Thin vs. thick: cold-engine protection

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What helps a lot on modern cars is their use of a coolant-to-oil heat exchanger. It's function is two-fold: It helps warm the oil up faster during engine warm-up, and also helps cool the oil when the oil is at full operating temperature, as long as the radiator works well and keeps the coolant temperature under proper control.

Funny thing is when I bought my Tundra a few years ago, my co-worker almost talked me into bypassing either the coolant to oil heat exchanger or I think it was the coolant to transmission fluid heat exchanger. saying it dumps too much heat into the ATF arguing that the car also has a separate ATF cooler with radiator so the heat exchanger is not needed for our weather in WA ... he had done it but I said no. I would have to trust the Toyota engineers on this one and the car is under warranty but he did it anyway.
 

Gokhan

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If your oil started at 8Cst, per university 101, your wear would be incredibly higher.

In the early cold stages, it's thick, providing ample oil film thickness. When it's hot, the additives are there...it's the "warmup phase" that the wear occurs, queue Magnatec...

I am not sure if this is as simple as the "higher viscosity is always better thanks to the Stribeck curve" argument.

What I am worried about is not the Stribeck curve but things like cavitation and oil starvation at very high viscosity, which are not well-studied. There is a paper, which says that a low-viscosity lubricant does a lot better than a high-viscosity lubricant in reducing cavitation in a sleeve bearing.

 
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I am not sure if this is as simple as the "higher viscosity is always better thanks to the Stribeck curve" argument.

What I am worried about is not the Stribeck curve but things like cavitation and oil starvation at very high viscosity, which are not well-studied. There is a paper, which says that a low-viscosity lubricant does a lot better than a high-viscosity lubricant in reducing cavitation in a sleeve bearing.

If cavitation was a big thing in everyday passenger vehicles, the journal bearings would be damaged from cavitation errosion. How many times have you heard of or saw erroded/damaged journal bearings talked about or shown here, or anywhere else?

If lower viscosity helps mitigate journal bearing cavitation and thicker doesnt, then you'd think engines using xW-40 and xW-50 wouldn't made it to high mileage before rod knock shows up. Or that all cars would have accumulated bearing cavitation damage from cold starts. They don't seem to suffer from that, and if they did inspection of journal bearing surfaces would show oil cavitation damage.

IMO, oil cavitation is the last thing I'd be worried about when it comes to choosing what oil viscosity to use. I'd be more concerned about the "W" rating based on where I drive, and the HTHS viscosity.
 
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To add ... motorcycles rev to 11,000+ RPM running 20W-50 all day long, and I never hear of their rod or crankshaft journal bearings being damaged by oil cavitation.
 

Gokhan

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With my plug-in hybrid, idling is out of question. In fact, I don't turn on the ICE (turn off the EV mode) until I pass the first traffic light with the car traveling 20 mph so that I don't waste fuel. Therefore, 0W-16 is the answer, and bring on the 0W-8 for the next-generation Prius Prime!

 
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To add ... motorcycles rev to 11,000+ RPM running 20W-50 all day long, and I never hear of their rod or crankshaft journal bearings being damaged by oil cavitation.
I'd love to see an engine teardown from one running 0w-8 or 0w16, babied until the oil reached operating temperature just to play it safe. My bet is as the grade of oil gets bumped up in that application the wear decreases. ;)
 
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If oil cavitation was going on here, I don't think an engine like this would last long ... but they do. Bet it's not running anything thinner than a xW-40 ... robust HTHS is a must in engines like this (things get going after the 1 min mark). 8 and 16 oils have their place, like in Gokhan's application where there isn't much demand or stress on the engine.

 
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Cool video! When I see that, for some reason I want to use an even thicker oil. lol

What's causing the dust? Is it oil vapor, droplet or mist ... and is it caused by heat or rapid movement?
what's the technical name for it?
 
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With my plug-in hybrid, idling is out of question. In fact, I don't turn on the ICE (turn off the EV mode) until I pass the first traffic light with the car traveling 20 mph so that I don't waste fuel. Therefore, 0W-16 is the answer, and bring on the 0W-8 for the next-generation Prius Prime!


after reading this, I want to use only 0W synthetic for when the engine and oil are cold.

The following is from the link provided by Gokan:

"The second problem is that most engines are constructed of aluminum these days which has a much higher coefficient of thermal expansion than the old cast iron structures. Thus, under extremely cold conditions the clearance between some of the key mating parts gets very small.

Of particular concern is main bearing clearance where the main bearing housing (the engine block) is constructed of aluminum and the crankshaft is constructed of steel. Thus, the aluminum housing contracts much more than the shaft and the bearings get extremely tight. The combination of reduced bearing clearance and lack of oil flow is a recipe for metal-to-metal contact and rapidly increased wear. "
 
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The clearances are larger on a cold engine . When the metal warms up it expands and the clearances get tighter. The oval pistons get round when the expand measure a cold piston
 
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Cool video! When I see that, for some reason I want to use an even thicker oil. lol

What's causing the dust? Is it oil vapor, droplet or mist ... and is it caused by heat or rapid movement?
what's the technical name for it?
Blow by. The pistons displace as much air going down as they do going up.
 
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after reading this, I want to use only 0W synthetic for when the engine and oil are cold.

The following is from the link provided by Gokan:

"The second problem is that most engines are constructed of aluminum these days which has a much higher coefficient of thermal expansion than the old cast iron structures. Thus, under extremely cold conditions the clearance between some of the key mating parts gets very small.

Of particular concern is main bearing clearance where the main bearing housing (the engine block) is constructed of aluminum and the crankshaft is constructed of steel. Thus, the aluminum housing contracts much more than the shaft and the bearings get extremely tight. The combination of reduced bearing clearance and lack of oil flow is a recipe for metal-to-metal contact and rapidly increased wear. "
Exactly the opposite.
 
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If the AL surrounding the bearings " contracts more" that means the bearing clearance would be larger, not smaller.

If I understand the article correctly, bearing housing is aluminum and the crankshaft is steel ... and the aluminum contracts more relative to steel and as a result, the clearance is further reduced and very tight.

btw, I cut & pasted the info from the article provided by Gokhan and it's not my summary.
 
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Saying the bearings are aluminum is too simplistic. They are specialized alloys, silicon aluminum, or other metals blended in. Aluminum comes in many ways.
 
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Blow by. The pistons displace as much air going down as they do going up.

Picture (video) is worth a thousand words!
Didn't know there is that much blow by!
Is that typical in car engines? I now feel the need to change the oil even sooner. :)
 

Gokhan

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The problem is not only because the shaft and housing have different thermal-expansion coefficients. Even if they have identical thermal-expansion coefficients, during warm-up, there is a large temperature difference between the two before the steady is reached, which reduces the bearing clearance because the shaft runs a lot hotter than the housing. Combined with a very thick oil, this could result in oil starvation and wear during warm-up.

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As long as you are using an appropriate winter grade according to the manufacturer you shouldn't worry at all. I used to be freaked out by "cold start wear" and only used 5W oils for a while and also let my car idle for 5 minutes in the morning in temps above freezing 95% of the time.

Viscosity is the least of my worries now as it has little to do with wear as long as oil is there. Shannow had provided us with some pretty significant data and proofs over the years. Different metals expending at different rates however might be a more relevant subject.

IMO, waiting 15 seconds to a minutes just to be sure that oil pressure is there or until it comes off high idle is enough, followed by being gentle with your car until everything's up to operating temp.

My owner's manual's chart says i can use 20W50 down to -5°C and i'd have no problem doing that.
 
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