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Chasing Overheating - Again!

A different electric fan

by Graham Pring

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This article was first published in 2009.
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Back in Chasing Overheating I looked at the problems I was having with my V8 Cobra replica in the overheating department. The car, equipped with a 383 cid stroker, had a major overheating problem when the car was stationary. I experimented with different electric radiator fan pitch angles and changed the bypass arrangement, but finally solved most of the problem by converting the single pass radiator to a triple pass design.

But while the cooling system performed much better, in just the wrong combination of circumstances – hot weather, idling and with all the heat sinks (engine, gearbox, oil and coolant) up to operating temperature – there could still be a problem.

Time for some more work!

Fans Again...

If by altering the fan blade pitch angle I had perhaps doubled the air flow through the radiator, as evidenced by increasing the current draw of the fan motor from 7 to 15 amps, then what else should I look at? It certainly felt like the airflow had increased a lot from the original fan flow, although testing had indicated that the cooling capacity had not changed much.

But where could I find a local (Australian) fan setup that would flow anywhere near the numbers 16 inch ‘monster’ (3000 cfm) fans were flowing?

I checked the Ford Falcon EL/EF and GM-Holden VT Series I and II dual fan setups and decided they would probably be fine if they could be made to fit, however after measuring the physical size of both setups, I found I did not have room to fit either unit.

But out of interest I checked the power usage of the genuine EF/EL, GM-Holden VR/VS single fan and VT Series I/II at a friendly wreckers and found a few large differences. These measurements were done by connecting the fans to a battery and running them in an open area so that there were no airflow restrictions on the entry or exit to the fan systems.




VT Series I (Dual fans)

Small 5 blade


Large 4 blade

9.4 (15.4 total)

VT Series II (Dual fans)

Small 7 blade


Large 5 blade

12.5 (25 total)

EF/EL (Dual fans)

Small 6 blade


Large 6 blade

13.0 (26 total)

VR/VS (Single fan only)

Large 5 blade

14.0 (total)

If I’d had a free choice, I would have preferred the Falcon unit as, when sitting behind it, it certainly felt to have the highest airflow.

Available Space

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The radiator in the Cobra is mounted vertically and doesn’t quite fit the profile of the car - the lower corners of the radiator protrude below the bodywork. This layout is what made mounting the above fans so difficult.

Searching the web revealed that SPAL make a dual fan setup, complete with fan shroud and mounting system, which pulls 2780 cfm. A photo of the unit showed that it has one-way flaps in the shroud. These are often used in current model cars; they allow good airflow through the shroud when the car is at speed, but at low speed are pulled shut when the fan is working – the best of both worlds.

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The SPAL design looked good, especially as the size was great for my radiator’s dimensions. The current draw was listed as being between 25 and 36 amps, depending on airflow restrictions, fitment and other items such as rated and actual current.

I emailed the Australian distributor and had no response, so I hit the web and ordered one, a dual 11 inch Fan Puller type (SPAL [Correggio, Italy] Advanced Technologies – 30102052) from the States and asked for it to be sea freighted as I figured I could live with the current fan setup until it arrived after winter.

Junk BMW Fan

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While waiting for the SPAL fan to arrive, I acquired a second-hand Bosch 16 inch fan and motor from a BMW X5. The fan was from a 1999 model and was being thrown out because the fan speed controller was defunct.

After cutting the controller out of the loom and removing the fan blades, I checked the resistance of the motor and it appeared to be about what I thought it should have been, or almost zero ohms, so I thought it was worth testing.

I held the motor in my hands while connecting the power leads to a battery. I was remembering the fact that my original fan motor pulled 1 amp without the fan attached and I wasn’t sure this motor would even work.

When power was applied, the motor immediately leapt out of my hands and onto the floor, as the torque reaction on start-up was nothing like what I had expected. Holding on very tightly and getting an assistant to connect the battery, resulted in a reading for the motor of over 7 amps - more current draw than my original 16 inch fan drew with the blades connected to the motor!

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The BMW motor and fan were then mounted to my original 16 inch fan mounts and shroud and tested for current draw using the 20 amp multimeter previously used. After the initial gulp, the reading settled at 16 amps so I thought it was worth a trial fitting.

Fitting the BMW Fan

The radiator and fan unit were then installed in the car, using the existing 30 amp relay wiring and 25 amp fuse.

I then tested the current draw in the car (18 amps) - so if it didn’t blow the fuse on start-up, then it would be OK.

