At the time the photo was taken, the longer hose went through the intake (my intake has provisions for this) and to the lower radiator hose. This isn't ideal IMO because it skips going through the normal radiator for additional mixing and cooling. Ideally, you would run it into a sandwich plate under the thermostat which regulates the flow and goes into the radiator for additional cooling and mixing. 

That said, I saw a good bit of additional cooling (no longer overheating at all) from just having it run back into the lower radiator hose. 

I found a small Derale that I intend to use if I can.

They make a variety of fluid coolers.

//derale.com/products/fluid-coolers

I don't understand fluid dynamics fully, but certanly rate of flow through the system is a factor.

What are the guidelines for changing the pulley size? Is the purpose to slow the rate of flow down when spinning at high RPM for a high % of time?

   sir.. i don't like the cut of your jib. lashing out like that...sigh..

First piece of advice from you that people should heed....

too big radiator and too long radiator hose used for the auxilliary radiator might jeopardize the good flow of coolant throughout the cooling system, the mechanical water pump with tiny impeller might not be enough to pump enough volume of coolant, it's always a  good idea to install a heat resistant electric 12v water pump midway to help push the coolant throughout the system.  it needs to be tapped on a independent thermostat attached to the engine block. 

you need to install the electric water pump in the lowest point of the auxillary radiator.

if you don't want to listen it's up to you.

don't use candy thermometer for heaven sake.

Does anyone have a water flow diagram?  I assume the flow to the cabin heater core is a parrallel path which can be opened and closed and still allow flow through the rest of the circuit.  I'm now curious if I run the aux radiator by using the cabin heater core path, and if I have a sufficiently low pressure drop aux radiator, would I then be robbing too much flow from the current engine radiator and in effect reducing the heat transfer there?    Or robbing flow from the engine block channels (which would be even worse)?   I guess ideal would be to be in series with the engine radiator so there is no robbing of flow. 

Thanks, looks nice.  Where is the longer hose from your aux radiator connected to?

Well, the one I built for my hot-rod Moke is probably a generic ATF cooler, but does have vanes built through the "loop" that runs inside the "radiator" portion.  These provide resistance to fluid passing through.  Before I put a youtube link here,  there are two main reasons a mini will get hotter even with good airflow.....  The mechanical fan can be installed backwards, reducing it's efficiency, OR- the heater is bypassed and HOT water is routed right back into the water pump via the lower radiator hose, meaning it never goes through any heat exchanger.  If not running a heater,  you must not simply loop that hot water from the head right back into the water pump!

This auxiliary rad set-up works fantastic!  This fire breathing Moke used to have me driving with a very nervous eye on the temp gauge at all times.... and that was WITH a supercool two core radiator, the 11 blade fan (installed properly!)... This aux cooler makes a huge difference keeping the engine cooler even on hot Sacramento summer days!  You can feel a vast amount of heat being dispatched into the air flow, the in and out hoses have a very noticeable difference in temperature!

https://youtu.be/vrymzzq6Olk

I had the same issue as you. 998cc, Living in Texas (Dallas area) and a good amount of highway driving. The engine would just keep up at highway speeds because of the airflow, but don't you dare slow down below 30. Heaven help you if you have to stop. The heatsoak was over the top and would cause an overheat quickly. I even fit an oil cooler to drop my oil temps because the heatsoak was causing the oil to get too hot as well. I believe I went to the parts store and picked up a heater core for a 90's Chevy Astrovan. It is meant for coolant, the right temps, and the right pressure. You'll be glad you bought a heater core instead of an oil cooler just as jeg pointed out.

Here's what I did. Very simple, and easy to remove and re-route to the original heater when winter comes around.

Check these out:

//www.calverst.com/articles/Cooling-How_it_works.htm

//www.calverst.com/articles/Cooling-Controlling_water_temp.htm

"I'd like a pound-sterling for every time I've seen an oil cooler doing duty as an auxiliary coolant radiator.  They simply do not work at all efficiently.  Their design makes them grossly inefficient as the coolant 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."

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My extra rad is a GM pickup truck heater core...mounted one in a removeable frame in the Cdn. Mini 1000 sedan and strapped one to the grill in the Aussie Mini Window Van.

Space available differs for roundnose Minis from Mark I/II to Mark III on, especially thickness, so look at the space available closely.

Here in Canada, there are a large number of heater cores available in the catalogues in the parts stores....some for 1/2" hose, some for 5/8"...

There is a OZ built 1973 Mini Clubman GT for New zealand market being auctioned now (see details on the Mini Collective on FB).  These GT's had a factory installed extra radiator (last photo).

The extra fan only helps in traffic, it does nothing at speed. If you are running hotter at speeds you may want to change out the radiator, i am not sure what you are using but the super 2 core radiators work well here in Florida with the 11 blade plastic fan. Also make sure everything is timed and adjusted right. A small heater core from a Chevy Astro has worked well for others on the board but if everything is set up correctly with the right radiator and correct thermostat you should not need it. Let the flaming begin LOL.

get the radiator from Honda Silverwing or the Suzuki Burgman 400.... from ebay / there's one with harness and fan for only $65.. has a very good coolant flow.

im buying one soon.

I used a heater core from a Mini for my aux front mounted rad.  Others have used motorcycle rads or various other very small heater cores.

I don't know the answer to your specific question though, sorry.

My mini has the side mounted radiator, and the Texas heat is a little much for it, especially during highway cruising (first car I've had which gets hotter the faster you go).  So, I'm thinking of adding a small front mounted auxiliary radiator by redirecting the coolant flow from the cabin heater core into a smaller front mounted radiator that would help out at highway speeds (and hopefully avoid me having to add a fan).  Problem is I can't find a good small antifreeze radiator that I can easily fit up front.  I can easily find small oil coolers however.  Question is, can I run antifreeze through oil coolers, or would there be corrosion issues, etc.