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MG Midget and Sprite Technical - Optimal coolant temperature ?

Good morning Getlemen
What would be the best coolant temperature to offer the 1275 engine.
Asking in another way, lets say the temp gauge was not equipped with figures, but as most, a green area and a red area. In which temperature range would the green area be?
Jan Kruber

Hi Jan. Paul Hunt has lots to say about temperature gauges here Yes he is talking about MGBs and V8s, but the same rules apply to the Sprite/midget.

So, according to him (and I would agree) the middle of the scale should equate to approximately 82C, and the H or red zone will be above 100C, possibly as much as 110C full scale. Of course the coolant won't boil at 110C because of the antifreeze mixture and the pressure in the system.

My V8 has a dual gauge marked in Fahrenheit. One hot, hot day in France the gauge climbed above the top of the scale at 230F (110C) and went into the oil pressure zone, but the coolant didn't boil. I use 4-Life coolant which has a very high boiling point - quoted as 180C.
Mike Howlett

I've had two gauges in my current Midget one brand new. Now I'm not saying either of them gave accurate readings but they gave readings that other Spridget owners also got.

I can't remember what readings I got on my previous Spridget as it was over 20+ years ago but I think they were the same.

IIRC you don't run your Midget in winter so like me you probably have an 82c 'water' stat (not that it matters after the stat is fully open).

Imagine the combined oil water dial as a clock face - then the car going along normally on open roads in normal conditions (i.e. steady driving above 40mph but not 100mph in 30 degree sun) the bottom hand will be at about 7 (or 25 minutes to the hour if you want).

I glance and expect to see 5 passed 7 on the dial when running.

Nigel Atkins

sorry forgot red

Nigel Atkins

if you want to compare numbers on the other two gauges see below but bear in mind those three guage might show different needle positions when fitted to the same car. If you wanted to you could calibrate the gauges so that the usually needle position sit at N / 82c / 180f.

Treat the gauges as just that, a gauge. If your car is running well note the needle position and only worry if it moves from that position without a good reason.

A chap in the local club has a Rover V8 engine'd road going Cobra rep and says his gauge shows 60 for normal oil pressure now that would unusually very high but if that's where his needle sits and the car runs well then fine as that is his gauge of normal reading on his dial.

Nigel Atkins

I think that the thermostat of choice has more to do with having a safety margin, than of representing the optimal temperature for the engine. Optimum engine temperature will be higher than that. I believe that most road going Midgets are operated below the ideal temp for power and efficiency, providing a bit more lee-way for the owner's peace of mind when heat produced by the engine begins to exceed cooling.

On modern cars with more sophisticated engine management systems, they usually run at a much steadier temperature with virtually no variation once warmed up, and this is rather higher. My VW was consistent at 90degs on the guage, whilst my Volvo runs at a very steady 92. BMWs run hotter than that but go to amber warning at 118 and red, 'limp mode' at 122

Optimal temperature is the hottest possible without boiling or cavitation.

Modern temperature gauges show normal over a large range of temperature - designed like that just to reassure the driver.

As such, and even with our "old style" analogue gauges they mean very little except to warn of impending failure and a warning light could do that just as well.
Chris at Octarine Services

All right, it is with a back thought I asked the question. And as usual, very interresting answers.
In my Midget 1275 there is a "standard" thermostat in the thermostat housing, 82 deg. C. In the past 3 years ownership, the gauge allways showed the figures 82C to 87C, depending on the outdoor temp. and the driving behaviour.
Slowly driving or idling would show the temp rising, off course. And when shutting off the engine would show an increased temp on the gauge momentarily.
This winter I discarded the old radiator due to a leak. I found an aluminium radiator at a very fair price, couldn't resist it, and decided to swap for an electric fan too. I bought a 9" Revotec blower fan and fitted it in front of the radiator. I found a switch, 87-82C and fitted it on an adaptor welded on the bottom tube, near the radiator outlet.
I took the car for a test drive the other day, outdoor temp. 16C. When warmed up, the gauge showed excactly the same figures as previous with the pulley driven fan, when driving, 82C. When home in the workshop again, I let the car idle for some minutes, just to see when the fan would be switched on. The fan was switched on when the gauge showed 98C. The temp decreased maybe 5 degrees and the fan was switched off.
So it seems to work very fine.
I thought about switching to one with a lower range, preventing the temp to rise to as much as 98C.
I have read though, that 98C is not a problem. Chris seem to confirm that a high temp is fine. Should I swap the 82C thermostat for one that gives a higher temp and is on line with my present electric switch?
That's why I asked for the best temperature in the 1275 engine. 82C, 87C, 94C or 98C?
Because now, with the electric fan it looks as if I can control it.
........well at least until one of you guys tell me I can't......
By the way, the Revotec fan seems very effective and doesn't make that much noise.

Jan Kruber

I'm not sure I've ever seen an optimal temperature figure for power and efficiency and where you'd measure that from but you're certainly right as far as I'm concerned about more lee-way for the owner's peace of mind.

