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Why do we preheat?

DanH

DanH

Legacy Member
Mentor
Sam Buchanan said:
That is far more important than oil temp. If oil is warm but the remainder of the engine is cold then the issues inherent with a cold start won't be completely addressed. But if heating the oil transfers heat to the CHT probes we can assume the crank journals and cylinder bores have been preheated so the start won't put undue stress on the engine.
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Ironflight said:
Is the heat “enough”? Hard to say, but warmer is always going to be better than stone cold….
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I pulled these quotes from another thread, so as to not drift that product review.

Sam and Paul are respected sources. Their comments are fundamentally expansions on manufacturer's recommendations. Question is, are those recommendations uniformly valid? What are the unnamed inherent issues, and why is warmer better?

Examine the service instructions, and you'll find only a few specific concerns, notably (1) lubrication failure due to congealed oil, (2) poor fuel vaporization, and (3) battery and starter load. Note #3 is almost entirely due to #1, oil viscosity.

Here's the thing...those three issues are based in 1950's tech. A high percentage of the RV fleet operates with multi-grade oil, electronic ignition, and improved cranking components. There is no significant oil viscosity or lubrication problem with 15W, the EI produces full spark energy at near zero RPM, and users rave about their LiFePO batteries.

So, for purposes of discussion, set those three aside. What specific mechanical issues are addressed with preheat?

I'll offer an example. Consider the effects of CTE (coefficient of thermal expansion) on a cylinder. An aluminum piston in a steel bore means clearance gets larger when cold and smaller when we preheat. Specifically, given a 5.125" cylinder, a 60F temperature delta (70-10) only changes clearance a bit less than 0.002". Starting with increased clearance this small is no big deal, so is preheat really necessary for cylinder health?
 
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At freezing temps, you could have an “almost start” then the plugs freeze over from the little moisture from combustion.. then you almost HAVE to preheat. I don’t like seeing the oil pressure climb really slow.
 
DanH said:
Starting with increased clearance this small is no big deal, so is preheat really necessary for cylinder health?
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Isn't the thought there that the aluminium piston will initially heat and grow faster than the steel liner, reducing clearances?

Isn't the other concern (not cylinder related) the contraction of the case around crank bearings, with cold aluminum creating tighter bearing clearances? Arguably, that would be a bad thing at some point.
 
I have Slick Mags and a 680 battery. If I do not pre heat, it is difficult for me to start below 20 degrees F. If I just heated the battery it would probably make a huge difference. My truck, tractor, or car are not that thrilled starting in the cold either. I am really interested in the "why" and if there is a need.

I am new to taking care of an engine and I pre heat because logically it appears to be a good idea but Dans point about 15-50 oil sounds logical too. I have a 5" box heater ducted into one on my inlets and the AS oil heater in the oil. I will continue to heat until someone can convince me it does more harm than good.

The more I read, the more confused I get! I just added a dehydrator and I just hope it helps save my engine if I dont fly for a week or two. I would like to get to TBO but mentioned that and was told most RV pilots will never get that many hours so why worry. That makes no sense to me unless I plan to trash my plane when I am done flying it. I guess we just do the best we can.
 
From Lycoming's Knowledge Base article "Operating in Cold Weather"....

"For most Lycoming models, preheat should be applied anytime temperatures are at 10˚ F or lower. The exception to this rule is the 76 series models that include the O-320-H, and the O/LO-360-E. These engines should be preheated when temperatures are below 20˚ F. It is recommended that these guidelines be followed even when multi-viscosity oil is being used. In addition to hard starting, failure to preheat the entire engine and oil supply system as recommended may result in minor amounts of abnormal wear to internal engine parts, and eventually to reduced engine performance and shortened TBO time"​
 
With EFI, a hot electronic ignition, a dedicated starter battery, a modern gear reduction starter and 15/50 oil, I'm pretty far from the 1950's paradigm. But even with relatively mild winter temps I see in my hangar, there is a very definite improvement in cranking behavior if I have thrown a little heat into the oil. For that reason alone it is worth it for me to pre heat. The other is that I spend less time on the ground waiting for things to warm up, so I can be flying quicker.
 
