Matching the Thermostat and the Otter switch

[mark10337]

After the great thread earlier this year regarding thermostats, I finally took the plunge to check mine. I had noticed that the heater would rarely get warm enough when driving at motorway speeds to be at all effective and the temperature gauge would read between Min and N of normal. The obvious diagnosis for this was no thermostat was fitted, and lo and behold this turned out to be correct.

A quick check in the parts catalogue for the Series 2, and the standard part for a US car, engine number 7R6987-9 gave me C28067/1 - an 82°C Thermostat. The lower 74°C only being fitted up to Engine No. 7R5262. So I ordered it, and after cleaning up all the housings, fitted it snugly in its new home.

Today when running up the motorway, it had the desired effect and functioned exactly as it should. The temp gauge needle sitting way higher than usual, up on the L of Normal on the gauge, which I can now roughly calibrate to 82°C. I have read previously others have mentioned their cars run at that temperature and is considered to be quite a normal temperature.

I have Coolcat fans and a Coolcat radiator fitted with an otter switch also at 82°C. This means there is very little delay in between the thermostat opening and the radiator fans coming on. Before I used to enjoy the circuit being continually open and being cooled by airflow from the vehicle moving and rarely needed fans at all except in stationary traffic. The temperature also feels a little high, in that there is not much leeway to the overheating point, but I do have a good radiator and fans so not too much of a concern.

With the fans being used more, this puts more load on the battery and power consumption. So, the question is, should the thermostat and otter switch be matched at the same rating? I get the feeling I may feel more comfortable running a 74°C thermostat with a 82°C otter switch - or perhaps just for the much warmer summer months. Perhaps running a cooler otter switch and having lots of cooler water ready in the radiator would be better. Reading some of the links and previous posts though, running hotter is better, the trick is not to overheat.

[politeperson]

I fitted one one these some months ago. It has been 100% reliable in traffic jams etc.

http://www.fossewayperformance.co.uk/fa ... placement/

[christopher storey]

Mark : in general terms , you are nowhere near the overheating point at 82 degrees C. Bear in mind that with a pressurised system ( is yours 13 lbs ? ) the boiling point of water at seal level is raised well above 100 degs C, and even allowing for the fact that in Switzerland you will probably be well above sea level at times , with the system in good condition you should not experience overheating . With regard to the problem of the fans running all the time , you might find it an advantage to disconnect the Otter and substitute a manual switch for the fans so that you can engage the fans only when you think they are needed. I have done this with my S1 car with some success, and I find it useful to anticipate a high temperature situation eg by turning them on when nearing the end of a fast motorway run when a traffic hold up is anticipated

[MarekH]

Dear Mark,

Most modern cars run 88'c thermostats, so you shouldn't consider 82'c as hot. The purpose of the thermostast is to maintain a minimum rated temperature, so 82'c in your case is the expected minimum temperature.

The Otter switch location and spec ought to be such that it cuts in when not enough cooling has been done by the radiator (as judged by the car designer). It wants to measure water temperature after it has passed through the radiator and cut-in if it hasn't returned cool enough water. If, as expected, the radiator (and fans) have done a good enough job, then the thermostat sulks and simply opens to the bypass more because you aren't hot enough and the whole cycle goes round again. (The s3 Otter location is at the radiator outlet - any other location is bad design.)

Also, it is wrong to think that the battery is working harder, So long as your rpm are high enough for the alternator to be working, then it is powering the electrics and charging the battery. You pay for the increased fan usage with petrol.

kind regards
Marek

[mark10337]

Hi Christopher

I'm running at 7lbs (C18484) at the moment which is apparently the standard for the Series 2 according to the parts catalogue. 13lbs I believe is the standard for Series 3. Saying that... I do have a 13lbs cap but wanted to ensure the system as a whole was functioning before upping the pressure.

I did exactly what you said, and fitted an otter switch override several months ago so that when hitting stationary traffic, or some of the mountain passes here I was prepared. That would also signify it is of advantage to have the water in the radiator, cooler than that in the engine jacket, so when the thermostat opens the flow to the radiator, the circuit gets water that is cooler. Saying that - it should already be cooler as driving along without it circulating would have cooled it. So it would also hint that an otter switch at a lower temperature than the thermostat would achieve almost the same effect as the override switch.

Perhaps someone more knowledgeable than me could explain why it is sometimes stated that the 82°C is for colder climates as opposed to 74°C in other climates. What difference would having a cooling jacket around the engine that is 8°C warmer make when an engine is up to operating temperature.

I'm still interested in what the original design from Jaguar is, and if the temperature of the thermostat and otter switch "should" be matched. From reading Marek's post, it would appear so as the objective is to maintain the constant temperature. Where the radiator cannot keep up with cooling the water that is being heated through normal air movement, the fans kick in. So is the answer to the climate temperatures, simply that the system was not thought to be able to cope with cooling air in a warmer climate quick enough, so the kick in temperature was changed. That however would mean only the otter switch should be lower, not the thermostat... oh my oh my.

[christopher storey]

The reason why a higher temp thermostat is used where ambient temperatures are lower is to prevent the engine being overcooled and also to provide reasonable heater output . If you use say a 74 stat with OAT of say 0 degs C there is a heat transfer capability of 74 degs between the heater matrix and the air input . That may well be insufficient to give proper heating in the car ( poor on Jaguars of that era anyway ) . If you increase the transfer capability to say 88 degs C then the heated air will be that much warmer . I suppose to calculate how much warmer we need to start from absolute zero, so that the increase between the 2 examples I have given will be from 273.5 + 74 = 347.5 K to 361.5K so that the increase in energy transfer is from 347.5/273.5 = 1.27 to 361.5/273.5 = 1.32 so that the heat transfer is increased by about 4 percent of the engines total heat output to coolant. I'm afraid my physics is now rather rusty but someone better at it can correct me

[MarekH]

Dear Mark,

You might be slightly overthinking things about matching the Otter to the thermostat. The radiator is a passive device with no control, no feedback and no idea whether it has a done a good job. It is spec'd to always "overcool" the water (whether directly or with fan assistance) and so it always returns water which is too cold. This simply forces the thermostat to then adjust and bias the water flow back towards the bypass. Essentially it's just a constant ping-pong battle between the two which is slightly out of phase in all sorts of ways. It is kind of nice to think that the Otter ought to match the thermostat, but in an analogue world, so long as the radiator/Otter forces the thermostat to always cycle somewhere between open and closed, then its job is done.

Sadly the thermostat doesn't know whether it is doing a good job either - it is just a water splitter which starts splitting at a minimum defined temperature. It can't tell that bad things are about to happen or offer any help when there is little or no water is flowing and it knows nothing about airflow.

Having said that, it does work quite well! If you look at a datalog of a drive on twisty A-roads, then the temperature graph looks like a wavy line moving slowly around by ~4'c above its base thermostat temperature although there appears not to be any direct correlation between load, rpm and the resulting output temperature. If you look at the same log of a car sitting on the driveway or at the traffic lights, then it looks like a sine wave, periodicity about 90 seconds and a rhythmic 6'c variation above the same base temperature.

The fans do have the dual purpose in providing some airflow under the hood for all the other components in the engine bay, so letting them run via the over-ride switch when stationary is probably no bad thing. The extra excess water cooling capacity deployed is simply thrown away by the thermostat which promptly rebiases the water flow towards the bypass.

kind regards
Marek

[erictheetype99]

Whilt I am a different 'Mark', I must say that was very well explained - I hadn't realised the thermostat reacted in such a broad way - I tended to think of it as either open or closed and not thought of intermediate partially opening/closing to regulate engine coolant temp. Many thanks.