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  #1  
Old 26th May 2008
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Default My last word on engine temperature

Hot is Good

Iím going to try to explain, once and for all, why you want to run your engine at the highest temperature that is sustainable. Every year, there are a hundred threads about thermostats, fans and overheating.
Thereís a lot of advice floating around the internet. Much of it is contradictory, much is based on the writerís experience. Iím going to try to explain this scientifically. As with all my advice and suggestions, itís your car, do what you want, I know mine will work tomorrow morning.

Heat Engine

The Otto cycle engine is one type of heat engine. A heat engine converts thermal energy into mechanical energy. It does this, adhering to the first law of thermodynamics, by exploiting the difference between high and low temperatures; the greater the difference, the greater the available power. Power = efficiency whether you use the power for the drag strip or fuel economy.

Since your heat engine exploits the difference between hot and cold, you get an increase in power when you increase the operating temperature of your heat engine. If your engine is very cold, you need a deliberate enrichment device to pump extra fuel into the heat engine. If you have fuel injection, the enrichment is a continuum, unlike a choke mechanism that shuts off, your ECU will guaranty that youíll have the enrichment needed for the temperature youíve chosen.


The heat transfer in your engine should be progressive. It is about 2000f in the combustion chamber 1500f at the back of the exhaust valve. 600f at the combustion chamber surface, 350f at the coolant passage in the head, 195f at the thermostat and 185f in the radiator. This assumes a normal 195f (89c) thermostat. A lower temperature thermostat will allow more coolant flow and suck more heat out of the combustion chamber. Note at this point that you can remove more heat from the combustion chamber (and buy the gas to do it) without increasing your carís total ability to shed heat; youíre just choosing to shed heat, youíre not increasing cooling capacity. The limit of your ability to shed heat is in the surface area of your radiator and the airflow, not in your thermostat (coolant or fan).

MYTHS

Cooler is Better
Actually itís worse, thatís why you warm up your car. The oil doesnít start to work right until itís 180f. Rich mixtures dilute the oil with excess gas. The pistons donít expand to fit the bore until theyíre hot.

Cooler Will Protect my Head Gasket
A 20 degree change in coolant temperature will not make your head gasket last forever. The things that cause head gasket failure are: corrosion, overheating, head warping, and high combustion chamber pressures (lugging, knock). Some say the number of heating and cooling cycles is a big factor.

Safety Margin
Some think that watching the gauge, and having more time to do so, will give greater warning of a problem, and more time to react. Thatís true to an extent. If you lose your belts and the water pump stops turning, you may get an extra minute before the coolant boils if you run cold; but no more than a minute. Keep in mind that the boiling point of a 50/50 mix of Glycol and water, under 10 psi, is around 275f; a long way from 195f.

These are facts about how internal combustion engines work. There is a market for all kinds of things: 82c T-stats and fan switches, coolant additives, auxiliary fans, Water Wetter, hood scoops, ad infinitum. The fact is, if your cooling system is not broken, it will maintain your chosen temperature in arctic or desert conditions. Your cars would all do it when new, if yours wonít idle indefinitely, with the A/C on, in the desert or the tundra it needs to be fixed, not modified.
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  #2  
Old 26th May 2008
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We might as well ask the mods to sticky this thread for the summer. However the same questions will be asked regardless of this thread's existance. I have been asking them for the past couple of years. Despite the OP's constant insisting of these principles. Now that I have more experience under the hood of my car, I understand much better the how and why of certain things. Noobs will always ask the dumb questions anyways.
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Old 26th May 2008
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Simpler explanation. Acceleration requires power, power comes from heat, heat expands the gases inside the cylinder and pushes the pistons down, the cooler your engine runs the more heat is being wasted in the radiator.Put another way, the hotter the cylinders can remain the less fuel is used to re-heat the metal of the engine on the combustion stroke and more heat is available to heat the gases inside the cylinder, which is where the power comes from.

