OK guys. I've been wondering about this for a while, and I hope a knowledgeable person(s) will step up and educate an ignorant oil person like me:

If the thermostat in our trucks is set at 194* (or so), why does it matter if the ambient air temperature we operate our trucks in is 20* higher, like say 100* vs. 80* ?

I understand how cold affects oil flow - if it's really cold, the oil is slower to flow, until the engine warms up.

But if it is hot outside (100* let's say) the engine is still 94* hotter, right?

Does the 20* ambient air difference between 80* and 100* REALLY affect the oil's critical properties, when it is usually operating at a minimum of 194* anyway ?

I'm not being a wise guy, I really want to know. It seems to me that the ambient air temp is less meaningful towards the hot side than it is towards the cold side, with respect to oil flow and lubrication.

As far as towing goes, I don't get that either. Lubrication in a modern engine is more dependant on FLOW and pressure than thickness. The more time oil spends in the pan (as opposed to in-between bearing surfaces) the better off the engine is. As long as you can maintain proper pressure for a given engine, thinner oil is almost always better. The loads presented by "proper" towing could not possibly require a heavier oil, could they?

Think of it this way: The Lightning boys put their engines through some serious stresses, right ? Going by what I can figure, using the most often quoted thought process on this board, Sal should be running straight 100 weight oil in his L. Would anyone disagree that his engine is subjected to WAY more stress than any regular F-150 pulling a 5,000lb. trailer ?

Just wondering...

OK. Kick my a$$ !!!!

Let the flames begin !!!!!

 

I think the confusion is that the air temp somehow effects the oil performance. Actually, hot air effects the engine performance. Hot air is less dense. This requires the engine to work harder to do the same amount of work. Working harder is harder on oil, the air temp is not changing oil viscocity as much as the engine is working harder to get the same horse power from hotter air. ( thats why intercoolers were invented and water injection works to eliminate preignition.) The oil experts on this board might be aware of some molecular change from running in hotter air but the way I have always been trained this is the reason.

 

You need to go to this website and ask that question;
www.bobistheoilguy.com
My head is still spinning from that question!

 

Come on, guys...

I expected way more activity than that !!

Not looking to start a war, just want to know what the general thought process is...

 

what is your question? im really lost. Ambient temperature should only really affect you during startup and the process to getting hot. Once your hot, thats it, the other end of the oil weight is whats important. However making oil that goes from say 5w to 100w isnt easy i dont believe. You can make 100w but the start weight would have to be in the 60-80 range at best. This is just an educated guess, nothing for sure, like the previous member posted, seem like a job for bob the oil guy.

 

I'm going to go with Pestcol's understanding of your question, and say that you are right in saying that the demp difference does not make a difference to the engine. The thermostat will maintain 194 no matter what the ambient temperature is, even if it is -40. The difference being two things, one the time it takes the engine to warm up to 194, and the hp needed to drive the rest of the drive train that will be affected by the ambient tempature.

The reason the truck has less power is due to less dense air. When gas burns there is a chemical reaction between gas and air producing a byproduct of CO2 and water. I may need some help on the chemistry here. COH + O2 burns producing CO2 + H2O.

You need the right mix of gas and air for the perfect burn. If the air is dense the computer will respond by injecting more fuel into the cylinders. As the air gets less dense on a hot 100* day you get less air compressed in the cylinder and the computer cuts back the fuel. With less gas/air mix you get power when the plug fires.

I don't think the engine works any harder because it does not have the air/fuel, and is not working hard enough. That's where the super charger comes in, but even that is forcing less dense air into the cylinders and will be affected.

 

Now on the question of towing & racing.

The problem with towing and racing is you are pushing the engine to work hard enough that cooling becomes an issue. That is why people run a 20w-50 oil in an engine designed for 5w-30.

If you run at WOT the oil can heat over the 194* mark. When that happens the oil continues to thin out. If it thins out enough it will thin out of the viscosity range. Say your engine is designed for 30w and it thins down to a 20w. It's now too thin and may not be able to support the bearings.

Race cars run a 50w knowing it will thin down to a 30w and the oil is where it needs to be. I forget the temperatures, but as I recall over 230 is where they start to get concerned.

Also, it's not as much of an issue for 1/4 sprints as it is for a 200 lap race.

The same problem applies to towing. Granted it is not advised to tow at WOT, but towing is more like a 200 lap race where the engine works hard for long durations, and it is possible to run at WOT up a hill for a long period of time.

Keep in mind, that the temp inside the cylinders is well over 200*. I saw the numbers posted once and I remember the top rings close to 1000*, and the valves over 1000*.

Did you see the comments about the dragsters that run so hot they have to have special plugs so they do not burn up?

That would be interesting if I can find it again.

 

this is an extremely interesting question and well written i might add. i've wondered about this as well. intuitively, it seems to me that the engine might experience higher oil temperatures on a hot day or when towing for extended periods. the engine is definitely generating more heat when towing because it is working harder. much of that heat is managed by the coolant and the thermostat as you suggest. but some of that heat has to get transferred to the oil as well. does the coolant also manage oil temps? probably; but i'd bet not totally.

we're towing a pop-up tent trailer from minnesota to utah in september. i've been using amsoil 5W-20 for 30M miles. i'll have to confess that i'm switching to amsoil 5W-30 for the towing vacation; for the very reason that i'm expecting higher oil temps from the towing, heat, mountains, and extended drive time.

interestingly, the ford service manager has indicated that the engine will knock if i put in the heavier oil but i don't believe a word of it. i've never heard of an engine knocking because of a slightly heavier grade of oil.

 

I have never heard that an engine would knock with thicker oil either. I think the guy is confused. If it knocks it is because of detonation, sometimes moving up to the next octane level will cure that.