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I love this line of discussion. Perhaps we can bury another OWT.

I need to see more than 58F on my oil temp "gauge" since that's the lowest it reads. I want to see it move in case the it's not working. The fastest way to do this on cool mornings is to preheat. It rarely goes much below freezing where I live so two 100w electric personal heaters gently placed on the cowl exit ramp connected to a remote control gets the job done in a couple of hours - I just start it before bed for an early flight.

Engine Heaters from below.PNG
 
A great question and discussion!

I'll throw out this quick response and then give it some real thought...

The crankshaft "floats on a cushion of oil". In a perfect world the crank journals and bearings never even touch. Anything that helps create that perfect world is a good thing.
 
Don't know if you were responding to my comment, (camshaft) relies on splash lubrication from rod bearings and oil fog from the general mayhem going on in that section of the case.
 
Kyle Boatright said:
Isn't the thought there that the aluminium piston will initially heat and grow faster than the steel liner, reducing clearances?

Isn't the other concern (not cylinder related) the contraction of the case around crank bearings, with cold aluminum creating tighter bearing clearances? Arguably, that would be a bad thing at some point.
Click to expand...

That is the rationale for my comments. How much heat is needed for safe bearing and cylinder clearances? I don't know but I assume if the engine is as warm as it would be on a nice spring day we are good to start. So I hook up the sump heater, get the entire engine warm....and don't worry about it.
 
A Very Good Question!

Dan - I agree with you that this is a great question to ask…are we just relying on old information and wive’s tales, or is there a sound basis?

I guess I am partly relying on my experience growing up in the frozen north, where it was not uncommon to drain the oil out of the Cub at the end of a flying day, heat it on a stove in the morning, and pour it back in before flying. Old days, old engines, old oil…..

I also rely on a bit of my own experience with engines over the years. For instance,. Louise’s RV-6, when we lived in Texas, had ignition issues when it was trying to start in the winter after sitting on the ramp in College Station. If it was above 40 degrees, no problem - below that temp (if it had sat all night) the impulse coupler didn’t want to snap. Old Slicks - we replaced them, and things got better, but 40 was still sort of the magic number we used, and it agreed with the old Lycoming manuals. Maybe the lubricant on the impulse couplers got stiff….

I do know that if I am pouring in a quart oil 15w50 that has come from inside the heated shop, it empties into the engine WAY faster than a quart that has been sitting in the cold hangar - so even the modern oils get pretty stiff when they are cold. A warm quart is probably three or four time faster than a cold quart.

That’s all anecdotal, with some reference to old manuals. So yeah - I’d be happy to have some real engineering data as well! I just flew across the Sierra for breakfast in the -3. I had a 150 watt shop light under the oil pan all night, and while the oil was nice and warm(ish), the PC680 battery back behind the seat was less happy than after the plane had flown and then sat on the warm ramp in Auburn while we ate - the subsequent start was better. Warm batteries crank better than cold ones.

I can tell you that I also buy into the Lycoming party line of “the engine is warm enough for flight when it can take full throttle without stumbling” - which is at a surprisingly low oil temperature that would frighten many people here on the forums…. ;)

How you gonna do an experiment on this one Dan?

Paul
 
Things have changed with the multi-grade oils.
When I learned to fly in an old six cylinder C172, it had an oil dilution pump.
It was used prior to shut down, pumping fuel in to the oil pan to "improve"
the viscosity of the oil befor next start up.
If it was very cold we removed the battery and brought it inside.
With multi-grade oil and lithium battery, starting is not a problem.
Starting and take of can be done hard and brutal, push to throttle forward as soon as the engine runs without faltering.
If it´s 0 F, this is on the border to unsafe.
When it´s relay cold I preheat as best as I can.
Then I start and let it run on on low rpm for 5-10 min and then shut it down.
I wait 10 min before starting it again to let the heat from the cylinders make it´s way to the colder parts of the engine.
When I´m asking the the engine to deliver full power for a safe take of, I would like to think that I have done my part to make it safe.
 
