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MG Midget and Sprite Technical - Cooling Question

Guys a curiosity question on cooling, at what temp does a cooling system become inefficient i.e. the ambient air temp fails to cool the radiator enough? I know this is a bit of a physics question and there is probably a formula ..........?
We had an unseasonably hot weekend 39 deg C or in old money 103 deg F , ambient air temp with a strong wind which was equally as hot.
The frog was okay at low speed as the electric fan cut in and although running pretty hot it did cool the rad temp down. However at motorway / highway speeds (where the fan shouldn't matter) it was still creeping up especially when rpm was above 3500. Once it crept up it stayed there, it was reading pretty close to 210 deg F but i know my gauge reads about 10 deg F over so not the end of the world especially as a 7 psi rad cap means boiling point would be raised to the region of 233 deg F .
That said not ideal, I may have to get a rear mounted elec puller fan to allow a bit more air flow.
Anyway back to original question was I at the critical ambient air temp where the radiator and cooling system start to have little effect ?

Ed H

The thermostat controls the temperature of your engine, not the radiator, and the best type of thermostat you can get is the original bypass type, which regulates the water flow around the engine a lot better than a simple thermostat. Thet are not cheap, but well worth the money.

You might advance your timing 3-5 degrees too, and run "hotter" plugs, these steps will also reduce your engine temperature.



Stat is an original brass type, plugs are slightly harder but only by 1. I might advance it a little more and maybe jump up 1 more in the plug range. Still curious to find answer to original question though.
Ed H

have a look in the Archives I think FRM has explained this in a reply about coolant
Nigel Atkins

The cooling effect of the radiator depends mainly upon the difference in temperature between the coolant and the outside air but also in part to the heat transfer coefficient of the cooling medium and then to how clean is your radiator inside and outside( fouling factors). Another factor would be whether your water pump is in good condition or not.
The heating effect of the engine depends upon factors such as the RPM, driving uphill or down, what gear you're in and general state of tune as well as the factors mentioned above.
All these variables mean that the critical temperature that you're asking about, which will occur when the heating effect equals the cooling effect, will vary from car to car and day to day and I believe could only be determined for a specific car by empirical means.

That probably doesn't help you does it !!
M J Chapman

if this carries on I'll suggest a thorough cleaning of the whole cooling system including the engine block, heater matrix(?) and rad and offer my simple method

oil is also cooling so a clean and efficient is also required with oil in good condition perhaps with a more suitable weight for the heat and less restistance for the engine, gearbox and r/axle

then of course other resistances, perhaps brakes catching a little

all on top of what's been put already
Nigel Atkins

110 degrees F. Outside temp Was were i started seeing the max out...that was 2 summers ago when i tried to make the drive to wisconsin for midget 50.

It took an hour to hit the 210 mark and stayed there untill i turned it off

That said... It really is an individual thing, no 2 cars are just adding an oil cooler i was able to reduce the water temp back to a constant 180 to 190

Wat freaked me out was watching the oil pressure drop as the water and oil temp climbed

Btw... I truly belive your right about exchanging the electric fan from a pusher to a puller...the rad is small compared to most, so any measurable obstruction can be a game changer

Prop and the Blackhole Midget

Australian built 1275cc Midgets were fitted with a 10row oil cooler as standard equipment, so the designers were well aware that the cooling system was marginal in a warmer climate.

If you are running anything better than a standard camshaft in your car, a 13 row cooler (MGB/Cooper S) is a good upgrade. I suggest that 25% of the engine cooling in an A-Series engine is done by the oil !!.

Most people have never seen a bypass type thermostat - it is cylindrical, not a flat disk. They were available from Moss USA a couple of years ago, but seem not to be available now - just the inefficient non-bypass type.

If you don't have a bypass type, fit a restrictor in the bypass hose with a hole no larger than 3mm, or plug the bypass if you choose.

Check that you have a 160F degree thermostat too - it helps.

My 1275 still has the original pistons and I have owned it since 1983 - 110,000 miles on the clock.