It is often overlooked that the internal combustion engine uses the "waste" combustion gases as the working fluid unlike external combustion engines like steam engines. Originally, steam engines were condensing engines which used steam to create a vacuum when the cylinder was cooled. This was thermally very inefficient. One of James Watt's brilliant ideas was recapturing waste heat from the "condensing" engine by also using the expansion forces available in the steam engine before condensing the steam to create a vacuum. However, it was Richard Trevithick who invented to modern high pressure steam engine which uses even more of the available heat for power by not condensing the steam at all.
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Old 27th May 2008
mike gibbons 1957 mike gibbons 1957 is offline
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MY BRAIN HURTS
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Old 27th May 2008
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We definitely needed a thread like this. Jim always has the wisest thought in mind and has everyone's best interest in mind too! I agree, this thread should be a sticky.
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Old 27th May 2008
mike gibbons 1957 mike gibbons 1957 is offline
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Quote:
Originally Posted by justin.graeff
We definitely needed a thread like this. Jim always has the wisest thought in mind and has everyone's best interest in mind too! I agree, this thread should be a sticky.
sorry i did not mean to belittle thread,just woke up ,all a bit complicated first thing in a morning,,
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Old 27th May 2008
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Quote:
Originally Posted by mike gibbons 1957
sorry i did not mean to belittle thread,just woke up ,all a bit complicated first thing in a morning,,
Oh, that was in no way directed at you! I was merely making the statement that Jim had a great idea about clearing up the heat issue. However, I do agree with you, the first time reading through my mind kind of went in circle
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Old 27th May 2008
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Why don't we get numbers on our temperature gauge? Would they really be that ugly printed there? I kinda wonder about actual numbers sometimes.

Is the mid-point of the gauge intended to correspond to the stock thermostat's rating?

On our newer VW, the temperature gauge climbs steadily to the mid-point during warmup (shown as "190"), then holds exactly there no matter what (In fact, it does this so quickly I am inclined not to believe it). My c900 climbs pretty steady towards the middle, and then depending on what you are doing may fluctuate between 1/3 and 2/3. The 9000 seems to climb fairly quickly out of the blue section, then moves slowly up to an operating point of about 1/3 up the gauge. The best of course are the cars with no gauge at all, and you just get a little light that comes on when your head gasket has already failed.
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Old 27th May 2008
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Numbers would imply that the gauge is accurate which it isn't. It seems many car manufacturers are using an electronic gauge that displays "normal" rather than "actual" temperature to avoid customer complaints that their engine is running too hot.

To use a gauge with numbers you'd have to know what range of numbers was correct. Most people would have no idea.

Modern pressurized cooling systems using glycol water mixes are designed to run from about 90 C to about 120 C (about 190 F to about 250 F) with no problems whatsoever.

As has been pointed out, the hotter you can get the engine to run the more efficient it becomes.
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Old 27th May 2008
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Quote:
Originally Posted by Superaero
It seems many car manufacturers are using an electronic gauge that displays "normal" rather than "actual" temperature to avoid customer complaints that their engine is running too hot.
Doesn't that defeat the whole purpose of a gauge? I don't doubt it though. Like I said, the VW just jumps up to 190 and sits there. I assume it will move if the temp is actually getting close to boiling?

I wonder if people are getting engines confused with electronics...
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Old 28th May 2008
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In 1973-74, when I worked at a Jaguar Dealer, we hated the gauges on the E-types that had numbers.
In traffic, with the A/C on, the V-12 E-types would run at 210 degrees (F) like they were supposed to. Invariably, new car owners would bring their cars in and complain that the temperature was boiling. We explained that it was normal, customers didn't believe us and spent the rest of their lives fearing overheating and telling their friends that "the mechanics can't figure out why my car is overheating".
When the XJS came out in 1975 (US), there were no numbers on the gauge. Nothing else had changed (except for the Fuel Injection instead of 4 Strombergs), but there were no customer complaints.
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Old 28th May 2008
justin.graeff justin.graeff is offline
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Don't most cars today stay pretty much right where they are supposed to be until they are really about to over heat? Don't they do that so consumers don't freak when they see the high needle?

When my Saab goes over half, I used to worry but not anymore!
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Old 29th May 2008
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Correct. I owner a 1986 SAAB 9000 Turbo for 17 years and 200,000 km and it never overheated once while I was driving it.


Modern cooling systems work perfectly as long as they are maintained properly. The temperature gauge can show a faulty thermostat or a cooling fan failing to come on but the numbers on the gauge would be irrelevant. Relative movement of the gauge tells you what you need to know.
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Old 30th May 2008
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Quote:
Originally Posted by justin.graeff
We definitely needed a thread like this. Jim always has the wisest thought in mind and has everyone's best interest in mind too! I agree, this thread should be a sticky.
I added a link to this thread in the FAQs. Thanks jim for taking the time to make this guide, this is much needed information.

