Thermostat

[mgcjag]

I thought that the point of these thermostat posts was to find a thermostat that actually worked to close off the bypass as jaguar intended.......and give users the option to fit a correctly working part.....seams like a sucessfull thread.....

[MarekH]

Dear Steve,

Close it off by all means, but if the radiator/fan can't do enough to make it cycle, then it won't help anyway.

This all started because one chap had a water pump impellor that was a poor fit and complained about overheating when idling a non-moving car.

The point of these threads is to help people understand how stuff works.

kind regards
Marek

[mgcjag]

Sorry to disagree Marek...but the point of this thread that i started was to find a correct thermostat to fit the S1 and S2.....

[MarekH]

I'd agree with you that the slot size and depth, sleeve size and temperature rating are all going to interact together - no one item can be "correct" without matching the others, but the "correct" thermostat will be one that cycles under all conditions. There may be more than one design that achieves that. Once you have achieved that, no amount of chopping or changing will have any effect other than to make it cycle slightly differently than before.

This is because the thermostat only divides the water, it doesn't control how much water and it doesn't actually do any cooling.

Consider whether ANY sleeved design can EVER block the bypass or can only ever restrict flow.

kind regards
Marek

[Heuer]

Well armchair theorising can't beat observation and practice! Haynes, Hassan and Baily designed a superb engine that went on to power Jaguars for almost 40 years, a remarkable feat of engineering by anyone's standards. Their design included a dual action thermostat that closed the by-pass and opened the radiator when the coolant temperature reached 74?C; a 87?C thermostat was available for "winter use" but of little value to most of us. It seems reasonable to me we should at least find and fit the specified thermostat and look at the results, although this has been all but impossible for the last 25 years because the sleeved thermostat has not been available. This means practically every Jaguar saloon and sports car produced up to 1968 and currently on the road is not seeing the benefits of a correctly specified cooling system. Owners appear to have accepted this status quo either through idleness, ignorance or self justification and woe betide anyone who offers an argument. Colin, from Wat-Jag, was telling me he was abused royally and loudly at Stoneleigh for displaying the skirted thermostat and was talking (I quote) "a load of bollocks". Why this person became so agitated over a replacement part is lost on me.

Having fitted the sleeved thermostat on Sunday I observed first hand how the by-pass hose heated up and then, through the magic of a lump of wax, rapidly cooled as the radiator hose took over. A small thing but I was in awe of something that had not happened on this car in over 30 years. It seemed like a small victory over complacency.

In the last 12 months we have seen the re-introduction of the correct fluted side lenses and slotted screws, the production of the correct HT conduit and other previously unobtainable parts yet no one puts finger to keyboard to decry their availability and dismiss them as nonsense. Interestingly Steve had the same experience when he single handedly pursued the re-introduction of 120? TRE's. Must be an engineering thing?

Installing the correct thermostat is simple and easy to do, can cause no harm and gives you the confidence your car is operating as Jaguar intended. SNGB offer a skirted thermostat that works on both the S1 3.8 and 4.2's with no modification. Wat-Jag offer an alternative solution which requires some machining of the 3.8 elbow but then allows the use of a readily available Land Rover thermostat. Wat-Jag also offer a machined thermostat that fits the 4.2 manifold without modification. If you have access to a lathe you can do these modifications yourself and fit the ?5 LR thermostat. If you have an S2/S3 make sure you have a double poppet thermostat of the correct length. If you employ others to do restoration/maintenance insist they fit the correct thermostat.

Never run the engine without a thermostat - by design it puts back-pressure on the water pump to prevent cavitation at higher revs. The cavitation decreases water flow and erodes the water pump housing.

[tinworm]

Thats a very good post David , and sums up everything I think !

Barrie

[MarekH]

David, consider what has actually happened.

You probably used to have a water divider which on a cold engine was dividing 100:0 (bypass:radiator) and on a hot engine, maybe 8:92 (bypass:radiator) and on average would settle (let's say) around the 20:80 mark, moving up and down around that mark as wax changed state.