Ironflight said:
I can tell you that I also buy into the Lycoming party line of “the engine is warm enough for flight when it can take full throttle without stumbling” - which is at a surprisingly low oil temperature that would frighten many people here on the forums…. ;)
Paul
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I had an experience years ago departing RTS (Stead) on a very cold morning in my Citabria. AEIO-320-E2D. The engine started fine cold, and it is a fairly long taxi down to the departure end of 25 (now its 26). I did a quick run-up and then departed. The engine took full throttle without hesitation and off I went. Until I turned cross-wind. At that moment, the yellow low-pressure light came on my oil pressure gauge and got my attention. I pulled the power back to idle, turned for the runway, and landed back on 07 (its 08 now). By the time I looked again after touchdown, the oil pressure was normal again.

My hypothesis at the time was that the oil in the engine got warm enough to close the Vernatherm and the oil in the oil cooler was very cold. So when the Vernatherm closed, the oil pressure dropped severely because of the very reduced flow through the cooler.

I don't know that that is what actually happened, but that is what I thought at the time. No harm done apparently, we ran that engine to 3200 hrs before needlessly overhauling it.

Since then, I have always waited until my oil is up to 125F before departure. By that point, there is enough warmth inside the cowl that the oil cooler should be getting warmed up some too.
 
Kyle Boatright said:
Isn't the thought there that the aluminium piston will initially heat and grow faster than the steel liner, reducing clearances?
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Preheat makes piston to wall clearance smaller.

Isn't the other concern the contraction of the case around crank bearings, with cold aluminum creating tighter bearing clearances?
Click to expand...

Quantify to illustrate. Let's start with 0.0011", the book minimum clearance for a thick wall bearing. We'll assume the dimension was taken at Standard Day temperature, and call it 60F to make the numbers easy. Further assume the steel bearing shell does not resist a diameter reduction when we lower the case temperature to 10F. Clearance will reduce to 0.00026". The example is extreme, but offers some support for preheat given there may be engines assembled at 0.0011". Middle of the clearance range (0.0011 to 0.0041) would be 0.0026". Clearance would be 0.00176" at 10F.

Main bearing clearances may be a concern in the Arctic, but the numbers don't suggest a need to preheat at 35 or 45F, as many seem to do.

BTW, here's a chart from the Aeroshell book. I've removed all the oils except 100W and 15W-50. At low temperature, the 15W-50 exhibits about 10x less kinematic viscosity, but they're identical at 212F or so. Can't imagine why anyone would use 100W when it's cold outside....or hot for that matter.
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Why do I preheat?

Well, easy answer really :D
Gone past a certain age, say 60+… do you jump outta bed and get into your running outfit, then go direct WOT into your 10 mile jog?
No? Then why should your engine designed almost 100 years ago do so?
Yes, to assure good lubrication. To all parts of the body, or engine…

PS
My bird’s happily lives in a heated hangar, so no preheat, pull her out, quick run-up, and off we go. 75ºF oil T being more than enough… let’s burn that gas flying :cool:
 
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DanH said:
Preheat makes piston to wall clearance smaller.
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I think that would depend on if you did a quick preheat or let the entire under-cowl mass (minus the hot spots on the sump) heat to 100F or wherever it stabilizes. That's your generic "Airplane sitting in a sunbaked metal hangar the Summer" situation.

Cold starts, where 10F Aluminum pistons are exposed to combustion temperatures, should briefly result in higher delta T (and smaller piston/cylinder clearances) than a start where the piston and cylinder are both at 100F.
 
What’s really interesting to me from the oil viscosity chart Dan posted in Post #15 is not so much the difference between 100 and 15w-50, but the difference in viscosity with temperature of the 15w-50. The viscosity is 40 at 70 degrees, but three times thicker (120) at 40 degrees, 300 at 20 degrees, and 500 at 10 degrees.

So if we don’t preheat until it’s colder than 40 (typical Nevada warm winter mornings), we’re still seeing three times thicker oil than at 70 degrees (Alabama temps). And if of we don’t preheat at my old Minnesota temp (a warm winter morning) of 10 degrees, it’s more than ten times as thick as at 70 degrees. That’s a pretty big difference….. and a really useful table!