Last edited by Stefano; 30th May 2008 at 07:59 PM.
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Old 31st May 2008
Norman Lovie Norman Lovie is offline
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Nice one jim, always thought of it as

Run engine cool = more energy from the combustion wasted trying to heat it up = less usefull work
Plus - combustion efficiency is critical to combustion chamber temp, this is designed for your engine and the fuel you use, so stick to it
Go below it and loose efficency (less of the fuel mix converted to usefull work and otherwise pumped out the ex)
Go above it and risk loosing your engine (if it increases passed the controlable combustion temp of the fuel mix)

Maybe it gets a bit confused when you consider " air inlet temp" - in this case heat is bad
So whilst your trying to keep the engine at its optimum temp, your trying to keep the air inlet temp as cool as possible
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Old 31st May 2008
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I am just curious then, that is the point of a cooler thermostat? I mean they are offered by a reputable companies(the ones which make regular thermostats) for the same price as regulars, therefore there is no incentive for the manufacturer to make a thermostat which is of no use at all, right? From everything which was said above, I understand that thermostat does not increse the cooling system efficiency, therefore it shouldn't matter what climate you live in, right?

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Old 31st May 2008
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Manufacturers make low temperature thermostats and a lot of other useless junk because people buy that stuff.
In the olden days, when alcohol was used as anti-freeze, it would boil off at high temperatures. In those days, people used 160 degree thermostats and had a radiator shade to warm it up in the winter to get more heat. Old habits die hard, they're passed down as folk wisdom. How many times have you heard people with modern fuel injection say that their car won't start right unless they pump the gas 4 times, no more, no less (or other nonsense)?
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Old 31st May 2008
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Quote:
Originally Posted by Jim Mesthene
Manufacturers make low temperature thermostats and a lot of other useless junk because people buy that stuff.
In the olden days, when alcohol was used as anti-freeze, it would boil off at high temperatures. In those days, people used 160 degree thermostats and had a radiator shade to warm it up in the winter to get more heat. Old habits die hard, they're passed down as folk wisdom. How many times have you heard people with modern fuel injection say that their car won't start right unless they pump the gas 4 times, no more, no less (or other nonsense)?
Hey, when I was living in Russia I used radiator shade on one of the trucks in the winter. It did help. It also had fins in front of the radiator which you could close. It did help...I am not sure if the car had a thermostat though.
What I am not understanding is that Saab itself sells two thermostats, 89 and 82 degrees. Interestingly, 82 is cheaper then 89.
http://www.eeuroparts.com/productdet...s=1&code=10654
http://www.eeuroparts.com/productdet...ts=1&code=5892

So, why would saab itself make useless thermostat and sell it at a price lower then the standard if there is no use for it??

They are not going to make anymore money of it, so there is no incentive there. And if you think about it, there are all of the costs which go into setting up a different manufacturing process for the different thermostat. NO matter how low, there is still price that saab pays(or the company which makes thermostats) for the extra variety in their product line, therefore, lower temp thermostat does not make any business sense. The only answers to the above question, assuming the lower temp thermostat is useless, are these:
1) Saab managment is dumb, and developed and still supports a product which doesn't bring any money.
2) There is some conspiracy theory going on, where lower temp thermostat will hurt the cars, and saab will make money on selling the extra parts you will need to fix the car.

Now, I find both of these kind of unlikely.

Don't get me wrong, I understand the logic of what you are saying and think it is right. However, i have no idea why saab would make such products then, which makes the whole picture incoherent. However, if there is a use to the lower temp thermostat, then the whole business model is coherent.


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Old 31st May 2008
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And just looking for parts for volvo, the exact same story, two thermostats, one 87 one 90, the lower temp being cheaper. Now, the stupid managment and conspiracy theories look even less likely.

http://www.eeuroparts.com/productdet...ts=1&code=7830
http://www.eeuroparts.com/productdet...ts=1&code=4380

And for BMW:
http://www.eeuroparts.com/productdet...ts=1&code=9943
http://www.eeuroparts.com/productdet...ts=1&code=9942

Klim
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Old 31st May 2008
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SAAB never approved low temperature thermostats for US spec. cars. There may be applications for other markets that I'm not aware of. SAAB only trained me in US Delivery models.
An aftermarket supplier such as eEuroparts sells all the products that the OEM sells, not just what SAAB recommends (see all the spark plugs, performance parts, etc). You can't assume that if eEuroparts lists the part, it's intended by SAAB to be used on a standard US market car.
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