You now have a different water divider which runs from 100:0 (bypass:radiator) cold to (hopefully) 0. 1:99.9 (bypass:radiator) hot and it moves up and down around a slightly different average mark maybe a little bit more quickly than the old one did.

Yes it is a thing of great wonder, but so what?
Why am I not impressed?

Well, the heat produced by the engine has not changed and the radiator capacity has not changed and the water pump speed has not changed.

So what is going on?

All that has happened is that a similarly rated water divider is now going to yo-yo from 40:60 to 1:99 and on average is probably still 20:80. Remember the heat input and the heat extraction have not changed - it is still the same engine producing the same heat and the same radiator extracting the same amount of heat - you've simply made it cycle a bit differently and sometimes a different proportion of the water reaches the radiator and a different proportion reaches the bypass.

Perhaps with a different spec 'stat it may now yo-yo about a different point - but on average the total heat produced and the total heat extracted still match one another.

Now in your case, your car has such a powerful cooling system that it didn't overheat, even from 87'C. Changing the water divider hasn't affected this at all. You had spare cooling capacity aplenty beforehand and you still do now.

I'm completely relaxed about the reintroduction of fluted lenses, screws and conduits. At the end of the day, these are all cosmetic.

Understanding physics isn't cosmetic. When in doubt, the man at Stoneleigh should have just drawn a box around the thing to analyse and labeled all of the arrows going in and out. In this case the "heat" labels on the arrows would have been the same in both cases, so there probably wasn't much to get agitated about.

kind regards
Marek

[Heuer]

On the road a properly specified thermostat will never be fully open or fully closed, it will increase or decrease the flow of coolant to the radiator based on demand, keeping engine temperatures optimal by leaking coolant progressively through the by-pass. The 'super-heated' by-pass water is added to the radiated water and the resulting temperature causes the thermostat to react accordingly.

The skirted thermostat used in the XK engine does however have another trick up its sleeve (no pun intended) and that is it can increase/decrease flow to the radiator without opening the by-pass. This is because the by-pass slot is 4mm wide whilst the thermostat skirt is 15mm wide and as a result will only ever allow water through the by-pass when the engine is cold or below ~65?C. So the thermostat is controlled solely by the flow of radiated water through the system, by-pass water is never added - by design. Jaguar referred to it as "quick-lift operation" in their Service Bulletin. That alone makes the investment in the correct thermostat worthwhile!

[PeterCrespin]

Having been the one (I think?) who pointed out that practically no 4.2 owner has the correct thermostat today, I feel a bit guilty. Not least because this statement is true:

Well armchair theorising can't beat observation and practice!

It is a true statement that argues, of course, against the largely theoretical concern I raised and the solution we're now urged to adopt.

Your own observation and practice with your car, and that of practically everyone else with a Series 1 car in good condition (including our hot climate brethren on Jag-lovers) shows that incorrect poppet thermostats are rarely an issue in observed practice. You did not think you had a cooling problem until it was pointed out things weren't technically as they should be. You would have jumped in your car and zoomed down to Biarritz or Ventimiglia in a heartbeat with the thermostat you had. QED.

Haynes, Hassan and Baily designed a superb engine that went on to power Jaguars for almost 40 years......Never run the engine without a thermostat - by design it puts back-pressure on the water pump to prevent cavitation at higher revs. The cavitation decreases water flow and erodes the water pump housing.

The XK engine that Bill Heynes [sic] designed could not and did not run with a thermostat for thousands of miles, 24 hours at a time under the most extreme conditions. Not quite the same thing, but you get my drift. Never say never because absolutes can bite you in the bum.