Paul
 
I preheat simply because my fuel system has a carb and no primer. The accelerator pump, while cranking is all normal after 30 mins of 1500 watt preheat ducted spaceheater.

I typically give it 90 minutes to try and get more heat soak. I do this once in the 40s.


30AH SLA battery- no preheat needed to crank down to the 20s for cranking power I have started fine, cold under 30 when needed, just two to four 1/2 throttle pumps after 4 blades.

Just seems worth the time and effort. Phillips Victory AW 20/50 oil, field wound starter, mags, carb, unheated hangar.
 
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Ironflight said:
What’s really interesting to me from the oil viscosity chart Dan posted in Post #15 is not so much the difference between 100 and 15w-50, but the difference in viscosity with temperature of the 15w-50. The viscosity is 40 at 70 degrees, but three times thicker (120) at 40 degrees, 300 at 20 degrees, and 500 at 10 degrees.

Paul
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I did an oil change in the <relative> cold a few weeks ago. The new oil poured out of the bottles like molasses. It was a good reminder of how thick that stuff becomes at near-freezing. Camguard, on the other hand, poured pretty freely. I need to look up the viscosity of that stuff...
 
Electricity is cheaper than avgas. If you're going to idle the engine longer than normal to warm it up, might as well have done that warming with electricity and leave the gas in the tanks.
 
Dan 57 said:
Why do I preheat?

Well, easy answer really :D
Gone past a certain age, say 60+… do you jump outta bed and get into your running outfit, then go direct WOT into your 10 mile jog?
No? Then why should your engine designed almost 100 years ago do so?
Yes, to assure good lubrication. To all parts of the body, or engine…:
Click to expand...

The Lycoming O 145 B2 Horizontally Opposed Engine was introduced in 1938, 85 years ago.
 
gasman said:
The Lycoming O 145 B2 Horizontally Opposed Engine was introduced in 1938, 85 years ago.
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...and the O-145 in my 1940 T-craft WILL NOT start unless above 65 degrees or so. In the summer, its one or two flips of the prop - in winter, my arm is worn out before it will pop.
 
I do a continuous "preheat" all winter. I blanket up the cowl and prop and keep everything under there at 70 degrees all winter. I want to be sure the engine is always well above the ambient dewpoint. I also keep the oil dipstick lifted up and to the side, so that the dewpoint within the engine can move towards equalizing to the ambient dewpoint, which in the winter will be extremely low (un-heated, non insulated hangar).

I've started the engine a few times when the engine was in the 20F range - it didn't sound too happy about it, even with 15-50 oil. Good enough reason for me to prefer starting a 70F engine.
 
AlexPeterson said:
I do a continuous "preheat" all winter. I blanket up the cowl and prop and keep everything under there at 70 degrees all winter. I want to be sure the engine is always well above the ambient dewpoint. I also keep the oil dipstick lifted up and to the side, so that the dewpoint within the engine can move towards equalizing to the ambient dewpoint, which in the winter will be extremely low (un-heated, non insulated hangar).

I've started the engine a few times when the engine was in the 20F range - it didn't sound too happy about it, even with 15-50 oil. Good enough reason for me to prefer starting a 70F engine.
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Alex, what's your source of heat to keep that engine at 70°F? during a Minnesota winter?
 
I'm not so sure we should set aside the 3 issues you mentioned from this discussion. Battery power and ignition, ok, I'll go with that but not lubrication. It's not 15wt oil but instead, 15W/50 and there's a big difference. I think lubrication is the biggie in this discussion.

Here's my anecdotal experience... The only time I have preheated an engine is when my wife and I took our air-cooled VW bus on a 5 month / 15,000 mile trip across the country in the winter of 04/05. I became intimately familiar with operating that engine in various temperatures and altitudes. We were using 10W/30 and straight 30 Wt oil and I changed it prolly 5 or 6 times on the trip. When the temps got down to single digits, I was forced to buy an oil preheat device and it made a huge difference in overcoming internal engine loads. The other thing is, low oil temps rob horsepower and I can say that with certainty. When all you have is 65 HP you definitely notice the difference and I had good instrumentation on board. I struggled through most of the trip trying to raise oil temps.

