:Any tribologists please chime in any correct all inaccuracies:
Our engines have several different lubrication needs but yet we ask 1 oil to cover all those needs. I'm on a very long conference call most of which doesn't pertain to me so I'm gonna do some typing.
This is my understanding of things:
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Let's use Cem's setup as an example. His engine is doing 2 things that are
abnormal from a stock engine.
1: Added power from forced induction and
2: very high oil temps due to track conditions which creates super thin, very low viscosity oil.
Before I continue, not everyone knows that oil system pressure does not separate or lubricate bearing/rod surfaces... nope, that is done by a process called
hydrodynamic lubrication. I've talked a lot about pressure in the past so I'll only cover a little toward the end. The best analogy I've ever heard for the hydrodynamic wedge is the hydroplaning tire. Think of a tire driving on a wet patch of road, if that tire speeds up fast enough it won't be able to evacuate the water in front of it, the water will create a "wedge" in front of it and once that happens the tire will ride up and upon that wedge and now the tire is hydroplaning (aka: lubrication has occurred).
The crankshaft, as the journal rides inside the bearing, a hydrodynamic wedge is formed in front of the leading surface which it rides upon and lubrication occurs. Oil
flow is the replacement of oil as it's being pushed out of the journal. Unlike the hydroplaning tire, we
always want the wedge to exist!
But let's go back to that tire, more importantly a tire with some tread because our bearing and journal surfaces have 'asperities' or roughness to them as well.
Let's say the road has 1/8" inch of standing water and now that tire will hydroplane at 20mph. How do we get the tire to hydroplane at a lower speed or any speed?
A) Remove the tread (like making your bearings/journals smoother)
B) Change the fluid so there's 1/2" of standing water (aka: thicker oil)
C) Reduce the weight the tire supports (reduce cylinder pressure or engine power)
Well, we can't really do
A because the engine's already assembled. Nobody wants to do
C because less power?.. no way!! So the answer is
B, we have to do it with the lubricant.
For any speeds below 20mph in this scenario, what is that treaded tire doing? That's right, it's making contact with the surface. Just like a tire traveling at a high rate of speed will hydroplane more easily, the faster an engine spins, the more easily it creates and rides upon its hydrodynamic wedge. Therefore at high rpms the engine can be safely lubricated with very low viscosity fluid, but slow that engine down (like slowing that treaded tire down) and at some point there might not be enough "water on the road" (viscosity or film strength) which would allow the journal to interface with the bearing, or in this analogy, the tire to have traction.
So Cem is right, running around all day at 7000rpms is just fine at high rpms using a thin oil even at high heat. But can I also be right? Well, the answer really is:
It Depends.
Hydrodynamic oil film thickness is directly proportional to the viscosity of the oil.
The Forced Induction allows his engine to make more torque at lower RPMs than OEM so his pistons can push down on the crank with much more force at lower rpms and we KNOW that at lower RPMs the hydrodynamic wedge is not as easily formed and now we're pushing it harder into the lubricant. So if I put more weight on the treaded tire I can now travel through the patch of water at greater than 20mph before hydroplaning (aka lubrication) happens. Another way of saying that is increased load makes it more difficult to for the hydrodynamic wedge to form. On top of that the extra heat put into the oil has thinned it significantly so now the treaded tire that's also carrying extra load, is now traveling through a thinner patch of water which makes far more difficult for the hydrodynamic wedge to occur.
Now what this means is that it's possible that at some point as the 5w-30 heats up to be very hot it may become so thin that if he applies engine load at a low enough RPM (which he has more load available to apply because FI) the hydrodynamic wedge may not form and premature wear on his crankshaft bearings or rod journals could occur.. or worse. I don't know where that point is however. It could be that at 260F oil temp it won't, maybe it won't occur at any load or rpm until the oil is over 280F...
But we do know that high load and low rpm needs high viscosity/film thickness. High rpm and load needs very little viscosity. My concern for his engine wasn't at high RPM. It was at extreme oil temp combined with low rpm load. There's also a lesson here.. Don't "lug" your engine on your cool down lap.
*"Lugging" is applying lots of pedal at a very low RPM.
Oil pressure is a crude measuring tool but is useful for gauging viscosity if you also know your oil weight and oil temp. If I'm at 260F on a 0w20 and I make 30psi at 7k rpms, the pressure is something that I can use to make a determination if I have adequate thickness for hydrodynamic wedge at
all RPM. Some people say 10psi for every 1000 rpms, others say different things.
So in Summary:
Cem can run the thinner oil and be fine especially if his engine oil temp cools quickly after a session and he doesn't apply high load at lower RPM when the oil is at it's hottest. But I don't know where the line is here so I see "risk" in this equation. That is why I think Cem would benefit from a
slightly thicker viscosity mainly due to the large variety of demands in addition to being FI.
I lug my street car around at low rpms for fuel economy so I like my 0w-30 for milling around town. In my race car I like running a thinner oil just like Cem but my goal is to maintain a minimum of 50psi or greater and the car doesn't see load below 4k. Since I've already addressed the relief system (increased pressure
and flow), my only option (per the rules) to achieve my target PSI is via oil viscosity which I try to run the thinnest I can without being below 50psi @ 7k rpm at my hottest temps.