I think the fundamental thing to guard against is to regard the functioning thermostat as a cooling aid. It isn't. It's a warming aid. Yes, at the very limits of cooling it is suboptimal to have an open bypass, and yes, many Jags are no longer factory-fresh and need all the reserve cooling capacity they can muster, but armchair theorising followed by thermostat replacement hasn't improved your car's overall thermodynamics or heat-shedding capability in any practical way. In fact if thermodynamics were the criterion, you've lost out by running at lower temp because more of your fuel's heat is going out the radiator core instead of pushing the piston down.

Happy motoring!

[Heuer]

Your own observation and practice with your car, and that of practically everyone else with a Series 1 car in good condition (including our hot climate brethren on Jag-lovers) shows that incorrect poppet thermostats are rarely an issue in observed practice. You did not think you had a cooling problem until it was pointed out things weren't technically as they should be. You would have jumped in your car and zoomed down to Biarritz or Ventimiglia in a heartbeat with the thermostat you had. QED.

More theory then :) I do not know how the specified thermostat will affect cooling and car performance because I have yet to drive it over any distance in varying traffic and weather conditions. To say the incorrect poppet thermostats are 'rarely an issue' presupposes we have a benchmark to compare them to, and we don't because the specified thermostat has not been available for so many years. Maybe the car will be faster, quieter, cooler and nicer to drive - maybe not - but at least we will have before and after comparisons on which to base bold, now substantiated, statements from the luxury of our armchairs!

[64etype]

Thanks to this thread I was able to order the correct thermostat for my 3.8 which has been modified with an add-on air conditioning kit. I have little doubt that the cooling system will be taxed to the max in the American Southwest. I want every drop of coolant passing through the radiator after initial warm up, as only then can I accurately determine if the modified configuration has adequate cooling margin. Very helpful thread from that perspective.

[MarekH]

On the road a properly specified thermostat will never be fully open or fully closed, it will increase or decrease the flow of coolant to the radiator based on demand

Having fitted the sleeved thermostat on Sunday I observed first hand how the by-pass hose heated up and then, through the magic of a lump of wax, rapidly cooled as the radiator hose took over.

I'm not sure this phrasing has been helpful.

If YOUR bypass hose cooled rapidly, it'd suggest that the bypass WAS actually shut off by its sleeve.

I can tell you from looking at an old datalog from my car that standing at idle on the drive, the engine temperature yo-yos between 85 and 81.9'c over 130 seconds and then between 82.9 and 84.7'c over the next 92 seconds. Generally, it takes about 120 seconds to go high-low-high.
Bumping the revs from 900rpm up to 4000 over 80 seconds sends the temperature up to 89'c - it peaks 40 seconds after the throttle is closed off back to 900rpm and the temperature comes down to 85'c which is where I shut the engine off. Interestingly, it then heat soaks back up to 89'c some 70 seconds after shut down, but restarting the engine sees it drop to 79.9'c whence the yo-yoing starts afresh. What I can't tell you is "how open" it was at any point because I don't measure it, but...

I'd say it's a safe bet that the thermostat can go fully open simply because of the time lag.

It's also misleading to to say that the thermostat controls the flow of water - the water pump does. This is why I keep calling it the water divider.

Lastly, it certainly doesn't respond to demand. If it did, then there wouldn't be a problem! When you are overheating, it'll be fully open and demand is still increasing. It can't and doesn't help you.

It's "job", if that's the right phrase, is to try and keep the temperature at or above its rated figure. When Mr Thermostat has done this, it starts to share some of the engine's water with the radiator. It has no clue what the radiator will do with whatever it gets but it tries to share as little as possible.

Anyone who wants to look at datalogs from my car should download a free copy of Megalogviewer or MegalogviewerHD and pm me.

kind regards
Marek

[PeterCrespin]

I do not know how the specified thermostat will affect cooling and car performance because I have yet to drive it over any distance in varying traffic and weather conditions. To say the incorrect poppet thermostats are 'rarely an issue' presupposes we have a benchmark to compare them to, and we don't because the specified thermostat has not been available for so many years.

Righto. I thought you'd posted that a recent swap to a sleeve had dropped like-for-like running temps 10C, or some such?