I think it's all about the oil and warm oil does a better job in every respect than cold oil does. Good discussion!
 
Ironflight said:
.. and a really useful table!
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Here's the useful part. 15W-50 at 32F has approximately the same viscosity as 100W at 70F. Flow to the bearings (the lubrication concern) is the same. Viscous drag (the starting issue) is the same.
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AlexPeterson said:
I do a continuous "preheat" all winter. I blanket up the cowl and prop and keep everything under there at 70 degrees all winter.
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If I recall correctly, corrosion rate doubles for every 10C increase in temperature above 0C...and below 0C, inland corrosion rate is close to nil. If I lived in Minnesota, I'd store it cold.
 
DanH said:
Here's the useful part. 15W-50 at 32F has approximately the same viscosity as 100W at 70F....

....Viscous drag (the starting issue) is the same....
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Chart or no chart, that certainly has not been my experience.

There must be more to the warm vs cold oil than viscosity that makes the engine spin easier on the starter.
 
R2600 engine

Since I have operated these engines, I might have an opinion: we always turn the engines over at least 9 blades on every start. No heat is used. We changed out one engine as it was quite old (in the big engine world) - it was happy at that point (it always ran). I noticed no difference between new engines and old engines. Using our standard system getting the things going, when everything was serviceable - the engines always started and ran. Of course there are different mechanical things that might have helped ‘em NOT starting, temps in the teens did not bother ‘em, and the screens were always clean. We did not ad power until the oil was warm - all ~32 gallons of it. We idled the engines at 1000 or so, and they happily clanked along to min temp. Normal idling speed was about 1000RPM until 100F on the oil temp gage.

If I were to cold start my 550 in the Rocket below in the teens, I would turn it over at min 6 turns, then turn it over and let ‘er rip as a normal cold start and warm it up until 100F.

I did have a problem once on one engine on the B25 - the dang spark plugs plugged up with ICE and we had to get heat to get that one engine running. That reminded me of the small tractor we had when I lived up north - it iced up just like the dang R2600 did. The crew had to take the plugs out and warm ‘em up and we did get going that day.
 
MacCool said:
Alex, what's your source of heat to keep that engine at 70°F? during a Minnesota winter?
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Howard - I use a Twin Hornet 22 stuffed in the oil filler access door, blowing downward. I also have a Reiff system, which I used for many years, utilizing a line break thermostat on the end of a power cord. I stopped using it simply because the constant cycling on/off of the sump pads tended to loosen them over time.
 
DanH said:
If I recall correctly, corrosion rate doubles for every 10C increase in temperature above 0C...and below 0C, inland corrosion rate is close to nil. If I lived in Minnesota, I'd store it cold.
Click to expand...

You are recalling the Arrhenius equation correctly. If the temperature were more stable, I'd agree with you. But, the day/night temperature swings of the inside of the hangar are quite large. On a clear, calm night, the inside temperature of the hangar might be 5 to 10 degrees F lower than ambient, which often puts it below the dew point. On sunny days, by mid afternoon the hangar temperature might be 30 or more degrees warmer than ambient. Keeping the engine temperature at 70F during the winter when the dewpoints are in the 20F and lower range just doesn't seem like trouble to me. Water vapor remaining in the engine after shutdown should tend to migrate out of the breather and oil filler tube to the much lower ambient dewpoint. Obviously I have no data on whether any of this matters a whit...

I also rarely fly unless I can get the oil temps up in the 185+ range for a half hour minimum.

Of course, the main reason is that I like maintaining the temperature is that I want the airplane to be available at a moment's notice when the whim to fly hits, without wishing I'd turned on the pre-heat a bunch of hours prior. There are many days when the temperatures are below zero F overnight around these parts...

There was no sign of corrosion inside the engine after 10 years of doing this when it was overhauled in 2011.
 
