When I got my car it had 25mm cooling pipes which I suspected were meant for a 998 cc engine and looked a bit small. They were pretty rough so I changed the lot for a 32mm set up which is the diameter of the water pump and radiator stubs on my ex MG Metro 1275A+ and radiator. I notice that when the cooling fan cuts in it stays on for ages (I have an 82 degree thermostat and have also replaced it just in case) and I'm wondering if the coolant is flowing through the radiator too quickly and not being cooled. Does anyone agree?

I'll be taking the front off soon to fit a brake servo and when I'm at it I'm thinking of getting a couple of reducers to create a short restriction just after the radiator but haven't a clue what to narrow it down to - any suggestions?

Getting the cooling system right seems to be a bit of a black art (or at least the science is beyond me) and I certainly don't have a definitive answer, but I offer a few thoughts based on own experience.

A standard Mini (or Metro) cooling system is perfectly capable of cooling a Mini, so you shouldn't need more capacity, or more water, in a GTM.
Even with 25mm pipes you've almost doubled the amount of water in the system, therfore the water is now spending a relatively large amount of time in the pipes and a small amount of time in the engine and radiator.

I originally had 25mm pipes like you and found that although the engine cooled OK in 'regular use' as soon as it got hot it didn't cool down again. Similar to your findings I think?
I believe what is happening is that there's sooo much water in the system that it keeps the engine cool until all the water has heated up to engine temperature and then you're b*gg*red!

My son who knows a lot more about hydrodynamics suggested thinking about it like this:
Consider that you have 2 buckets.
One bucket is being heated up (the engine) the other is being cooled (the radiator)
What you're aiming for is to squirt the hot water from the 'engine bucket' into the 'radiator bucket' where it can cool down... and then squirt the cold water from the 'radiator bucket' back to the engine.

Thought of like that, what you need is smaller pipes not larger ones, so that the water can 'sit' in the engine being heated up, then 'sit' again in the rad being cooled down.

FYI My current arrangement is 15mm pipes, a Golf 1.6 rad and fan/housing and a small electric pump (Vauxhall heater pump I think)
The system works effectively although I have wired the low speed fan and pump to run continuously.
I have also arranged for them to continue for a few minutes after the engine is shut down as I had problems with microboiling in the head.

As above really, you can go too big on the pipes, I run 25mm pipes and can keep my Honda under control, when racing the engine usually stays running most of the time even on hot days and when using the nitrous it puts extra heat in but seems to handle it ok.

Neil

Thanks for all that - I seem to have opened another can of worms ............

I must admit that I hadn't considered the extra coolant capacity contained in the pipework thinking it to be a good thing but, to use Dino's bucket analogy, I would have thought that the water squirting (smashing technical term ) would be controlled by the thermostat but, then again, I've drilled some holes in mine for when the heater feed is turned off. It's a complicated subject, obviously.

One final question, then - would I gain anything by fitting a reducer into the pipework or would it be necessary to change the whole lot?

My pipework was all 25mm but I fitted a Davies Craig electric pump which solved any cooling issues at a stroke, even when I put more heat into the system by using a Laminova oil/water intercooler to cool the engine oil via the water system.

Where is you fan thermal switch located in the cooling system?

Neil

The fan switch is screwed into the radiator

A sensible place for it so that should not be a problem. do you know the switching points of it, prehaps it has a very large temp switching range

Neil

I don't know the specified switching points, I'm afraid, although the fan comes in at about 85 to 90ish degrees on the gauge . It's the standard Metro switch and radiator and they're both new.

I agree with all the folk that have replied.

sitting in traffic on a hot day is the worse scenario for my rossa.
I have a 1.3 fabia radiator and a biggish fan that I switch on as soon as the traffic stops!
even in winter! (that's a bit of an exaggeration but not much)
nelly hasn't boiled over since I swapped to the fabia rad and manual electric fan switch.

in my experience is NOT the pipe diameter that matters at all - its a decent sized rad
properly cowled in with a big electric fan.

The heat-soak that you get off a fat cast iron manifold is huge. ive gone to a medium bore
cooper style tubular manifold -selected cos it has the LEAST amount of metal in it.
so im zooming along the M6 in june in the sunshine (ok a fantasy I know!!) suddenly stop in a hug jam and nelly sits there for two hours. in the first ten minutes ALL THE HEAT in the exhaust manifold gets dumped into the head via the head bolts and nelly boils over. not with the lightweight manifold she doesn't.

the other thing ive noticed is the water heated manifold makes a HUGE difference to cooling.
the hot water that goes through the manifold gets cooled massively by the vaporising petrol.
itsa huge effect. try piping water to the heater first then back to the manifold and see how hot the heater gets . then try piping the water through the manifold first then the heater.
on my nelly the heater NEVER gets hot if the manifold has cooled the water first . .
I get much better economy by feeding the hot water to the manifold first then to the heater.

if you disconnect the water heating from the manifold your car will run a lot hotter
and it will not run right unless the weather gets to 25 centigrade (its never does that here in Manchester) - you need that water heating to vapourise the petrol enough. Ive wasted a lot of time trying to run my car without water heating on the manifold - a total waste.
my thinking was air density and power but none of it works well in a cold climate with unleaded petrol.

Interesting what you're saying about the water heated inlet manifold. When I had my Scooby the aim was to cool the air as much as possible so that the greater density gave more power and I applied the same logic to my Rossa by cutting off the water hose stubs on the manifold! As far as I know none of the Minis had a heated inlet although some Metros did but all the tuners seem to offer them so there must be something in it - I'll have to rethink that one. My cooling fan is operated by the thermostatic switch on the radiator (MG Metro) and I also have an override switch fitted just in case... The front end is off the car just now (along with the roof, interior, back end, etc etc) and I've made up a 15" long 25mm restrictor from bits I had lying around in the garage and it will be fitted anyway although I'm not hoping for very much.

cooler air is denser - agreed.

im gonna start a new thread about this . . . .