I ran the car a few times to heat it up and cool it down to make sure all the air was out of the cooling system, then decided to go for a drive.

After getting the car’s gearbox, engine, coolant and engine oil up to temp, I stopped not far from home to take some readings. In turns out it’s lucky it wasn’t very far from home as the system ran well for a while with the fan sounding and operating well, and with the airflow under the car feeling a lot higher than I had remembered than my previous fan - but then the car started getting very, very hot.

I lifted the bonnet and found the fan had stopped working!

But why?

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Further investigation showed that the fuse had melted inside the distorted fuse holder. I couldn’t get the fuse out of the holder, so I let the car cool down and ran home to get some parts!

The fuse-holder had to be broken to get the fuse out so I went and purchased a 5AG inline fuse holder and a 40 amp fuse. I ran new wiring from the alternator to the fuse holder and then tested current draw - 18 amps.

Something didn’t add up! Why would a 25 amp fuse melt if it was handling only 18 amps?

Instead of using the in-line ammeter I pulled out a clamp-type ammeter and checked current flow through the single line from the fuse holder to the 30 amp relay, this time recording 22 amps.

This meant that, at the higher end of its reading capacity, my series flow ammeter was restricting current flow through the multimeter itself. The clamp-on meter has an educated guess at amperage by checking the electrical field around the wire, not physically measuring the current through the wire like the inline one, so poses no restriction to current flow.

Checking the wire size and flow capacity of the wire revealed that the wire was rated for 15 amps continuous and knowing it had flowed at least 22 amps continuously, I doubled the number of wires and connected another relay in parallel with the existing one.

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Further testing revealed that I was dealing with a fan which did not blow a gold, 40 amp 5AG fuse on start-up and used about 32 amps at times, depending on voltage supply to the unit.

In short, this was one serious fan!

More on-Road Testing

Road and idle testing indicated that the BMW fan had solved my overheating problems in these sorts of conditions, with the fan now being turned on by a thermo switch at about 100 degrees C and turning off at about 87 degrees C coolant temps.

With a reading of over 31 amps, I was a bit worried that there was an airflow restriction in the input to the radiator fan setup, causing the increase in amperage, so the next test was to check this and see if the current draw came down.

I put the number plate holder (370 by 120 mm) into the grille opening and the amperage was the same.

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I put a 330 by 330 mm square face flannel (what else would you use one for?) in the intake and got the same result, so the relatively small intake area was not of concern.

Maybe the answer was at the rear of the fan, where there was a large cross-member that I had put across the ladder frame chassis to hold an item which I later removed.

A few pieces of string on the fan shroud and aerodynamic testing was underway.

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To my untrained eye the bits of string were telling me that the air coming out from the fan was rotating as it exited the shroud. There was also very little action at the extremities of the blade length, while the inner section of blade had little flow and that was circular in motion, and the outside of the blades produced not much flow compared to the centre section of the blade length. So while interesting, the tuft testing didn’t tell me a lot that was immediately useful.

The only way to test if the cross-member was making a difference to airflow was to cut it out, remembering that this cross-member was not originally a structural unit or part of the chassis.

Out with the radiator and fan again and I removed the cross member, then applying a quick coat of paint.

While I was doing this I thought I may as well do some aerodynamic improvement to the leading edge of the front suspension structural members which were now readily accessible.

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Out with some plastic pipe, cut it in half and applied with silicon to the cross-members to shape the leading edges.

There was now a large space between the radiator and the frame cross-members without restrictions, so any decrease in load on the fan should have been noticeable on the ammeter.

All back together and the system bled, testing resulted in an ammeter reading virtually the same as that when the cross-member was in place, namely around 31.5 amps. Therefore, my aero changes behind the fan appeared to have achieved little in smoothing the flow.


Prior to the triple-pass conversion, the coolant would take 12 minutes at idle to go from 90 to 102 degrees C. After the conversion, I stopped the test after 20 min 45 secs as the temperature had reached only 95 degrees C.

Retesting at idle after the bypass restriction was removed, gave a time of 12 minutes to get to 95 degrees C, and 30 minutes to 100 degrees.

With the upgraded 30+ amps BMW single 16 inch fan in place, the time to heat up to 90 degrees C averaged 42 minutes.

That’s a massive improvement on the 7 - 15 amp, 16 inch fan used previously.

Hmmm, do I even need that SPAL fan that’s on the boat....?

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