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Factory 11-blade plastic fan, one of the most efficient fan ever fitted to a Mini. Pushes a lot of air with very little noise. If you need more, then fit a tropical fan, part# 2A998; they push even more air but are substantially louder than this 11-blade version and will not fit with the 4-core radiators.
Cooling: Controlling water temperature
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The introduction of the Cooper S proved to be a testing time for the Mini's systems, but conveniently provide a guideline as to what the standard cooling system was capable of - that used on the 'S' was marginal to say the least! It wasn't uncommon for many S's to spew water from their overflow pipes when ever it was doing anything other than a steady 70 miles an hour, over-heating eventually caused through water loss. Perhaps some deductions can be made from the following…
There are a number of elements involved in controlling water temperature. Some confusion over what to sort first when over-heating occurs leads to wasted time and money, and possibly terminal engine damage. Maximum power is usually generated from A-series engines at 70 - 75 degrees C (160 to 170 degrees F). The main problem with this on a road car is the oil's unlikely to get hot enough for maximum performance - the results outlined previously. Another being that the heater (where needed) will be grossly inefficient. So, excluding race-cars, the optimum temperature to aim for is 85 to 90 degrees C (185 to 194 degrees F).
Radiators. No amount of tweaking the rest of the cooling system will help if there simply isn't enough cooling capacity in the radiator. Water capacity used to be the answer, hence the production of four-core radiators. It's possible the improvement in cooling was a product of more surface area created by the extra tubes, but the inefficient airflow through the congested radiator area reduced its ultimate effectiveness. In reality, effective surface area's the answer, and why the latest after-market, super-efficient two-core radiators are the best. The standard radiator can just about cope with a standard engine in most cases. The exception appearing to be the fuel injected cars. They'll stand the limited modifications that can be made without problems. Perhaps it's the 'brain' compensating for it somehow in trimming ignition and fuel? If you're significantly increasing the power output, I strongly advise fitting one of the aforementioned two-core radiators. And ALWAYS have water flowing out of the heater tap take-off. If no heater or auxiliary matrix is used, plumb it into the top hose. If it's put back into the bottom hose it won't work properly, if at all. The water going into the bottom hose at that point MUST be below that in the main hose coming out of the radiator.
I'd like a pound-sterling for every time I've seen an oil cooler doing duty as an auxiliary water radiator. They simply don't work. Well, they do a little. Their design makes them grossly inefficient as the water flows through too quickly, and material spec causes minimal heat transfer. If you need to run an auxiliary radiator, use a heater matrix. See 'Cooling - How it works' for hook-up details.
An expansion tank could be the answer if your motor runs at the right temperature, but is prone to spewing water out the over-flow at odd occasions. Usually when come to a standstill after tramping-on a bit. Water passes into it when over-flow occurs when hot then is drawn back in again when cooling down. Make sure the pressure cap's fitted to the expansion tank and a flat, plain cap on the radiator.
Fans. They're there to cool the engine whilst at low speed. Fact. Once above 35 mph or so, it's airflow through the radiator that does the cooling. Electric fans only help up to about 30 mph, so fitting one won't cure hot running at speed. The fan creates a barrier to airflow at speed; trimming the blades down in length (NOT removing blades) can often help. Generally the standard plastic fans are the best all-round as they are aerofoil shaped, cutting power consumed, increasing airflow, and quietest running. Two-blade fans are good but noisy, four-blade fans made up of two two-blade fans more so of each. Six-blade 'export/tropical' fans better, but noisier! Both eat horsepower.
Water pumps. One good thing that came from 'S' development - an improved water pump! Unfortunately, the water pump has fallen into the oil pump syndrome - biggest is best! True for road cars spending most of their time at low-ish rpm under load, but not for high revving engines. The A-series pump is essentially centrifugal; it's pumping capability squaring with engine rpm. The design's such that maximum efficiency's around 2,000rpm, so at low speed it's hardly moving any water. At 2,000 rpm it's pumping all the water needed to cool the engine, so higher rpm just means it's sapping power. If your engine spends all it's time north of 3,500 rpm or so, a deep impeller pump is costing power, and may be causing cavitation, reducing cooling efficiency. To mediate the A-plus motors got a bigger diameter pump pulley (first seen on the Ss'), and should be used where possible on modified road engines.
I would very strongly advise against the use of the after-market water pumps that have the 'folded tin' impellor as opposed to the cast iron one on the original equipment types. They are grossly inefficient and have a tendency for the impellor to fall off at the worst moments! There are some about with plastic impellors. They seem OK, but I haven't put one to test on a race motor yet. All I can say is I haven't seen a road car with one fitted that has failed yet.
Recent testing has seen the growing popularity of electric water pumps. These have to be the ultimate answer, as their pumping capacity remains constant, as they're completely independent of engine speed. Consequently cooling efficiency is far greater. The only two drawbacks being their initial cost, and installation, as adaptors have to be made up to blank-off the water pump mounting hole. Both, however, are well worth it - the results are outstanding. Not to mention the fact the water pump consumes power to drive it and reduces accelerative power output - to the tune of 4 bhp on a small-bore engine and 2 bhp on the large-bore ones! A further benefit is that the pump can be left running with the engine off after a race/hot/long journey to reduce the problems associated with the 'heat-sink' effects of non-circulating coolant at stand-still. For further information on electric pumps, see relevant article.
Coolant additives. Too many folk seek solace in antifreeze. They keep adding more and more in the hope it'll solve their problems. Whilst a small amount of antifreeze does help marginally as it breaks down the water's surface tension (waters only real drawback as a major coolant), in large amounts it actually makes matters worse (see 'Cooling - How it works' for further information).
The only additive I've ever tested that actually lives up to expectation/recommendation is Redline's Water-Wetter. This stuff basically breaks down water's surface tension without affecting its cooling capability. This maximises water's wetting capability, getting as much water against the metal surfaces of the water jacket as possible. Consequently it prevents the hot-spot syndrome outlined in 'Cooling - How it works'. I always use the liquid product (they do it in crystalised form too, but I'm not so keen on that). Temperature reductions in the order of 8-10 degrees have been experienced. Brilliant stuff. It also acts as a corrosion inhibitor - effective enough to stop ALL corrosion on the block water jacket walls, and the water pump impellor/housing. Lubricates the water pump seals too. Most impressive. For the racers even more good news is it doesn't make your slicks slippery if it gets out of the cooling system.
For further cooling information, see 'Lubrication - Temperature critical'.