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MG Midget and Sprite Technical - How hot is too hot?

Yesterday I helped out with the local carnival procession as someone had dropped out and a friend on the organising committee asked if I'd drive the mayor in the Frogeye. It has a 1275 with standard mechanical fan - you know what's coming next...

Walking pace for too long and temp climbs and climbs. There were some downhill bits where I could switch engine off but temp constantly reaching about 220F on gauge (don't know how accurate it is but normally shows 170 to 190). Radiator is re-cored vertical flow Frogeye with 4lb cap. 25% to 30% antifreeze so I reckon boiling point is maybe 230F. No fuel evaporation occurred with mechanical fuel pump so engine ran and re-started fine with decent oil pressure. Heater fan kept on.

Driving home about 0.5 mile the temp soon dropped to normal. Rad needed about 2 pints of coolant adding. I did a dry compression test with about 3/4 throttle and got 160 all cylinders.

But how hot is too hot? What are the chances of HG damage? Should I worry?
Bill Bretherton

Bill

My 1275 Frog has the same cooling spec although it does have an oil cooler but no thermostat.

If the temp dropped back to normal on running and the engine is performing ok with decent compression on all cylinders, I would not worry too much.
One comment though, the rad cap pressure is normally 7LB??.
Just keep an eye on the coolant level, if it does start disappearing with no obvious leaks then further investigation may be needed!!

A few years ago I was touring in Northern Spain and the ambient temp was 32 degrees. Going up a long narrow winding hill the temp gauge got up to 220 so I left it to cool off for 1/2 hour. A pint of coolant and all was fine with no futher issues.

Bob
Bob Beaumont

I reckon that's ok, and no damage done. A higher pressure cap would raise the boiling point, giving you a wider margin and less coolant loss, but of course it doesn't lower the engine temperature. Heater on full, including air flaps open is definitely a good emergency move even if it did risk melting the lord mayor's chain of office. I presume it was a chocolate one?
GuyW

Bob, Guy

Thanks, that's reassuring. I did a few miles earlier and it performed as normal so seems OK. Radiator coolant level not falling. Probably should get higher rated radiator cap.

If I do similar type event again I'll fix a pusher fan to inside of grill (I'm inclined to keep mechanical fan as "you know it works").

I didn't open the passenger flap as mayor was in full regalia and pretty hot already! He knows about classics as his daughter has an MGB and he was watching the temp gauge as closely as I was!

There was an American "Hudson" ahead of us which overheated and wouldn't re-start for a while - I didn't have that problem.



Bill Bretherton

Couple of weeks ago I did my first long drive and went to Nick Mason’s open day, when he get his amazingly collection out around the grounds of his very nice house.
Took about 2 hours to get there. It was a hot day and we had to go through parts of Bristol and Bath. Over time the temperature gauge got hotter and hotter. Was hovering around the 212 mark and I was getting worried, but we made it had a great day.
It was the same on the way home and during traffic in Bristol went above 212. I was waiting for clouds of steam to appear from under the bonnet but it made it home with no issues.
I did feel it wasn’t as smooth or happy as normal but that might just have been my anxiety.
Anyway since then have been trying to find out if there is anything I can do to improve the cooling.
I did some testing. One was boiling some water filling a cup and putting the thermostat and a digital thermometer in it. There was a difference between the two. See the photo.
I will try again to put both gauges into a boiling kettle. Just wasn’t much room with everything still attached.
The readings on the left are the digital ones.
I also removed the main thermostat and out that into a boiling pan and it did open at the correct temp.
I was also given a digital endoscope so fed that into the radiator to see what it was like on the inside and it was very clean following all the flushing I’d to remove the mouse larder of peanuts whilst the radiator was in storage.
I guess you just have to get used to your car and what its limits are.
I’m going to speak to a local classic radiator company to see what options there are to improve the cooling ability whilst retaining the original look.
Don’t really want to put in an electric fan.
I’ll let you know how I get on with the radiator guy.






James Paul

James

You'd think 212 was OK as the coolant boiling point will be somewhat above that depending on cap pressure and antifreeze concentration.

I don't really want an electric fan either but they are, of course, very useful when stationary in traffic.

Nick Mason's place sounds interesting!
Bill Bretherton

Bill--it sounds like your fan is too small to me---Your car should be able to sit and idle all day without overheating
You say you have a std. fan, is that std bugeye pressed steel or std. 1275 plastic
You need the six blade plastic job to keep a 1275 happy in the traffic

https://mossmotors.com/434-560-fan-plastic
William Revit

Willy
It has the 1275 plastic fan! However, I think the rad is original (1958) and doesn't have a shroud - maybe that would help?
Bill Bretherton

Bill

A shroud does make a difference as it helps funnel air through the radiator especially at speed. I believe it was an early upgrade by the factory to reduce overheating. Mine has it but still gets warm in traffic!!!
Bob Beaumont

So does the new plastic fan fit the original fan pulley on a 1098?
James Paul

Bob
Thanks. Does yours ever get to 212F or above?

James
I'm not sure, hopefully someone will answer.
Bill Bretherton

Bill

I doesn't just idling but it has if the ambient temp is high (plus 32) In cooler conditions it runs about 190 in traffic.

James

Yes the 6 blade plastic does fit.
Bob Beaumont

That's good to know thanks.

My issue is if I take it easy say around 50mph it never gets near 212 even when idling, usually 190 to 205. Perfect!

If I push it to 65mph for a bit or climbing hills it starts to rise and rise getting closer to 210 and then on to 212. But slowing down doesn't really bring the temp down to 50mph levels. It just stays there. Perhaps I am not driving at a slow speed for long enough.