Taltruda said:
At freezing temps, you could have an “almost start” then the plugs freeze over from the little moisture from combustion.. then you almost HAVE to preheat. I don’t like seeing the oil pressure climb really slow.
Click to expand...

water and therefore ice is a conductor. Ice on a sparkplug should provide no issue in creating a spark to ignite the fuel air mixture.
 
Toobuilder said:
There must be more to the warm vs cold oil than viscosity that makes the engine spin easier on the starter.
Click to expand...

The battery is also temp affected. THe colder it gets, the less current it delivers and it is a pretty steep curve. This is why they created a unique cold cranking amps specification in the 70's to help buyers understand how effective the battery will be in cold climates. The oil pump is a pretty small load when considering what is required to spin an un-started engine. Usually the bigger issues are starter and battery inefficiency. Not saying thick oil isn't a hinderance, just that it is not as significant as most think.
 
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Kyle Boatright said:
What happens when it bridges the spark plug gap? That's the problem - a shorted plug.
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While that is true, it would seem impossible to build .018" of ice in the few seconds of running that the poster suggested. He said that it would spark and start, but THEN freeze over from combustion moisture and stop running. Remember that each and every combustion event creates ~2000* gas and it would seem quite improbable for ice to form in this environment. The water vapor in that combustion gas is also 2000*. Don't care how cold the plug is, water vapor at that temp is not going to form ice.
 
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Oil pressure

My brother had a built VW when I was a kid. Dry sumped and everything. That thing needed to be warmed up at idle not because it was cold outside, but simply because the oil pressure would be too high when rev’s up.
 
lr172 said:
The battery is also temp affected. THe colder it gets, the less current it delivers and it is a pretty steep curve...
Click to expand...

Not applicable to my situation. My starting battery is well behind the firewall and not influenced by my oil pre heat method.
 
Toobuilder said:
Chart or no chart, that certainly has not been my experience.

There must be more to the warm vs cold oil than viscosity that makes the engine spin easier on the starter.
Click to expand...

Lubricity vs viscosity. Improved lubricity is the reason I use synthetic oil in all of my vehicles and winter rolling stock. Easier engine turnover at -20° F (and below) is the main effect I'm looking for.
 
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AlexPeterson said:
You are recalling the Arrhenius equation correctly.
Click to expand...

When they dug Glacier Girl out of the ice, the crew was able to fire the guns. Low temperature storage does wonders.

Go to 36:40 https://youtu.be/OeXy0IdCELk

Keeping the engine temperature at 70F during the winter when the dewpoints are in the 20F and lower range just doesn't seem like trouble to me.
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We need only remain a little above the dew point to stop condensation. Typically, doing so requires only ambient or a bit more. Next 10 days in Minneapolis, below.

And remember, air at 20F or 70F can have the same water vapor content.

There are many days when the temperatures are below zero F overnight around these parts..
Click to expand...

Can't knock it. Returning to topic, at zero F, the preheat requirement appears to have some physical basis. Lycoming says preheat when less than 10F. But at 32F, or 45F, with multigrade, EI, and adequate cranking systems? Is there really a need?
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Dan, the days circled in blue are the concern. The overnight temperatures are cold, and then a rapid rise in dewpoint the next morning/day before the engine can warm up. I've seen frost all over the plane (inside the hangar) on days like those. (I think you mentioned a while back to use a fan to try to minimize this.)

It would be interesting to plot the engine internal temperature, hangar ambient temperature and dewpoint.

Glacier girl was happy perhaps because of a lack of oxygen, not just the cold.
 

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AlexPeterson said:
Dan, the days circled in blue are the concern. The overnight temperatures are cold, and then a rapid rise in dewpoint the next morning/day before the engine can warm up. I've seen frost all over the plane (inside the hangar) on days like those. (I think you mentioned a while back to use a fan to try to minimize this.)
Click to expand...

Gotcha. Yes, sometimes we get a few days of cold air from the Midwest, then a warm wet flow from the Gulf. Circulating air keeps the airframe surface temperature close to ambient.