 

Never heard of one knocking with thicker oil myself although I have heard some valve trains quiet down a little after switching to the lighter oil.

Another thought I have on the heavier oil would be heat would take longer to dissapate. It could be possible that engine temps could increase, but should stay within the range of the cooling systems ability to cool.

tom

 

My question was related to the fact that MANY people on this board quote that they live in so and so place and it is over 100* in the summer, so they run 5W-30 or 10W-30 or whatever, in the belief that it is better for the engine than the 5W-20 Ford specifies.

I was simply inquiring why these folks think it is better, if the engine's temperature is set by the thermostat?

 

I'll take a stab at this question.

Does the thermostat really keep the engine operating at one temperature? Ie, the thermostat stays closed until the antifreeze hits that temp, then opens keeping it there?

Not exactly. The thermostat allows faster warmup of the anti-freeze and maintains the operating temp of an engine ABOVE the set point, not at it. Ie: if th temp is above the set point, antifreeze moves out of the block and into the radiator to be cooled. That doesn't mean the antifreeze is that temp - it can be much warmer than that.

It goes to the radiator and the heat is removed and then the antifreeze is returned to the engine, where the heat exchange process repeats.

The fixed piece in the system is the radiator size. It can cool only so much based on the airflow over it since it is fixed in size. However, the temp of the air going through it varies with outside temp. At higher outside temperatures, the effciency of the radiator is diminished since there is less differential between the hot anti-freeze coming through the radiator and the air gong over/through the fins. Thus the antifreeze returning to the engine is not as cold, meaning it doesn't cool the engine as well, leading to higher operating temps in the engine - which means hotter oil.

I think we get insulated from the fact that temps in the cooling system really do fluctuate much more than the guage you see in our trucks. If I recall, our gauges barely move so long as the temp is within about a 50 degree range.

My old Jeep has real guages in it, and depending on outside temps and driving habits, the temps vary quite a bit - all with a 195 thermostat.

Short story is that thicker oils were generally more resistant to breakdown at higher temps. Engines operated in HOT weather generally do run hotter, and thus an use the additional protection.

Modern oils have closed this gap, but there are still good applications for thick oils.

 

Oh, I see what you are saying now. Why is it more important for those who see 100* days in the summer to run a 10W than it is for those who only see 80* days when the difference is only 20* and the engines run at 194 in both instances.

I would take this angle on it. If you start your engine in a city that sees a lot of 100* days then you have none, zero, nada, cold start engine issues. That's a warm start. The engine is up to operating temp as fast as you can roll the window down, and back up again.

On a cold morning, what is the temperature, 72? It is not necessary to use a 5w or 0w for a cold start at 72*. So you might just as well run a 10w-30 which has less viscosity improvers and will hold it's viscosity better, and be more durable in the long run at dealing with the heat.

Granted it is true that most engine wear happens at start up, but that 10w is going to flow much better at 72* than 0w flows at 32*, and we have truck owners on this site who live in cities where 32* is a hot summer day. There is at least one on this site who knows from experience the coldest temp our visor thermostat will display.

As for oil temp difference in different temp ranges I don't know. Good question. If two trucks start in St Louis and one drives to Texas and the other to North Dakota will the oil in the engine going south be hotter than the one going north?

As I understand the thermostat attempts to hold the engine's temp at 194, and the oil will be close to that temp, but at WOT you are pumping a lot of BTU into the oil and it will heat up.

I think one option I would like to see offered on our trucks is a set of gages, oil temp, trans temp, coolant temp.

Back to the point of the question. If you only see 80* temps in the heat of summer, it is more likely your oil will remain in the engine untill the cool days of fall when the 5w flows a little better providing better protection on a cool start. I think the same theroy applies in revers. Those who experience extreme heat know that 80* is not a lot of heat to deal with.

 

Interesting question and answers. If I understand the question correctly as to the 20 degree temp differential being critical, the answer is no in most cases. Most folks assume that all Xw-30 viscosity oil is the same thickness- it is not. Some are just barely a 30w at temp and some are almost a 40w at temp but are packaged as a 30w because it falls in between the allowable viscosity at temp. A 30w oil can be a minimum of 9.30cST all the way to 12.49cST. Most 5w-20s are right at the maximum threshold for a 20w which is 9.29cST. Motorcraft 5w-20 will almost always test 8.90 to 9.10 which means it is almost a 30w.

The word flow. Too many folks have no clue when using the word. I love the statement that synthetics flow better. Really? If you have 2 liquids of equal viscosity at say 20 degrees, which will flow better? Synthetic oil or concrete? They will flow at the same rate. Obviously, the synthetic oil will have better pumpability characteristics but they will FLOW the same. Flow and pumpability are not the same thing. When looking for a winter oil, check out the cold start pumpability rate, not the flow rate as it is virtually meaningless.

In regards to towing. Most folks use a heavier oil because it provides better protection- right? Maybe in your daddys day but todays motor oils offer much, much more protection than your dad ever had available to him. Old habits are hard to break so most folks will continue to use the heavier oils. Will they do any damage to their engines? No, but far too much emphasis is put on viscosity. Going to a heavier oil is only going to effect the gas mileage. Todays lighter oils will give you all the protection you will need and because of the spec concerning 5w-20, it will also run cleaner as there are less allowable deposits which results in less engine wear.

 

Labnerd, good point. I was concerned that my using 20w in my example would be related to the use of 5w20 which was not my intention. Thanks for clearing that up.

I do agree with your point on "flows better" and "pumpability." I think the term "flow rate" comes from an attempt to explain concepts in terms that seem more understanable to someone with average knowledge of oil and oil related terms.