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MG MGB Technical - Measuring Coolant Temperature.

Hi,
I have recently fitted a Kenlowe fan to my 74 1/2 B, for reasons of quicker warm up, HP etc. I wish to set the thermostat by water temp and not just by the temp gauge (30 years old) that the instructions tell me to do. Question. Do i remove the radiator cap, run the engine, wait for the fan to cut in (as i have it set at the moment) and check the water temp in the top of the rad. Keep adjusting so the fan cuts in at the required temp. Question 2. At what temp should the fan cut in? Thanks, Chris.
Chris82

That's a lot of faffing about, unless you suspect something wrong with your gauge reading, which should be fixed independantly, just set it so it comes on about 1/3rd the way between N and H. Factory electric fans come on about half-way, which is about 90C/194F.

Where is the sensor? If in the top hose near the rad if you try to set it lower than that to keep the temp gauge needle on N the cooling fan will be fighting the thermostat, and likely to stay running when isn't needed, blowing through an already cool radiator.
Paul Hunt 2

Chris, your method should work fine, just be careful not to splash and coolant when it's hot. I think the temperature setting for the fan switch depends a lot on where the sensor is placed. If it's in the block or head I like it to be around 190F or the same as the thermostat, if it's in the radiator then I'd want it to come in a little lower, say around 170-180F. I like to give the fan a little 'lead time' to cool so that the peak temp reading doesn't get higher than 212F if possible. My theory is that you can't 'overcool' the engine if you're running a thermostat, but you can easily under cool one, so adding cooling lower in the temperature range can't hurt.
Bill Young

>My theory is that you can't 'overcool' the engine if you're running a thermostat,

Depends on the thermostat! Here's what Peter Burgess, well-known MG tuning guru, says in his book "How to Power-Tune MGB 4 Cylinder Engines": "The ideal 'stat is 88 degrees Celcius (190F)."

Here's what the late, great Smokey Yunick said in his book "Power Secrets": "It is easy to see how overheating can be a problem, but I think some racers overlook the fact that it is possible to 'overcool' the engine. Some guys go to great lengths to keep the engine temperature down to 180 degrees. And, though the engine doesn't overheat, they don't realize that they're putting energy (heat) into the cooling system that could be used to produce power at the crankshaft. Running the engine at 180 degrees will drop the overall horsepower by 2%-3%. For max power the cooling temp should be at least 200 degrees...."

HTH!
Rob Edwards

If running an 88C thermostat you don't want your fan cutting in at 90C.

What I did when I used an adjustable kenlow, in the top hose, was set it at 100C. That easy to do as you take the cap off, wait for the radiator water to start boiling and set the fan to come on at that temperature. With a pressure cap on the actual boiling point will be a fair bit higher than 100C. I didn't get any boiling but there was quite a bit of expansion loss.

A research fellow at Warwick University who does quite a bit of work for the motor industry told me that this was all wrong and that the fan should be switched based on the temperature of the water going back to the engine. Only then can you tell if the radiator needs the help of the fan. I am afraid that he did not give me a target temperature at the location.
David Witham

Thanks for all the replies. The only reason i want to do it this way is for accuracy. If i can keep the engine at its best running temp then it must be better in the long term. It has a 180f, 82c thermostat fitted, centre fill rad (no expansion tank) and the Kenlowe thermastat probe is fitted in the top hose. From all the different replies im still unsure. Chris.
Chris82

I don't get the stuff about overcooling... firstly within reason the energy heating the coolant is waste already inasmuch as it has been used to heat the head/block, and secondly a temperature difference of 20F is going to be pretty small relative to the heat being generated from combustion so the difference in combustion temperatures between the two running temperatures is going to be minimal? And the engine is only producing max power at say 5500rpm under full load which probably implies a decent airflow through the radiator due to motion (and in any case much more than the fan is going to deliver).

And I don't get the stuff about fan setting temperatures... the temperature of the water entering the radiator is determined by the thermostat, the temperature exiting the radiator will change with airflow (ie whether the fan is on or not) but colder water leaving the radiator will reduce the bulk temperature of the water in the engine which in turn will close the tehrmostat?