I twice tried the 88c stat and although I know others have got along well with it I couldn't settle seeing the gauge needle higher than before and know my gauge under reads. If the gauge wasn't there and a light instead as Chris suggests I'd probably be fine, sometimes you can have too much information, same with when I had an oil temperature gauge in a car and drove in winter.

If you can't smell or see coolant coming out then you're OK but often by that time you could be on the path of a stoppage and I've had I think 5 HGFs so I've had my fill of them.

Nigel Atkins

I didn't see your post until after mine.

See my previous post.

Did I cover the range at which 'water' thermostats actually operate and their variances in your thread about fan switch? (or forgotten/ignored/dismissed/lost).

Personally I think it is better to control the temp sooner and perhaps lower to give that peace of mind as well as physical margin.

Also bear in mind at the moment you rad is very clean outside and in and there's no debris built up around your facing fan and brackets so the rad is getting full air and coolant flow.

As the water stat seems to operate as you want I would consider a fan switch that comes on at a slightly lower temp perhaps an 84-79 or 85-80.

16c is different to 30C+ weather but why not run it as it is until the warmer weather to see if and how much you might want to reduce it.
Nigel Atkins

You did send me the "waterstatrange" in a PM
Jan Kruber

Guy / Chris
You seem to vote for higher temperatures. My gauge is in Fahrenheit, but the figures are converted here to C. We do not know how accurate the gauges are, but as mentioned I have a driving temperature of 82 - 87C depending of the outdoor temp. This is without the fan switching on when driving. The electric fan is witching on at approx. 98C when idling. My thoughts was to narrow the range. At first I thought I would fit a switch with a lower range, but still higher than the driving temp, so that it will not switch on all the time.
But reading elsewhere, and it looks as if you two guys too confirm it, higher temp is better. So now I consider to swap the thermostat for a 88C one. This would narrow the operating temp range, wouldn't it?
Denmark certainly is more like a cold than a hot climate anyway.
Do you think a driving temp of lets say 92-97C is better for the engine than 82-87C?
Or should I just let it as Nigel suggest?
Jan Kruber

Unless the cooling side of your system is super efficient, the car will probably run at quite a lot higher than your chosen thermostat temp. It is better to think of your thermostat as being the minimum operating temperature, rather than the actual running temp.

Conversely, the fan needs to come on at near to the maximum running temperature that you want to achieve so that it acts as an 'insurance' against overheating. On a properly matched system the fan should only come on fairly rarely, or as a response to specific difficult conditions such as stationary traffic or a long uphill climb in warm weather.

It's quite good to wire in an override switch for your fan so you can also switch it on earlier, before the thermostatic switch operates. Then in some circumstances you may anticipate hotter driving situations and give the cooling system a head start.

I run an 88 degree thermostat in the V8 - it has twin electric cooling fans which cut in around 95 degrees.

Even in the heat of Italy, stuck in a motorway jam, the gauge never moved beyond 3/4 - normally just over 1/2.

If you have access to an infrared temp sensor then it is worth checking what the actual temp of the head is and comparing that with the gauge reading.
Chris at Octarine Services

Chris, I have an IR temp sensor and I will check it out.
Where do you have your fan switch, on the inlet or outlet side of the radiator?
Jan Kruber

You would normally have the fan switch in the top tank, or inlet pipe.

If you have it in the outlet, the coolant will already have been cooled by the radiator by the time it reaches the switch, which means it would have to be very hot in order for the fan to come on at all. Either that, or fit a much cooler switch.
Dave O'Neill 2

You would be better off with a thermocouple surface temperature probe for an accurate reading. The problem with IR thermometers is you need to know the surface emissivity which can vary widely depending on the surface condition, although most oxidised surfaces are in the 0.9 - 0.95 range. There are charts online giving emissivity values for a range of materials.
David Billington

Emissivity - that's a good word for a Pointless 'words ending ..ity' round!

'Make a note of the word emissivity, I like it and want to use it more in conversation.'

John Payne

My fan switch is in the top of the inlet manifold (V8) - just on the engine side of the thermostat.

For a 4 cylinder it should be in the top hose or top of the radiator.
Chris at Octarine Services

Yes, I always thought the top of the cooling system was best for a fan switch. Revotec make a very nice adjustable unit you can plumb into the top hose. I have one on my V8.
Mike Howlett