Safety Fast


Thanks for comments I think I experienced what Prop did which is the planet earth was just too hot on the weekend !!
I think I will bite the bullet and fit an oil cooler at some point to help on the hotter days and put on a puller fan to expose more of the rad than the pusher. To answer Nigel no the brakes don't drag the oil is fresh and the engine and rad were flushed in the last 6 months.
I've run the car in france on some pretty hot days 30 to 35 without any issues but I guess the extra 5 degrees probably made the difference and pushed it up just that bit furher.
Ed H

Tony the cylindrical thermostat you mention, i'm correct in assuming its not the sleeve that you can use instead of a thermostat for racing purposes ? see pic

Ed H

I was going to post this earlier but got called away then -
Nigel Atkins

In the earlier post I was just widening things out a bit with brakes ect. and not really meaning your car

I expect you may need to fit an oil cooler to deal with the hotter days in future, if you do I'd suggest an oil thermostat too but the following points still apply

I expect it probably is - but the fresh oil (and filter) does need to be good quality and the right grade for your use

(stick with this next bit) I do more than just flush the rad and engine block but even so when I had to refill it again after only a few months (don’t ask why please) I was amazed that I still got more crud out of the engine block drain and this was on a recon engine that had been flushed prior to these two times and without a huge mileage using fresh coolant mixture each time

could it also be the fuel getting hot and bubbling causing it to run lean which would in turn make it run hotter – and as said before mixture settings against the hotter weather

final thought – have you a fibreglass front end, the car I mean as they hold more heat
Nigel Atkins

not doing well tonight - I missed this off the John Twist post -

I think this is the Moss part Tony meant -

coincidence as I caught up with the JT videos the other night

Nigel Atkins

Nigel ,
Yep decent oil 20/50 not supermarket rubbish, steel front end and personal preference I always air on the side of caution and run the car a little on the rich side. Coolant I'm using is actually an inhibitor that does not have glycol in it, so works more on the properties of water so no increase in boiling point but better heat transfer reccommened for motorsport use. Nulon Ultra Cool.
I've drained the system recently and it all came out with no debris whatsoever and still looking as green in colour as it went in.
As an aside does anyone know will a high flow water pump GWP134 fit a 948 in place of a GWP132?

Ed H

If your overheating, it is because the rad is limiting, not the stat as the stat would be max open. So a lower set stat does nothing.
That said, 210 F is no problem. Just use the right viscosity oil.
Water has better heat transfer than glycol mix.
Art Pearse

If you where running glycol.. id suggest a mix of 30/70 ... 30 gylco and 70 water with a couple of caps of water wetter

Also drill an extra 1/8 inch hole in the stat face

Along with a puller fan, a bit rich on the carbs and the timming advanced a bit, and a 13 row oil cooler, id think your ready for summer

Prop and the Blackhole Midget

perhaps a very good 20w-50 or 60 might help

I don't want to be repeative but my coolant came out clean both times but both times when I cleared the engine drain hole out with some wire there was quite a bit of what I called crud, like scale flakes but dark coloured, both times, and I'd added a cleaner to first series of flushing and back flushing

sorry can't help with info about pump, good luck with all

I couldn't cope with 29 degrees so my car wont have to whilst I've got it :)
Nigel Atkins


Here is a note from the MGOC spares website ref the GWP134:

"Water pump GWP134
Cross flow radiator- Check vane to block clearance if fitting to earlier engines"

and from the Moss website:

Note: check clearance between vane & block before fitting"

Doug Plumb


Thanks it points me in the right direction after a bit of net searching it sounds like with some gentle grinding you can get the high flow cast impellor pump to fit. Has anyone out there done this to a small block A series or is it not worth the hassle of trying ?
Ed H

from an earlier post:
"Australian built 1275cc Midgets were fitted with a 10row oil cooler as standard equipment, so the designers were well aware that the cooling system was marginal in a warmer climate."
- you don't have one? - you need one, fit one!
what more to say.
David Smith


You could always send some of that heat over here, it's been a bloody cold spring!!
Paul Walbran

GWP 134 will fit (Impellor 13 mm), GWP 154 will not fit
(impellor 15 mm) but an angle grinder will do the job in 2 minutes.


Left rubbish (> 212 o F) only works in winter.
Right the original (worked for 40 years than started leaking)
Under GWP 134 ( stil works at 160 o F max)

Flip Brühl

Thanks for info did you have to grind block or pump? And in your opinion because it sounds like you've tried all the pumps is the upgrade from a 132 to 134 a mod worth doing
Ed H


No I did not have to grind the 134 or the block, it was a big improvement together with changing from 100% coolant to 50% coolant 50% water and mounting a plastic fan. Before the change I always had to open the heater in Paris traffic jams, driving uphill etc.


Flip Brühl

This thread was discussed between 22/10/2012 and 25/10/2012

MG Midget and Sprite Technical index

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