I take as the 'benchmark' your experience to date and that of many other owners, including well-known 3.8 drivers using poppets in hot climates without problems, albeit sometimes with CoolCat fans or alloy rads etc.

AFAIK your observed practice has been to drive the car hard and not have overheating problems. Sounds like an adequate consistent long-standing observed practice baseline for all practical terms. I suppose for a true comparison you'll just have to retrace all those fab French holidays and revisit the numerous chateaux with Jill.... all in the cause of science you understand.

[ralphr1780]

Marek, which engine/car are you referring to in your post?

[MarekH]

Dear Ralph,

I have a v12 etype.

My observations can be applied to any car running a wax thermostat and a water pump run by a pulley in the conventional manner because the general principles of what makes the water hot and what removes the heat are no different for my car than for anyone else's.

Essentially, my view is that if your car has only a marginal cooling system then the further blanking off of the bypass might be helpful, but a car with a properly maintained cooling system will almost certainly show no material benefit from any advice offered.

kind regards
Marek

[Heuer]

Although Jaguar thought differently otherwise they would have deleted it! They changed the design twice and the thermostat specification four times which kind of suggests they were pretty keen on the idea.

Marek - your data is based on a static engine in still air. Would it not be more representative to have dynamic measurements with the car on the road given 95% of an engines cooling is done by moving air. I doubt you would see as much cycling of the thermostat on the road. Also you are not comparing like with like - the V12 has a different thermostat (dual poppet), different radiator, twin fans and different bonnet. Did your car have the correct Jaguar specified thermostat?

[ralphr1780]

Thanks Marek, it was just for my better understanding of your detailed educative description. I am back from a skiing week with still some schnaps between the ears.

I suppose your thermostat is then a 74?C type, which would be fully open at the observed running temperatures indicated in your post. Correct?

My understanding, is that the water cooling is solely the job of the radiator. By blocking the bypass, almost all of the water is circulating through the radiator, hence optimal cooling efficiency.
When the bypass is open and the thermostat is closed, all of the water is circulating through the bypass circuit, hence no cooling.
When both bypass and thermostat are open, part of the water is circulating through the radiator and part is circulating through the bypass, hence less than optimal cooling efficiency.
If I understand correctly your comment, you mean that in the first and third scheme (all other parameters same) the thermodynamic exchange would be the same?
Cheers,

[MarekH]

Dear David,

The principles most certainly are the same:-

Thermostat:- below the specified temperature, the path to the radiator is blocked, save perhaps for a jiggle pin. Above the specified temperature, the thermostat starts to share water with both outlets and this always progressive.

Radiator:- a passive device which provides cooling.

Otter controlled fans:- an active device for additional cooling which cuts in if the water exiting the radiator exceeds the designer's deemed safe operating temperature.

Water pump design:- belt driven. Amount of flow is rpm dependent.

Same principle of operation:- At or above operating temperature, the thermostat will share some of its water with the radiator, the rad/fans return water which is cooler than the thermostat spec and the thermostat readjusts to restrict flow to the radiator. With a closed or closing thermostat, the temperature then rises until it excceeds the thermostat spec and the process repeats.

There are many variables which affect how quickly it adjusts and readjusts and thus whether it does or doesn't reach a steady state of opening:- rate of melting/freezing of wax, the engine rpm, manifold vacuum, the orifice sizes and/or sleeve to housing clearance, sleeve depth, airflow. What is clear though, is that the rad/fans must always have excess capacity and overcool and this forces the thermostat water split to back in favour of the bypass. Essentially, the thermostat always wants us to be at least as hot as its spec, but when it achieves this, it scores an own goal because it opens to the rad & fans which promptly always slap it down. Interestingly, this means that the rad/fans have excess cooling capacity and the thermostat systematically throws this away every time. So if your cooling system already works, an even better cooling system has no benefit - the thermostat undoes all of the incremental upgrade!