It would be interesting to plot the engine internal temperature, hangar ambient temperature and dewpoint.
Click to expand...

Yes indeed.

Glacier girl was happy perhaps because of a lack of oxygen, not just the cold.
Click to expand...

Umm, yeah, about that open dipstick... ;)
 
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DanH said:
And remember, air at 20F or 70F can have the same water vapor content.
.
Click to expand...

Hold On!

Made me dig out my Thermo text, pretty dusty, its been a while,

Consulting the standard psychrometric chart,

100% saturated air at 20F has the same amount of water vapor as 17% relative humidity air at 70F. Both about 18 grains of H2O per lb of dry air.

So, yes, air at the two temperatures can have the same water vapor content, but they behave profoundly differently.

100% saturated air at 70F holds 108 grains of H2O per lb of dry air.
 
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rmarshall234 said:
Here's my anecdotal experience... The only time I have preheated an engine is when my wife and I took our air-cooled VW bus on a 5 month / 15,000 mile trip across the country in the winter of 04/05. I became intimately familiar with operating that engine in various temperatures and altitudes. We were using 10W/30 and straight 30 Wt oil and I changed it prolly 5 or 6 times on the trip. When the temps got down to single digits, I was forced to buy an oil preheat device and it made a huge difference in overcoming internal engine loads. The other thing is, low oil temps rob horsepower and I can say that with certainty. When all you have is 65 HP you definitely notice the difference and I had good instrumentation on board. I struggled through most of the trip trying to raise oil temps.

I think it's all about the oil and warm oil does a better job in every respect than cold oil does. Good discussion!
Click to expand...

So was that you that I had to pass on Hwy 120 heading west from Lee Vining toward Tioga Pass? Geez, there ought to be a law.....
 
DanH said:
Here's the useful part. 15W-50 at 32F has approximately the same viscosity as 100W at 70F. Flow to the bearings (the lubrication concern) is the same. Viscous drag (the starting issue) is the same.
.
Click to expand...

Just to be sure, the purple curve on your chart for 100W is for SAE 50 wt oil, as in Aeroshell 100w, right? (not to be confused with SAE 100 wt oil).
 
scsmith said:
So was that you that I had to pass on Hwy 120 heading west from Lee Vining toward Tioga Pass? Geez, there ought to be a law.....
Click to expand...

yeah, sorry...it didn't take me long to figure out to just keep the curtains of the back window closed. When my wife asked, I said " I know they're pissed off, I don't need to see it too". :D
 
scsmith said:
100% saturated air at 20F has the same amount of water vapor as 17% relative humidity air at 70F. Both about 18 grains of H2O per lb of dry air. 100% saturated air at 70F holds 108 grains of H2O per lb of dry air.

So, yes, air at the two temperatures can have the same water vapor content, but they behave profoundly differently.
Click to expand...

Let's note they can have the same water content and not be saturated, for example, 15 grains.

In the context of engine storage, either 15 grain air sample will condense obvious liquid water on metal with a surface temperature of about 19.6F. The dry bulb air temperature (20 or 70) doesn't matter. If Alex keeps his metal parts above 20F or so, neither will condense obvious liquid water on those parts.

Water in vapor phase also contributes to corrosion. Here I definitely need more reading, because it appears there are complicated interactions between three major factors...relative humidity, atmospheric pollution, and temperature.

Roughly, relative humidity needs to be high (like >75%) in order to deposit a very thin layer (~1um) of water on a metal surface. The number goes up for a polished surface and down for a rough surface. At first glance, that would not happen with warm air, a warm surface, and 15 grains of water per lb.

However, salts and other atmospheric pollutants deposited on the surface are huge wildcards. I've attached a snip below, from an NPL paper. Note the 35 RH for MgCl2. It's the "salt air" problem, corrosion which does not involve obvious water droplets.

Temperature we already talked about. Warm corrodes faster.

What am I missing?

Just to be sure, the purple curve on your chart for 100W is for SAE 50 wt oil, as in Aeroshell 100w, right? (not to be confused with SAE 100 wt oil).
Click to expand...

Yes. Original chart below.
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