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Old 05-04-2013, 09:21 AM   #1
Throckmorton
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Default Why is it OK to put cold oil into a hot engine?

You're supposed to change your oil with the engine hot. But why isn't it bad to fill the sump with cold new oil so it can then be sprayed into a hot engine? How does that not cause stress to metal parts? Shouldn't that be just as bad as adding cold water to your coolant system? Yes I know oil has half the thermal capacity of water, but it's not like it's orders of magnitude less.

Someone asked about this here and got funny responses http://www.m3forum.net/m3forum/showthread.php?t=324586

And of course everyone says to change oil hot so more drains out of the pan. But what's weird is Car Talk says to drain hot so more oil drains out of the engine. Huh? Once the engine is cold, all the oil that will drain down has already drained down. Oil doesn't migrate back into the engine as it cools.
http://www.cartalk.com/content/which...ne-hot-or-cold

There is lots of discussion here but no one mentions stress to metal by being cooled down quickly... It's like it never occurred to anybody. Honestly it didn't occur to me until this morning.
http://www.bobistheoilguy.com/forums...Number=1479704

Found another thread on this same topic, but it only got a few replies http://www.bobistheoilguy.com/forums...Number=1205028
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Old 05-04-2013, 09:38 AM   #2
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what is "hot"?? oil temps 150-200 degrees?.... and then putting in oil at ambient temp of say 70 degrees? not that big of a deal to be concerned with
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Old 05-04-2013, 09:51 AM   #3
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Quote:
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what is "hot"?? oil temps 150-200 degrees?.... and then putting in oil at ambient temp of say 70 degrees? not that big of a deal to be concerned with
Well let's convert to Kelvin. Operating temp is about 366K and room temperature is 294K.

So why is it bad to put 294K water in a 400K overheated engine? Does that extra 34K make the difference between destroying the engine and everything being fine? I don't get it.
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Old 05-04-2013, 10:20 AM   #4
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because it'll warm it up
edit: oh, interesting point.

The film left on the components is plenty for starting before the pump can push it through
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Old 05-04-2013, 10:31 AM   #5
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I change it cold when I can (almost all the time). I have no idea why you would want to change it hot, it would take longer to drain since it isn't right there in the pan. Not to mention dealing with hot oil is just not fun.
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Old 05-04-2013, 10:37 AM   #6
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You know, I think the reason car makers etc say to do it hot is they don't want you to crank up your cold car, drive onto your ramps, and change the oil without the engine having warmed up, because you end up with a lot of cold thick oil stuck up in the engine.
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Old 05-04-2013, 10:53 AM   #7
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It's mainly to make sure that the oil is thin, so more will drain out. I think it's not a bad idea to let the car sit for 10-20 minutes so it's not blazing hot when you put the cool oil in. I can see it being an issue if you were super hot after driving hard with the A/C on in bumper to bumper traffic and then dump six quarts of oil that was kept at 50F while the engine was still raging hot. But in the real world, I think the differences are narrow enough most of the time that it isn't really that much of a concern.
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Old 05-04-2013, 11:39 AM   #8
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You are not supposed to change it hot, just change it warm. Like just as the temp gauge leaves its lowest position shut off the car and change it.

The reason being the oil drains out faster when it has been warmed up a bit.
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Old 05-04-2013, 12:05 PM   #9
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Hot oil flows better.

Also as for oil vs water, its not just heat capacity but also heat transfer coefficient.
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Old 05-04-2013, 01:26 PM   #10
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Oil is also going into the lubrication system, designed to lubricate moving parts. Not the cooling system, designed for enough heat transfer to keep a loaded engine cool.
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Old 05-04-2013, 01:36 PM   #11
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Oil is not the primary heat transfer medium for the engine. It doesn't sit in large quantities next to parts of the engine that reach temperatures over 3,500 degrees Fahrenheit (inside the combustion chamber temperatures routinely get this high), rather it simply drains down into the oil pan.

In some cars, when you add oil, it just goes straight down into the oil pan, which does not experience anything close to the temperatures seen by the engine's head. Even when the oil filler is just a hole in the top of the cam housing, the oil doesn't sit in large quantities against the head and cannot drive the temperature of the heads down by any appreciable amount since it just rapidly drains back into the oil pan. There's not enough time for appreciable heat transfer to occur.

Coolant, on the other hand, sits in engine passages that are designed to transfer heat to it.

Also, as others have said, you're not supposed to change the oil when it's hot, merely when it's warm. I typically change my oil at least 30 minutes after I've parked from a drive that let the car get fully warmed up. That way the oil's still warm, but not scalding hot. Frankly, if the ambient temperature is above about 65 it doesn't really matter, you just don't want to be waiting for sub-freezing oil to slowly come out of the engine.

