Mark VII fan installation and testing in a 1972 Chevy C10 truck.

I had a Zirgo 16" 3000 CFM in my 72 C10 350/TH350 Non A/C for about a year and a half at this writing. It worked well. I like Zirgo fans they last and are trouble-free, that's why we sell them. One of our main product lines are relay kits that we make here. We have a single fan relay kit, a dual fan relay kit and custom kits; and, they sell very well with a good reliability record. But often I would get calls and emails for a relay that could run a two speed fan. We never had one and felt it may just be a short lived trend that would fade. A year later we now get even more calls and emails about two speed kits, and decided to take a serious look at what this is all about.

After many emails back and forth with 'Stan', from the HybridZ.org Forum, he clued me in on just who is doing what and what fans they are using. He sent me a ton of links and diagrams. I looked thru all of the web pages that I could find, and as it turns out for several years Mustang owners, Rock Crawlers and Rodders in general have been installing the 17" Two Speed Ford Taurus fans, and sometimes the bigger 18" Two Speed Lincoln Mark VIII fan. The main reason from what I read is HUGE CFM [Air Flow], plain and simple. These fans pull MASSIVE air, more than $300 aftermarket fans [reasonable, considering Ford spent millions developing the design. The next reason is that they come cheap.  At almost any junk yard there will be twenty or more 3.8L V6 Ford Taurus' or its Mercury Sable 'sister' cars with these 17" high CFM fans for anywhere from $15 to $40 [with the pigtail].

The bigger question was not where do I find it, but how do you wire it up. Most wire it straight to the high speed, and don't bother to use the low speed. But many wanted both High and Low speeds. Well I know how to do that and have always been pretty good with 12v power ever since I wired up the shop teacher's chair to the spark plug tester ! [that was an interesting conversation with my parents]. But anyway, I made up a kit to test following the diagrams that Stan and I shared; and, with a few tweaks got a fan, a pigtail, and here is what came of it all:

For this install we will talk about the 18" Lincoln Mark VIII fan and '67-72 Chevy trucks. This fan has been said to pull CFM anywhere from 3500 on low speed to Hurricane-Force 6 winds on high, can out-draw Clint Eastwood looking into the sun and may just well flip your car or truck over when it starts on high speed. But I think realistically its more like 3000cfm on Low and 4500cfm on High. It does draw a lot of [momentary] juice when it starts up on high with a big amperage spike; but, with a few changes to the wiring diagrams we eliminated that 100 amp spike or in-rush as some have called it. It's a momentary spike where the fan starts from full stop to full speed.

I do recommend a 105 amp or better alternator, because the fan draws 40 amps [continuous] on high running at 14V ... not counting any other accessories like A/C, or audio systems.

The RF24 fan that I am testing is 18" and has an RPM [not CFM] of 1350 on low speed and 1780 on high.   I don't know the true CFM.   It came from a '94 Cougar XR7 [a '94 Thunderbird LX will work as well]. The fan is 18.5" x 24", and with surprisingly little work, fit nicely in my '72 C10. As most of you know, the stock fan shroud drops into 2 clips on the lower radiator support on a C10. First, remove the old fan and shroud. You will need to get a piece of angle iron or aluminum 1/2"x1/2" cut to fit into that spot, and drop it into the clips. Next, you need to turn the fan upside down so that the wiring plug is oriented like the picture below, and then move the new fan all the way to the left of the radiator to clear the old fan pulley. It's tight, but it does fit with a GM belt drive assembly. You will need to notch the upper left corner of the new fan housing in order to fit under the radiator clamp. You will also have to cut off the upper most tabs [both sides] on the new shroud.

Mark VIII Fan

Make some kind of clamp or bracket for the top of the shroud. I used a piece of aluminum and gave it a few bends.
Mark VIII Fan

Mark on the bottom where the angle iron fits the fan shroud and attach it with some 10-32 bolts. I used two on the bottom with stop nuts. It's that easy!  You're done with the fan conversion install.

Mark VIII Fan

As you might notice, the shroud doesn't cover the entire radiator; but, it does meet the minimum requirements of 80% coverage [this fan actually covers 85%].

Mark VIII Fan

As you can see, our test Relay Kit is actually three relays and a 50 amp J-Case fuse. It's wired so that it always starts on low speed and steps through that low speed to get to the high speed.   It then steps down from high speed to low speed after it cools the radiator below 195*. The step-through reduces the huge in-rush [spike] of 100 amps because the fan is already moving. When the fan cools the engine below 180* it stops. The relay that we use will handle 150 amp in-rush, so 100 amp isn't an issue; but, it is hard on motors and relays to take that level of surge. We have 10AWG wire from the battery to the relays, and 10AWG wire to the fan.   The rest of the wires are 18AWG.

Mark VIII Fan

I told you that it was a tight fit. This fan cools better than any fan that I have ever used, and the price is comparable to aftermarket fans [often less].