I've come back and left it running on the drive for about 5 mins and it just stays running hot.

James Paul

IIRC when fiting a 6 bladed plastic fan a couple of points:

there are blower [Mini etc] and sucker [midget] type blades

fit the correct way round

use an ali spacer to get the fan close to the rad

I am happy to be corrected as these are just vague memories.
Doug Plumb

James

It should not overheat as you describe.It should run about 165/190 even when going at 60 plus. May be worth giving the engine and radiator a good flush out. The six blade midget fan will also help.

Bob Beaumont

"there are blower [Mini etc] and sucker [Midget] type blades. fit the correct way round"

Don't they respectively blow or suck regardless of which way around you fit them ?
T Carter

A cowling also greatly increases efficiency by reducing radial movement of air along the outer part of the blades, which is why modern set-ups have a full cowling. But half a cowl is better than none.

Nothing wrong in principle with 212 deg, provided the coolant isnt boiling or near to it. Most modern cars dont have the fan cut in until past that point. But they will be running 15+psi caps and 50% antifreeze. Combined this pushes bp to around 125C iirc.

Both these are important, i wouldnt be keen on going higher than 7psi on the vert flow rad due to the size of its header tank, but that is an improvement on 4. Upping the antifreeze concentration can be done easily and is worth it in hot weather for the increase in bp, even though heat conductivity is reduced a bit.

A low load idle temp increase is less of an issue than if under full load. In the latter case there are likely to be small pockets of local boiling around the ex ports, chambers,tops of bores if there isnt enough margin between coolant temp and its boiling point. And when there is gas rather than liquid against the hot bits heat transfer is drastically reduced.

Paul Walbran

The problem I've found when much over 195 ish, is the under bonnet temperature gets too high. I prefer to keep mine under 190, about 185 max.
anamnesis

Thanks Paul. Just to say that the car did 90 miles today on the local classic run and ran as normal, so must be OK I presume.

Generally runs between 180 and 190 Anam.
Bill Bretherton

The A-series engine has a operating temperature of 82 degrees Celcius - that's when the thermostat should start to open.

The thermostat will open and close, regulating the water flow to maintain the engine operating temperature around 82 degrees Celcius.

Attached is a early engine cutaway drawing showing temperatures within an engine - the A-series is almost identical in the temperatures quoted.

Also bear in mind that the oil does the same amount of engine cooling as the water - around 11% each. The rest of the heat generated by the engine (78%) is sent down the exhaust, dispersed by convection or by radiation.

While the water cooling is important, don't forget all the other ways you can remove heat from the engine.

Sitting in slow moving traffic can build temperatures in all components under the bonnet, and you simply cannot expect the water cooling system to take away all this additional heat. An oil cooler can help, but you really need that airflow around the engine to keep under bonnet temperatures under control.

I hope this helps answer your question about what is too hot for your engine.

Tony



A L SLATTERY

Bill
What thermostat are you running---i'd go for 75/165, if it's any higher than that it'll just generate unwanted heat that just keeps multiplying on itself-
William Revit

Willy, t/stat is 82C which seems fine for typical driving but I could try a lower one.

Tony, thanks, interesting diagram. What I was specifically asking was what's the highest temperature the A series will sustain for say 15 minutes, without damage to HG (say). This is assuming coolant is below boiling point. So, say, is 220F OK for a little while (that's what my gauge read and engine still seems ok).
Bill Bretherton

Bill,
As you can see in the image, the cylinder head in the area above the combustion chamber is around 150C - obviously this is way above the boiling point of the water so there is rapid heat transfer to the water in this area.

When you can hear water boiling it's usually in this area first - once the water becomes steam the head temperature can rise quickly to way over 300C. That's when you can expect HG damage - no so much due to the high temp, but the rapid escalation of the temperature and different expansion rates of cast iron, steel fire rings and the gasket material (and the head studs/bolts).

As long as the water is not boiling in the head, you have a stable situation. No one can tell you how long your HG will last once you have steam in there.

Safety Fast

Tony
A L SLATTERY

Tony
Thanks, that answers my question! I was too preoccupied with water temperature alone I guess whereas, as you have clearly explained, one has to consider what is happening in and around the head itself.
Bill Bretherton

At one time I used to swap the thermostat on my car, using an 82 in summer and an 87 in winter. My logic was that in winter I didn't want the rad to come into the cooling regime whilst the other heat paths were still adequate on their own.
GuyW

Decided to get my radiator upgraded. Was getting so annoyed with it running hot.

Took it to Arrow Radiators in Bristol and they said I had two options. Replace the core with new closely matching the 69 channels or for an extra £40 increase the efficiency by about 30% with a 99 channel core.

Went with the 99 core and did a test run yesterday. I have a 82 to 88 thermostat and I was pleased to see the temperature stayed around that.

They said the old channels were furred up but nothing they hadn’t seen before. I must have flushed it both ways about ten times since the rebuild, but I guess after 60 odd years not amount of flushing can increase the efficiency back to original specifications.

Total cost was £270 which included testing and painting it. A reconditioned one from Moss with 69 channels is £224. Upgraded aluminium one is over £500, so I think it’s a fair price.

I can now get back to enjoying driving it rather than worrying about it getting too hot.






James Paul

James
Sounds like money well spent! I used Arrow radiators but can't remember which core was fitted. He seemed to do a good job. He sprayed it black while I was waiting to collect and dried it with a bit of heat! I had the heater core tested there as well.
Bill Bretherton

This thread was discussed between 18/06/2023 and 22/07/2023

MG Midget and Sprite Technical index

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