RECOMMENDED THERM0STAT IN UK CLIMATE

[Retroman]

Hi Folks,

The workshop manual states that a 74 degrees C thermostat should be used in hotter climates and an 83 degrees C in cooler climates.

What are hotter climes? France Spain The Sahara perhaps?

Mine is currently bubbling along at 75 indicated. I have a Smiths non electrical gauge fitted so it must be relatively accurate

Am I right in thinking that the 10 degree higher option would help the engine run cleaner and more efficiently or are XK engines unique in this respect in running at peak efficiency at a lower indicated temp than most contemporary cars....... or does the sender position indicate a lower reading where it is located as opposed to what is bubbling elsewhere in the block etc.?

Appreciate Merek has written a post on temperature reading issues but cannot fathom an answer from it to my question.

What temp are UK drivers running their E's on??

Nigel.

[tinworm]

Once the thermostat has opened it doesn't matter what the rating is and the car will not run cooler because of it. The higher temperature opening thermostat just allows the engine to warm up faster - with the circulation closed off the head temperature will rise rapidly .On the E-type you can see the temperature drop on the gauge - and you know your thermostat is working !

Barrie

[MarekH]

The higher temperature opening thermostat just allows the engine to warm up faster

Err, no. It warms up at exactly the same rate regardless of the thermostat rating because it is the engine that is doing the heating and that is constant in this analysis.

The 74'c 'stat and the 82'c 'stat both behave identically below 74'c because they are both shut and blocking off the radiator. At 74'c the lower rated one will start to open and some of the water will be diverted to the radiator. The radiator (if in good condition) will make the engine run at between 74-88'c but exactly where within that range you are at any point in time is anyone's guess.

The 82'c one will let the engine continue warming up at the rate is was previously until the water gets to 82'c and then it'll behave exactly the same way as the other one did. Your water will settle at between 82-96'c, but again, it won't be obvious where within that range you are at any point in time.

The only thing the lower rated thermostat "buys you" is some warning time if the temperature is going higher than the radiator can cope with. My experience (with a v12) is that the temperature goes up 7'c per minute if you are stationary and at idle with no water going to the radiator. Under these conditions, water flow to the radiator is zero and airflow is at a near zero. (A car running at higher load and/or higher revs generates more heat than at idle so adjust your early warning time accordingly.)

I would consider the Sahara or the south of France in summer to "hotter climes".

So the long and short of it is that you'll know about 90 seconds earlier that your radiator isn't coping, if it can't cope, and you find this out at ~88'c rather than at 96'c that the temperature isn't going to drop like you thought it would.

kind regards
Marek

[tinworm]

I agree the engine will warm up at the same rate Marek and the engine won't remain at 84-88 degrees - the upper limit is just controlled by the cooling capacity of the system and the ambient air plus the work the engine is doing- but the object of the higher rate thermostat is to allow the engine to come off choke sooner (ie warm up quicker and by that I mean the head only)- something that affects cars in cold climates more than cars in hot ones. The cold coolant shock as the thermostat opens ( and as I said that shows on the gauge) is something to consider for the well being of your engine and a reason not to go to too high a range of 'stat though. Jaguar used the bypass loop to lessen that I believe - and that is blinded by the original bellows thermostat so divert most of the coolant through the radiator for normal running.
Engines that have a sludged up or blocked system will get by for a time (and you are right with that) with the lower (74 degree) thermostat as rate of temperature rise is slowed down for short or gentle journeys. In my experience with the 6's the higher (84 degree) thermostat is fine in the UK as long as it is the original type and NOS items are the best (I have still found)

regards Barrie

[christopher storey]

Your running temperature of about 75 degs C is about ideal for an XK engine. Jaguar designed the whole setup for the cars on the basis of a temperature while in motion of 70 degs C, and on all the earlier cars which I drive such as 3 different XKs and my own Mark IX , all run at 70 on the mechanical gauge, going up to 80 to 85 at idle after 3 or 4 minutes but rapidly dropping back again once moving

[MarekH]

There are some misconceptions floating about here.