I have given a couple of examples at static rpm (idle and zero rpm) and still air because removing as many of the other engine variables as possible shows best what is going on. In reality, the engine load, rpm and what have you are rarely static and so the effect of opening and closing of the thermostat is not readily visible above these other factors. Logic says it'll still try to cycle, because the radiator is a passive device and thus will always overcool. For this not to be so, the radiator must only be fed water on demand such that the exit temperature of the water at the Otter remains constant irrespective of load, rpm, etc.

For dynamic measurement, I can offer countless datalogs but you can only deduce how open or closed the thermostat is from the temperature and we already know it lags that because temperatures still keep rising for a minute when the fans cut in. On an eight minute drive over backroads, there were eight peaks and troughs with temperature running from 76 to 86'c but never going up or down by more than 4'c. Essentially the graph still looks like a sine wave but it's not as regular and predictable as the static test.

Comparing like with like:- a correctly functioning dual poppet will have a water split between the bypass/rad going from 100:0 to 0:100 hot to cold. A perfectly sleeved design would do the same and so the proposed modified sleeve designs probably approach that of the dual-poppet. Essentially, the less perfect the sleeve design, the more smeared out the effect will be of its water split, as it'd always have some component of bypass mixing in even when fully open. In reality, this simply means it'll be a little bit more open than a tightly fitting one and its cycling will be less "digital" - this is why I am dubious that these modifications are of any value except when dealing with an already dodgy cooling system.

The fact that the radiator, fans and under bonnet space are different is not relevant. I say this with confidence because the cooling system capacity in each case exceeds cooling requirements. When this happens, and it always does on a properly maintained system, the thermostat promptly closes off the water to the radiator. It doesn't make any difference whether you have 5% excess cooling capacity or 70% excess cooling capacity - the thermostat shuts and makes sure that your car gets back up to the required temperature because the rad/fans always overcool.

The "correct" Jaguar specified thermostat is a red herring, so long as (in my case) it is a dual poppet of sufficient length. The "correct" item is one that makes the temperature cycle, no matter what and we don't care HOW it achieves that. Remember that you are dealing with a self adjusting system. So long as the water coming back from the rad is too cold, and it always is, then it self adjusts back up to its design temperature. So long as your thermostat cycles, you don't have a problem, and any change you make will be self adjusted out the next time water has come around.

The other definition of "correct" is the temperature spec. This is also a red herring. You just cycle around a different number. If your rad and fans can't return cold enough water at 79'c, you'll just lose control of the system 9'c earlier than you would have at 88'c anyway.

The long and short of it is make sure your rad and fans can return water back at below the temperature of the thermostat spec. The 'stat will than act to throw away the excess cooling and raise the temperature back to its spec.

If your cooling system is marginal, feel free to make sure that the returning water from the rad has no mixed in bypass component by altering the sleeve arrangement, but this may not be enough to save you. At low rpm, you are pumping hardly any water anyway, so you either address that by pumping more water or else add a lot more airflow.

kind regards
Marek

[Heuer]

All this seems somewhat at odds with your posts on J-L where you went to some effort to both identify 'correct' S2/S3 thermostats and produce a Delrin insert to make sure the thermostat by-pass worked correctly: http://forums.jag-lovers.org/av.php?1403497m62

So the by-pass is important and valid for the S3 (one of which you own) but when S1 owners want to get the by-pass working as Jaguar intended we are all talking nonsense? :?

[MarekH]

Dear Ralph,

I run an 82'c thermostat so that sets the floor on the temperature.

"Optimal" and maximum efficiency probably aren't the right words. The radiator is a passive device and is designed to be a size which always means it cools too much, no matter what. If it doesn't return cold enough water, the Otter unleashes the fans and makes sure it does. It always overcools because it doesn't know how much to cool. The thermostat then adjusts to stop it overcooling subject to the engine heating up returning water.

You can think of the thermostat as always fighting against the rad/fans and the rad/fans always win.

The "optimal" amount of cooling is going on only when the temperature is not changing.

kind regards
Marek