There's clearly no point in setting the fan to a temperature less than the thermostat as no water goes through the radiator when the thermostat is closed. If you set the fan too close to the thermostat temperature then it will come on as soon as the thermostat opens, which might not be necessary. The fan is only needed if there is a prolonged period when the ability of the radiator to regulate the temperature without the fan (which also depends on vehicle speed and air temperature) is insufficient - so you need to give it a bit after 'normal' to see if it settles down but enough before 'hot' to give it a chance to work before the water gets too got. Hence third/half way between! If you keep losing water out of the expansion hose then it is probably set too hot. If you set it too cold then you will wear out the fan but not overcool the engine. Go for what Paul says!

Neil
Neil22

Hi again,
Niel22 I am in fact going as Paul says, but the only difference is that i want to be a bit more accurate. I have it set up at the moment exactly how Paul mentions but would like to know the best temperature to have the fan cutting in. I thought it was quite a straight foward question but it appears otherwise.
Chris.
Chris82

Chris I have a supercharged B which which should make it a little more critical in temp settings. Also living in OZ is a bit more of a challenge. With the stock cooling system in good condition I set the fan thermostat switch to enough above normal highway driving temp so as not to come on at all until conditions change to lift the temp about ten deg above normal, such as traffic, stop start, a strong tail wind and of course really high ambient temp. I find that nine out of ten times I will use the car without hearing the fan at all and the engine temp is kept constant by the thermostat. With the fan coming on and off there is a more sudden change of temp which is not good. I have no problems with my system. The main thing is to mount the fan so as to allow plenty of natural air flow through the radiator when the fan is not going. I run at 185* fan cuts in at 195*. I would run it about 5* hotter but head temp is a little more critical with the supercharger.
DENIS4

>I don't get the stuff about overcooling...

Running the engine hotter makes it more efficient -- that means you get more power AND better economy (all the time). The idea is that when you burn fuel, the energy released either turns the crankshaft or becomes waste heat. The energy will want to "follow the path of least resistance" and so if the engine is hotter, less energy will flow into the engine becoming heat, and will instead be converted into mechanical energy to turn the crankshaft. Once the energy is in the iron of the engine it's lost, so you try to prevent it from getting there.

Also, running the engine hotter makes it last longer. When you crank a cold engine, fuel atomisation is poor and some fuel doesn't get burned. This fuel escapes past the rings and makes it into the crankcase, diluting the oil. Also, water is a natural byproduct of combustion and it too gets past the rings and into the oil. Once in the oil, it combines with other combustion products to create acids. Oils have buffers in them to neutralise acids, but they can only do so much. By getting the engine to full operating temp as quickly as possible and running it as hot as is practicable, you drive out the fuel and water faster, thus saving your engine.

The above of course relates to the coolant thermostat choice, of course. As you mention, the fan thermostat should be set to come on above the coolant thermostat's opening point....

HTH!
Rob Edwards

Rob,

Sorry, when I said I "didn't get" the stuff about overcooling I meant that I "didn't agree with" the stuff about overcooling. The thermodynamics are way more complex than that. I didn't want Chris to think he should run the engine as hot as possible because, even if it did mean more maximum power in theory (which I don't think it does), it would mean less power in pactice and a whole bunch of issues.

Higher temperatures are good for economy because the fuel burns better so you need less of it. Higher temperatures are not so good for maximum power as knock occurs at lower pressure and hence the compression ratio needs to be lower. And the ignition needs to be retarded.

Higher cylinder head temperatures cause other problems in an MGB - overheating of spark plugs, fuel vaporisation, sticking valves, burnt valves, pre-ignition, burnt gaskets, warped exhaust manifold... just ask anybody who has put heat insulation on their exhaust manifold. Also, higher underbonnet temperatures mean higher intake temperatures which also robs power.

I'm not suggesting running an engine cold, there are downsides of that as you say. But running 20F hotter water isn't going to make more power.