The sole function of a thermostat is to set the minimum operating temperature of the engine. Just go with the manufacturer's recommendation and don't read any more into it than that.

The cold coolant shock as the thermostat opens ( and as I said that shows on the gauge) is something to consider for the well being of your engine and a reason not to go to too high a range of 'stat though

Thermal shock:- don't put the cart before the horse. The maximum temperature swing of the water is only ever going to be about 75'c. That's limited to the first radiator-full of water when a thermostat opens for the first time after startup. But this happens because heat is being generated in the cylinder head by a controlled explosion in a metal cylinder surrounded by a water jacket. The real thermal shock is coming from any rapid change in engine load and rpm. This happens all of the time whenever you are driving and you change the revs or change the workload.

thermostat opening.png (147.46 KiB) Viewed 13775 times

In this example, contrast what is going on between A and B (at idle) and then between B and C, when the car moves off. On the second graph, the exhaust gas temperature (white trace) jumps from ~280'c to ~430'c and that is just going from idle rpm to ~1400rpm. Expect temperatures to rocket to 600'c+ if you just decide to gun the works. The pesky little movement of the water temperature gauge is pretty insignificant compared to this and isn't really worth a second thought by comparison - this is just the result of the real thermal shock inside the cylinder head (which is part of normal driving) after the heat has conducted through the metal and after it has then been picked up by circulating water. Being worried about the effect, rather than the cause is sort of "secondary" in my opinion.

The third graph shows water temperature. CLT in white is the temperature exiting the back of the engine and there is an identical sensor called OtterCLT with a green trace at the bottom of the radiator. You can see a 61'c spread between the two when the thermostat first opens. Unsurprisingly, that spread narrows once the water has looped around a few times.

The fourth graph is water flow via the bypass and via the radiator. White and red are flow through the bypass (65000=no flow; 0=full flow) and yellow and green are flow through to the radiator (65000=no flow;0=full flow). Low numbers on the white and red traces give way to low numbers on the green and yellow traces - water through the bypass (and none to the radiator) is replaced by flow to the radiator as time goes on. Interestingly, both left and right thermostats (it's a v12) don't open at the same time and the flow of water isn't equally spread 50:50 between them. C'est la vie!

So the thermal shock being worried about is just the smeared out, delayed effect of the real shock of burning petrol in the cylinders. This can move the temperature about 500'c in a few seconds if you do a dyno pull or aggressive overtake. A one-off 8'c thermostat difference in opening temperature isn't "something to consider".

....but the object of the higher rate thermostat is to allow the engine to come off choke sooner...

Really? Really? I don't buy this very much. Yes - a higher ambient temperature in the engine bay will help atomise the excess fuel that is being delivered, but I can just as easily generate more heat more quickly by driving the car than I can by waiting on the driveway with the higher rated thermostat. The jump in the EGT trace clearly shows that. Once again, the water temperature is an "output", not an "input".

kind regards
Marek

[tinworm]

There are some misconceptions floating about here.

The sole function of a thermostat is to set the minimum operating temperature of the engine. Just go with the manufacturer's recommendation and don't read any more into it than that.

The cold coolant shock as the thermostat opens ( and as I said that shows on the gauge) is something to consider for the well being of your engine and a reason not to go to too high a range of 'stat though

Thermal shock:- don't put the cart before the horse. The maximum temperature swing of the water is only ever going to be about 75'c. That's limited to the first radiator-full of water when a thermostat opens for the first time after startup. But this happens because heat is being generated in the cylinder head by a controlled explosion in a metal cylinder surrounded by a water jacket. The real thermal shock is coming from any rapid change in engine load and rpm. This happens all of the time whenever you are driving and you change the revs or change the workload.

