Radiators
2 or 4 core ? Aluminium or Copper ?
Cooling System
Radiators are a Heat Exchanger. Heat from the Block and Head are transferred in to the Coolant, which then carries that heat to the Radiator, at which point it then transfers that heat to the Air. Simple enough, though it does involve transferring the heat at various points in the system. The more efficiently that heat transfer can occur, the more efficient the cooling system is overall.
While not the subject of this topic, one place that is so often over looked in this system is how clean the cooling jacket is in the Block and Heat, often they are rusty, which has very poor heat conductivity. The cleaner these are, the better the system can work. Clean and de-scaling the Head and Block are always a good place to start when looking for overheating issues.
I do read and hear often that the cooling systems in these cars are poor or under-rated. From my experience, the stock system has enough capacity for standard engines and even those modified for higher outputs, though perhaps under some limited conditions they maybe marginal.
The quantity of heat put in to the cooling system is around 30% of the total heat made in the engine, it's around the same amount of energy that goes in to driving the wheels. It does need to be kept in mid too that while you may have a cooking 130 HP monster with 20 PSI boost pressure under the bonnet, especially for street cars seldom will you be using all of those horses and when they are called on, the period of time that they'll be unleashed for will only be less than 30 seconds. Most legal driving around the streets in your Mini will only be using around 18 - 20 HP from the engine, adding a factor for starting from dead stops, hills etc, when averaged out, you'd be lucky to be using more than 35 HP or 27 kW. For most Minis, allowing for hot days and a margin, a cooling system rated for 30 - 35 kW should be adequate.
There is a mix of materials using in the standard cooling system, there's Cast Iron as used in the Block and Head, an 'Alloy' of a particular type used in the Thermostat Housing, Rubber which is fairly inert, but does provide both electrical and thermal insulation, an 'Alloy' used in the water pumps, likely similar to that of the Thermostat Housing, Copper & Brass in the Radiator as well as tin, from the solder and often in head gaskets.
The Thermostat Housing it needs to be noted is also a sacrificial anode. That is it scarifies itself (by electrolysis) in order to save the more expensive parts of the cooling system, like the head, block and radiator. Likewise, some water pumps are also sacrificial, but tend to be less so than the Thermostat Housings.
Another factor that needs to be kept in mid too is that the Radiator in these cars is electrically insulated, it is connected to the engine by Rubber Hoses and is mounted using Rubber Grommets. It is said to be 'floating', that is it is not electrically connected to Earth or anything for that matter.
Now on to Radiators.
'Cores' (Tubes)
Automotive Radiators appear to be referred to as having a number or 'cores', it's actually not the correct terminology and nearly all of them have only 1 core, the core being the assembly of the top & bottom plates, the tubes and fins. The slang term 'core' in fact refers to the rows of tubes when viewed from the top or bottom plate, front to back. Most standard Mini Radiators were 3 tube types, though some models for certain territories did receive a 4 tube.
Diving in a bit deeper, if look at how a radiator actually transfers heat from the coolant in the tubes (then to the outside air), it's only the coolant that's in contact with the wall of the tube that's going to transfer heat. The coolant that's in the middle of the tube can only transfer it's heat to the adjacent coolant and eventually find it's way to the wall of the tube, where it will eventually be dissipated. At lower coolant speeds, the coolant that's towards the centre of the tube may have some heat dissipated, but then the engines not making much heat at those speeds. At more normal driving speeds, this won't occur. This is why Radiators are made with flat tubes rather than round ones - to keep the cross section of the coolant as thin as possible.
With that in mind, if you calculated the surface area of a row of 2 vs 4 tube (core);-
A row of 2 tube 72.5 mm (typical)
A row of 4 tube 85.2 mm
Further, if you look at the arrangement of the tubes - and this depends on the design of the radiator;-
2 tube;-
4 tube (factory);-
You can see that the air passing through the core passes more directly over more tubes in the 4 tube design than the 2 tube. Granted, not all 4 tube are arranged this way ('staggered'), but the factory ones were. Of all the radiators I've tested, the staggered 4 tube have the most notable temperature drop. I've never seen a 2 tube radiator that had staggered arrangement.
Original Leyland 4 Tube Radiator Core.
Of all the Radiators I've tested, these do give the best cooling.
Aluminium or Copper ?
In regards to Copper vs Aluminium, for starters, Copper has a higher temperature conductivity, almost by a factor of 2, over Aluminium;-
Further compounding that issue is that a thicker section of Aluminium is needed for the same job over Copper due to it's poor fatigue resistance. The Factory Copper radiators had a wall thickness of 0.008" / 0.20 mm. The thinnest Aluminium type is 0.24 mm, though these have a high tendency to crack, more typical thicknesses are 0.28 to 0.30 mm.
Aluminium is a highly reactive material, especially when compared to Copper and Cast Iron, which are rather inert. Because of this high reactivity, the importance of the correct coolant and it's maintenance can't be over-stressed enough. Whilst coolant is always best to use, you can run for a very long time with just demineralised water when running a Copper Radiator, however it won't last too long at all doing this with an Aluminium based one.
Leyland first tried Aluminium Radiators with these engines back in the early 80's on the first Metros, but after about 18 months and the warranty issues they had (even with a 'proper' coolant), they swapped back to Copper. Rover eventually went this way with the MPi, though these used a different grade of Cast Iron in the Blocks, but even then, reading through the Technical Bulletins, they still had issues up to (and likely beyond) five years after they ceased production.
Examples of issues from using dissimilar metals
This Thermostat Housing (which as discussed above is normally a sacrificial anode) that's been run on an engine that had an Aluminium Radiator fitted and using the 'recommended' coolant. It had been on there for about 10 years;-
You can see the rust stain but otherwise, it's largely in good condition, no corrosion at all ! So here, the Block and Head have become to sacrificial anode and the thermostat housing will live forever !!
Subsequently, the block has been chemically cleaned (but at this point, not with caustic), you can still see the heavy rust in the cooling jacket. The guys tried a few different chemicals and this was the best they could get this one;-
Before;-
After;-
The cylinder head as found ;-
The Radiator was clean as a whistle, like new, no corrosion and no discolouration. This is all I have I'm sorry, I have lightened it up so you can sorta see inside it to a point;-
If you are contemplating an Aluminium Radiator, I'd suggest you have a long think about why you'd like to fit one after weighing up these matters here.
