those circumstances. For some reason very few people are not able to get this simple principal correct.
I think for 99% of drivers the best way to get the viscosity they need is to look in the owners manual.
 

OVERKILL

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Interesting. The specified oil pressure requirements for the two cycle Detroit Diesels was 4psi at idle (500rpm) and 30psi at high rpm (2250). Most heathy engines I tested ran 10psi at idle and 40-50psi at high rpm. The Detroit Diesel oil pump, and oiling system, was designed for flow and they determined these pressures would deliver the required flow.

The 10psi per 1,000RPM rule was just a rough rule of thumb for the Small Block Chevy for guys who were building them, why it is being applied universally here is yet another reason why these threads keep getting locked. They are full of incorrect information and personal opinion being presented as fact.

Applying the above logic more broadly, my wife's truck produces ~32psi hot at ~800RPM on a 0w-20. FCA spec's a 5w-20. They didn't spec 5w-20 so that I'd have 10psi at idle and 30psi at 3,000RPM, they spec'd it because their internal testing under the anticipated usage profile confirmed that it was a safe choice under the anticipated operating conditions the vehicle was to experience. Tossing 0w-8 in the pan to try and drop my oil pressure down would likely result in a rod leaving the party through the side of the block and I doubt the author here would be willing to foot the bill for that experiment.

This is the reason that when Honda started going below 0w-20, chasing 0w-16, 0w-12 and 0w-8, they made mechanical changes to the engines, such as wider bearings, which would be necessarily more tolerant of the reduced MOFT provided by the lower viscosity lubricant.
 
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Regarding 10 psi per 1K rpm:
that ratio is not universal.
I think I posted in another thread that my car service manual shows:
V8 @3000 RPM: 43-85 PSI.

also not trying to be mean but curious who are these "people"? (refer to the quote below).
I've never heard or seen anyone comparing the two (city vs. race) driving conditions!!!


"I will discuss driving in traffic jams in the Florida summer as well as racing in Sebring though there is little to no commonality. People lump these two driving situations together but there is little overlap.
 
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