thermostat opening.png

In this example, contrast what is going on between A and B (at idle) and then between B and C, when the car moves off. On the second graph, the exhaust gas temperature (white trace) jumps from ~280'c to ~430'c and that is just going from idle rpm to ~1400rpm. Expect temperatures to rocket to 600'c+ if you just decide to gun the works. The pesky little movement of the water temperature gauge is pretty insignificant compared to this and isn't really worth a second thought by comparison - this is just the result of the real thermal shock inside the cylinder head (which is part of normal driving) after the heat has conducted through the metal and after it has then been picked up by circulating water. Being worried about the effect, rather than the cause is sort of "secondary" in my opinion.

The third graph shows water temperature. CLT in white is the temperature exiting the back of the engine and there is an identical sensor called OtterCLT with a green trace at the bottom of the radiator. You can see a 61'c spread between the two when the thermostat first opens. Unsurprisingly, that spread narrows once the water has looped around a few times.

The fourth graph is water flow via the bypass and via the radiator. White and red are flow through the bypass (65000=no flow; 0=full flow) and yellow and green are flow through to the radiator (65000=no flow;0=full flow). Low numbers on the white and red traces give way to low numbers on the green and yellow traces - water through the bypass (and none to the radiator) is replaced by flow to the radiator as time goes on. Interestingly, both left and right thermostats (it's a v12) don't open at the same time and the flow of water isn't equally spread 50:50 between them. C'est la vie!

So the thermal shock being worried about is just the smeared out, delayed effect of the real shock of burning petrol in the cylinders. This can move the temperature about 500'c in a few seconds if you do a dyno pull or aggressive overtake. A one-off 8'c thermostat difference in opening temperature isn't "something to consider".

....but the object of the higher rate thermostat is to allow the engine to come off choke sooner...

Really? Really? I don't buy this very much. Yes - a higher ambient temperature in the engine bay will help atomise the excess fuel that is being delivered, but I can just as easily generate more heat more quickly by driving the car than I can by waiting on the driveway with the higher rated thermostat. The jump in the EGT trace clearly shows that. Once again, the water temperature is an "output", not an "input".

kind regards
Marek

Opinions vary.

[MarekH]

Most cars from the 1980s onwards run 88'c thermostats (because NO2 emissions are lower at higher operating temperatures). There must be millions of car owners out there who are twice as worried as Barrie about this thermal shock issue.

kind regards
Marek

[Series1 Stu]

Opinions vary.

True, but the laws of physics don't.

Regards

[mgcjag]

Hi Nigel...where are you seeing this info...hotter/cooler climates.......typically in other model manuals you see the requirements as 74deg for normal conditions and 82 for extreme cold climates......Steve

[Retroman]

Hi Steve
See below:
Jaguar Motors Workshop Manual including 4.2 supplemental and a section D and a good Old Oily Haynes Page 67
Cheers
Nigel.

[mgcjag]

Hi Nigel ....your original post was...."The workshop manual states that a 74 degrees C thermostat should be used in hotter climates and an 83 degrees C in cooler climates.

What are hotter climes? France Spain The Sahara perhaps?"

I was pointing out that the workshop manual dosnt say thermostat for hotter climates.....it quotes 74 or 83 for cold climates .....Steve

[dlgis]

Does it define cold climates? I would count the UK as a cold climate, but the missus thinks Iceland is a bit on the warm side

[MarekH]

This really isn't going to affect very much when you look at the bigger picture.

1/ Your etype will probably spend 350 days of the year sitting in a garage. Worry about the required maintenance due to lack of use first.

2/ Apparently, not all etypes obey the laws of physics, so the "wrong" thermostat chosen for the wrong reason will still work.

3/ The thermostat doesn't affect how quickly the car can be cooled down anyway. Worry about the ability of the radiator and fan to do their jobs. Those millions of cars with 88'c thermostats still run ok and do way higher mileages than our classic cars and they still work because the radiator/fan/coolant receives regular maintenance. They're not scared stiff on a knife edge because they are always 14'c hotter than your car. It's trivial. A car with a bad fan/radiator will overheat no matter what thermostat is fitted and no matter how classic it is.