Good evening Gentlemen
Since I have chosen to have the switch in the outlet of the radiator, I found a switch in the range as low as 80-72C deg.
I think it works quite good. I have been test driving with outdoor temp's from 12 to 23C degr. The fan is never switched on when driving, only when idling for a while, but at 23C the fan was switched on yesterday when driving slowly in line. I still had the Fahrenheit gauge in by that occasionand when. When the fan was switched on, the gauge shoved 210F or 98C degr. I'm not sure how accurate the instrument was, though.
The other day I found a Smiths dual oil / water gauge with temp. in celsius, 85,90 brand new. I couldn't resist buying it, since I'm a little tired of the Fahrenheit instrument in my Midget. So today I fitted it, very intertaining to work up under the facia, by the way....:-) Before fitting the new instrument, I checked it in hot water against a modern reference thermometer. I checked it at different readings from 75 to 100C. It is fairly spot on.
So now I have a brand new instrument with temp. readings I trust. As a bonus my oil pressure miracolously has increased by 15 - 20 lbs over the range :-)
Well then, today, with an outdoor temp of 13C I drove a test. with engine at operating temp, the temp reading, as allways, was steady, but only 75 - 77C degrees. The fan did not switch on. I let the engine idle for some minutes, to se when the fan would be switched on, with the 80/72C switch in the radiator outlet. When the reading was 92-93C deg. the fan was switched on. It continued for a minute or two and was switched of. 86-87C degr. could be read on the instrument.
Now Im excited to see which driving temp reading I will get when driving in 20-22C or even higher temperatures.
Regarding the steady 75-77C degr. Isn't it a little low, or normal with a 13C outdoor temp? It's the temp. though, that my engine has been operating in with this outdoor temp as long I had the car. Nearly 3 1/2 years.
Jan Kruber

It seems to me that with the thermostatic switch on the bottom outlet hose, your system is responding to the efficiency of the radiator, and not to the state of the engine. I suppose that if the outlet temperature is low enough, then switching on the fan is not going to improve matters. But you still don't know what the state of the engine is. Maybe add a thermostaic switch to the top hose to drive a warning light if it gets too high?

Guy, I believe I can se the instant engine temperature on the gauge any time, right?
I have an termoswitch adaptor to fit in the top hose and I could fit a switch, lets say in the 87/82C degr. in it, to see how the things behave, to compare to the present setup.
Jan Kruber

Jan, I think I misunderstood your description. I thought you had the temp gauge operated by the same sensor in the bottom hose. But that is just for the fan switching, yes?

Yes Guy, the switch in the bottom hose is exclusively for operating the fan. My instrument in the dashboard is the usual dual gauge you all know, showing the cyl. head temp.
I found fitting the fan switch in the bottom hose was a more "esthetic" look than fitting the large aluminium adaptor in the top hose. I was convinced it would a question of matching the switch range so that the fan would be switced on, when the coolant increases in temp. above the "normal driving temp.
In my oppinion, fitting the switch in the top hose or in the bottom hose will give the same result, when the switch is the correct match.
With the present bottom hose 80/72C degr. switch it looks as if I'm close to the goal, but as mentioned in the previous message, I think I will do a test with a 87/82C switch in the top hose.....just to gain experience.....
Jan Kruber

>> ... with an outdoor temp of 13C I drove a test. with engine at operating temp, the temp reading, as allways, was steady, but only 75 - 77C degrees. The fan did not switch on. I let the engine idle for some minutes, to se when the fan would be switched on, with the 80/72C switch in the radiator outlet. When the reading was 92-93C deg. the fan was switched on. It continued for a minute or two and was switched of. 86-87C degr. could be read on the instrument.<<

Nigel Atkins

Nigel....?....are you there? :-)
Is your fine graphic to visualise that my prensent setup works quite fine within the "green" area.....?
Jan Kruber

no and yes.

I put it up as representation of your findings to a centigrade gauge as it's easier (for the likes of me anyway) than trying to calculate and visualise it on a Fahrenheit or C-N-H gauge.

And yes it would fall withing the green marks on the other gauge reading but they are not your gauge in your car.

As I think Guy (or someone) has put before I do wonder if the gauge tube picks up any additional engine bay heat or whether that is factored in to the gauge calibration, perhaps that's why your gauge had slight variance to your modern reference thermometer or the difference of temperature to the transmission tube being in ambient air to engine bay heat(?).
Nigel Atkins

Good morning Gentlemen
Nigel, I have a Celsius gauge in now, brand new.
Yesterday's testdrive with an outdoor tep. 18 - 19 C degrees.
Temp reading when driving as on the picture

Jan Kruber

.....and after some mintes of idling, the fan was switched on, temp. reading as on picture

Jan Kruber

.....and after a minute or two the fan was switched off at the temp reading as on the picture
I thinks it looks fine, there's till a little gap from the driving temp and idle / slow traffic temp. which in my eyes mean that the fan only running when needed
Switch is in the bottom tube

Jan Kruber

it was very good of your passenger to take those photos for you, my mind can't get passed this and the cowl not being fully screwed together. :)
Nigel Atkins

Nigel, I know, some plastic in the cowl is broken so one of the nuts isn't existing. The cowl is unfortunately out of stock, >I will try to live with it :-)
Jan Kruber

Black elastic bands and call them USA crash protection requirement.
Nigel Atkins

This thread was discussed between 11/04/2019 and 01/05/2019

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