Neil
Neil22

On the subject of measurement. I've got one of these >
Ebay UK Link http://tinyurl.com/3x3xvc & I want to mount it on my dash to get a real time reading of the coolent [or oil] temperture as it rises/falls but the system as you know is presurised. Any idea where I could mount the probe [preferably in the coolent flow]get an accurate reading from the engine?

http://www.mgexperience.net/registry/GHD5465203G




Paul Morgan

What exactly do you propose to do with a thermometer having a top range of 70C?
For the record, while trying to figure out what a brand new late fan switch was doing, I measured temps at the top tank, just below the switch. This was with a precision thermocouple in hard contact in the tank gutter, covered with insulation to avoid heat loss. As the switch went through what proved to be random antics from c140F to c250F, I pretty conclusively proved that there is no boil-over up to 255F, with 50% glycol and 15psi cap.
Since heat transfer to air from rad is not a simple function, but improves drastically with temp differential from hot source to ambient sink, it is expedient to run the engine hotter rather than cooler, within limits of oil & metal temps. Certainly fan temp should be above full open tstat temp, plus a bit for dither in the switching. I figure around 210F is perfect - least fan use, least power loss, no problem with excess temps.
While reading fan switch temps off the lower hose makes theoretical sense, it also requires that you predict heat load in the engine in order to prevent running with the tstat closed. That's undesirable, since circulation is impaired in some areas if there is not water flowing at a good rate through the block, which can give local hotspots.

FRM
FR Millmore

Excellent work there, Fletcher.

Neil, running 20F hotter WILL make more power, you need to keep heat in the combustion chamber. That's why aluminum heads need higher compression ratios to make the same power as an iron head, because they dissipate heat faster. My short simple answer to your setpoint question is to set it at 100C (212F).


Paul K

Chris,


My brother bought me a wine temperature tester a couple of years ago for my birthday....frankly the wine in our house never sits around long enough to get too cold , or hot, but the tester has a long thin probe which fits fine between the fins on the radiator it's digital and reads well over 100c in 10ths of a degree.....I used it to set up the fan cut in temp, and to play with extractor fans on louvres on the rear if the bonnet (don't bother two 4" computer fans have no effect whatsoever.)

Those fancy kitchen shops sell wine testers....no idea on price however.
M Barnfather

Just a view from the practical side. There were times that I thought I had my elctric fan set just right. Then on the occasional run on the hot freeway, even at 75 MPH, the fan was coming on. I had the wiring set up to come on automatically or with a lighted switch. But no way to turn the fan off if it was coming on and not doing any good at 75 MPH. I should have just wired it to a switch and turned it on myself while caught in traffic.
Tom

Or just set the thermostat so that it only turned on the fan when it was required.
Paul Hunt 2

Hello again,
Many thanks to you all for your input. Reading the replies has been interesting but the last reply from Paul H probably hits the nail. 'Just set the thermostat so that it only turns on the fan when required'. Simple but correct i think. I believe my way of thinking on this occasion my have been to particular,(My wife keeps telling me its an age thing). Thanks again guys, Chris.
Chris82

Paul, my point was that the day was so hot that the fan came on at speeds of 75 MPH. It was a waste of the fan and the energy to run it, as the air was going much faster thru the rad then the fan could push it. Therefore being able to turn it off would have been nice, as it was doing no good at all, then give control back to it when I was going slow or stopped.

I see your point too..if I were to set the temp higher, I could still turn it on if I wished. But there were times on abnormally hot days that were less than an optimal balance for me. Maybe that's clearer.

A more efficient rad would have made this occasional glitch go away, I suspect.
Tom

You certainly shouldn't need additional cooling with a standard MGB at 75 mph *in free air*. But bowling along in multiple lanes of traffic the ambient air will be hotter than if you are on your own, as your cooling system is 'breathing' the hot air expelled by the cooling systems of those around you. Also if you are behind a large truck the volume or air going through the radiator will be much less, and you can expect an electric cooling fan to come on in these conditions, and with high ambients, I know mine does. The fact it *does* come on, *if* it has been set to the correct temperature, means it *is* required. It's not a glitch, it is doing what it is supposed to do. Yes, a bigger rad may stop it happening, but I suggest that would not be a sensible option, and as I say *certainly* isn't requied for a standard MGB.
Paul Hunt 2

This thread was discussed between 09/10/2007 and 16/10/2007

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