4/ Get a grip on reality. Look at the graph and realise that after the car is warmed up, the temperature just meanders about anyway and is not directly related to the exact operating conditions at any particular time. All that matters is that the radiator (plus fan) are in good condition. A car that does probably less than 5000 moles per annum isn't going to mysteriously wear out tomorrow because those 5000 miles were, on average, 8'c hotter or colder. Your car doesn't even care or have any control over the temperature until the fan comes on.

5/ The thermostat is a misnomer. It is really just a water splitter. It's the radiator and your right foot which is running the show. The actual temperature at any time is just an delayed average of the heat put in minus the heat extracted. All the thermostat does is stop the radiator from dropping the temperature below a certain value by limiting its water supply.

Stop worrying - this isn't a big decision.
kind regards
Marek

[Phil P]

In reply to Marek's comment of - Err, no. It warms up at exactly the same rate regardless of the thermostat rating because it is the engine that is doing the heating and that is constant in this analysis.

The engine is the heat source and the radiator cools the system. If you temporarily isolate the radiator using a thermostat, why wouldn't the engine heat quicker? The only time that I can see that having no effect would be if the operating temperature were below the thermostat opening temperature.

The other aspect is that the thermostat is not fully open/closed but opens and closes as the cooling demand of the system changes... i.e. engine output, cooling airflow, ambient temperature.

Yours,
Phil

[MarekH]

Barrie's original reply stated that the engine would warm up quicker with the higher value thermostat, but the thermostat starts life shut so that is why the engine warms up at the same rate regardless of thermostat spec, just as you pointed out.

Just think of temperature as being the equilibrium point that pops out from heat generation minus heat dissipation.

Heat generation is directly and immediately proportional to engine load and engine rpm.

Heat dissipation is mostly from water flow and airflow, but it takes time for the wax to melt/solidify and for any water to travel to/from the radiator, so the effect is not as immediate.

If, later on, you temporarily isolate the water flow, that in itself can have no effect on making it heat any quicker than it would have during warmup, but the temperature will continue going up higher than before, because the see-saw doesn't balance.

kind regards
Marek

[Phil P]

I still agree with Barrie's original statement that it will warm up quicker... given identical driving conditions. The data Nigel presented shows that the thermostat for cold climes begins to open at a higher temperature so isolating the radiator from the engine (heat source) for a longer period. The heat output is exactly the same but is managed in a different way. Additionally, whilst it may not be that effective, the objective of a thermostat is to control variation in heat output and this will continue whilst driving under all conditions....unless you're maxing it all the time.
Yours,
Phil

[tinworm]

I'm keeping out of it - there is only so many graphs a man can take.

Barrie

[MarekH]

If Jaguar had called it a "water splitter", would any of these ludicrous claims have ever been made?

Phil,
Explain to me why "maxing it out" will make it run hotter. Empirical evidence shows the exact opposite is true. You seem to have forgotten that there is more waterflow as you increase the revs so you get more cooling as revs go up. The radiator's ability to cool must always exceed the engine's to heat, otherwise you own a very expensive kettle.

Barrie,
Your opening shot was that a lower rated thermostat wouldn't make it run any cooler once it was open. Explain why we have thermostats of different ratings then. I think with a well maintained radiator you will run 8'c cooler, because the 74'c thermostat won't choke off the supply of water to the radiator until the radiator has driven the temperature down to 74'c rather than down to 82'c.

And "quicker" and "sooner" are not synonyms. You warm up to 74'c sooner than you do to 82'c, but you aren't warming up any quicker.

kind regards
Marek

[TGN]

I think you’re flogging a dead horse Marek

There maybe some confusion over the fact a car with a thermostat heats up quicker than one without.