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Old 05-04-2013, 02:55 PM   #12
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Putting hot oil in a cold engine is not something to worry about from the perspective of damaging the steel. The operating temperature of an engine is far below what is required to cause microstructural change to the steel. Heat treatment of steel generally always starts with an excursion to 727°C, the temperature at which the microstructure changes to austenite, from which all other microstructures evolve. If you never get over the austenitizing temperature, you're not effecting real microstructural change.

Also, the temperature difference is not really enough to cause a problem due to thermal shock- this happens with glass because it is extremely brittle: it's microstructure is such that it has no ductility at room temperature. Modern (Post WW2) steel is very ductile. The coefficient of thermal expansion of steel is 7.3 x 10^-6 in/in K. So with a temperature difference 72K, the dimensional difference would be 0.000525 inches per inch. Well, well within manufacturing tolerance for any engine. A 1" dia camshaft would become 1.000525. Any temperature difference in a monolithic piece of steel will cause some amount of stress, but the difference here is so minute that the stresses are nearly zero- not even close to what you'd need to shatter something like putting hot glass in water.
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Old 05-04-2013, 03:26 PM   #13
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As said, oil changes are done with engine wam, not extremely hot. And by the time you have the old oil out, filter changed and start to add the new oil, the engine has cooled off a bit. Never had any issues due to engine temperatures and oil changes.
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Old 05-04-2013, 04:07 PM   #14
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And before anyone mentions oil coolers, these are for race engines...with dry sumps.
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Old 05-04-2013, 04:14 PM   #15
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Quote:
Originally Posted by foghorn67 View Post
And before anyone mentions oil coolers, these are for race engines...with dry sumps.
My MR2 has an OEM oil cooler under the oil filter. It's nothing fancy, but it is an oil cooler.
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Old 05-04-2013, 04:18 PM   #16
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Quote:
Originally Posted by foghorn67 View Post
And before anyone mentions oil coolers, these are for race engines...with dry sumps.
There are a lot of wet sump engines with oil coolers out there. Hell, my Volvo has an oil cooler. Admittedly, it's a turbocharged engine, but oil coolers are definitely not limited to only dry sump applications.

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Old 05-04-2013, 04:35 PM   #17
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Quote:
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My MR2 has an OEM oil cooler under the oil filter. It's nothing fancy, but it is an oil cooler.
sorry, forgot about turbo engines.
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Old 05-04-2013, 06:26 PM   #18
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I'm a ThrockMorton right FWIW.
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Old 05-04-2013, 07:32 PM   #19
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I'm not really sure why this is even a question quite frankly. It sounds like someone obsessing about minutia.
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Old 05-04-2013, 08:36 PM   #20
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Quote:
Originally Posted by JulesMaximus View Post
I'm not really sure why this is even a question quite frankly. It sounds like someone obsessing about minutia.
Kinda my thought.

I'm thinking about it this way: taking a hot drinking glass out of your dishwasher and throwing cold water on it might crack it.

Throwing cold oil on it will not. For one, it's hard to throw cold oil at something, heh. Even if you dropped the glass into a bucket of oil, the heat transfer would probably occur slowly enough to not break it.

Now substitute 'cheap piece of glass' with 'big chunk of iron, steel, and aluminum,' and consider how slowly heat is being transferred to the bottle of oil you're pouring into the engine.
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Old 05-04-2013, 09:42 PM   #21
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Quote:
Originally Posted by foghorn67 View Post
And before anyone mentions oil coolers, these are for race engines...with dry sumps.
A lot of police spec sedans have them. That's because the idle a lot.
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Old 05-04-2013, 09:56 PM   #22
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Quote:
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And before anyone mentions oil coolers, these are for race engines...with dry sumps.
My wet-sump, non-sport motorcycle has an oil cooler.

So did my NA 89 MR2 (I believe jlee's is also NA).

There's an exception to every rule...
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Old 05-04-2013, 10:29 PM   #23
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The oil change instructions for my Jeep say to get the engine up to normal operating temperature.
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Old 05-04-2013, 11:16 PM   #24
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Quote:
Originally Posted by jagec View Post
My wet-sump, non-sport motorcycle has an oil cooler.

So did my NA 89 MR2 (I believe jlee's is also NA).

There's an exception to every rule...
Mine is turbo. The V6 is n/a, but I haven't gone far enough with it to know if it has an oil cooler or not.
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Old 05-05-2013, 12:22 AM   #25
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Quote:
Originally Posted by LTC8K6 View Post
The oil change instructions for my Jeep say to get the engine up to normal operating temperature.
The coolant comes up to temp much faster than the oil. If you shut the engine off as soon as the temp gauge is up to the "normal" range, the oil will only be warm.

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