Almost forgot. The two speeds come from separate temp sensors.  A 180* and a 195*.   The low speed keeps the truck at about 180-185* in traffic and daily driving at this point. The 195* speed [high] will come on when I'm towing. I have always had a problem while towing a 2300 lb trailer with a 3800 lb truck on it. It would get to 220-240* and as you would expect, the motor starts to ping.

I'll keep you informed on my next tow and how the set-up works out. I'll also let you know when this kit will be ready for sale. I expect a complete kit with a NEW fan (1 year warranty), two  sensors and the relays [with instructions] to go for about $260- $270 [which is less than most aftermarket fans ONLY]. There will be versions without the fan, and without the sensors for those that might run LSx [Gen. Mtrs. LS1, LS2 ...]  PCM's, which we expect to be under $85. Yes, we want this to be able to run off of your computer for those that had the stock two fan set-up controlled by a PCM.  We'll keep you informed on that, as well.

May 27 '09

It's been about two weeks and so far this kit is working very well. Low speed cools nicely and it has not gotten hot enough to hit the 195* sensor. I unplugged the low side so that the high side would come on, and so far it cools [literally] within one minute of the 195* sensor activating the high speed. You can watch the gauge go down in about thirty five seconds. Mind you, this is a stock 350SBC, so there's no big BTU's coming out of it. The next step is load-up a truck on the trailer and take it for a tow.

May 30 '09

The tow went very well.  Drove for about an hour with a 1950 Chevy 3600 truck weighing in at 3665 lbs and a trailer at 2300 lbs, with another 600 lbs in the bed of my C10.   Only in Texas would towing 6565 lbs with a 1/2 ton truck be considered normal!   While the fan was on [nearly all of that time on low speed] it never hit the 195* mark, which would have engaged the high speed side. So I'm happy with that end of the test. The kit worked without issue and the fuse held up despite being far less than the 100 amps everyone said that it would need.

The next step is to order up all of the parts for the first run of kits. I think by mid-June, we will be ready for sales of the Basic Kit with just the relay and one 180* sensor. I'm not quite sure how most people will want to use their high speed, so I'll make the 195* sensor optional.  So far, most that have asked about the kit have said that they want to 'toggle' [manually control] the high speed when needed.

June 15, '10

Its been a year and the fan and kit in my truck has worked flawlessly. No more over heating issues. I have gotten a great response from others that have purchased this kit and its a vast group that uses the Taurus and Mark VIII setup, Rock Crawler, general off roaders, Cobra kit cars, late model diesel trucks and hot rodders in general. We now have several hundred units in operation with very few problem. The failure rate is at about 0.5%, with 1 unit failing at eight months out and 5 with switch issues, 3 at install and 2 at almost a year. So that's a pretty good record. With the new switches coming in the next month and redesigned dual high side relay configuration and testing each switch and kit before it goes out it should drop to less than 1/2 % failure rate.

We offer a one year warranty from purchase on the kit and switches. This is when you buy it not when you install it, We realize you may buy it now and wait 6 or 8 month to install so we will work with you on an individual basis for extended installs. But as a rule we only offer a warranty from purchase for a year.

As I said above we have redesigned the kits and added single speed and an A/C option. The kit now has dual 60 amp relays for the high side and a single 60 amp for the low side. So now it can handle 120 amps of continuous load and a 300 amp surge. If your getting a surge higher than that your fan is toasted. We now carry the Mark VIII fan plug pigtail and a conversion connector for the Taurus pigtail.

July 6th 2010

I came across some very interesting test data that led me to believe that the Mark VIII fans cfm is more wishful thinking than actual truth. Don't get me wrong the Mark VIII and Taurus fans have cured a lot of cooling ills at an affordable cost but the real cfm is actually under 3000 cfm. yep UNDER 3000 CFM actually 2700 on high and about 1700 on low. WHAT? no way!!!, Prove it. Read on.

Here is an actual CFM test between a Spal dual fan set up and a Mark VIII 2 speed and a few other fans. With actual amp draws at peak and start up. Tom Dewitt of Dewitts Corvette commissioned the tests back in 2007.

Test data

Your back and scratching your head. Me too, I had a Zirgo 3000 that was about the same rpm and 2 inches smaller blowing about the same as the Mark VIII on low. So I just assumed the Mark VIII was pushing about 3000 cfm. It is entirely possible that Zirgo tests theirs at 0.0 static pressure or not up against a radiator at all. The Mark VIII on high just moved a great deal more air than the Zirgo so again assuming it was about 4000 cfm. The test shows three things, the Spal dual is a good fan, the Mark VIII isn't anywhere near 4500 CFM some think it is and the Zirgo 3000 cfm isn't any where near 3000 more like 1700 under the same conditions as a Mark VIII.

That said the Mark VIII on low speed does cool my truck better than the Zirgo at its maximum speed and my real life road test does also prove the fan is a good affordable cooling solution.



I have lately gotten many calls about the Mark VIII fans that people on ebay sell as Mark VIII two speeds for anywhere between $50 and $150, only to get it installed and have it turn out to be a single speed fan. First let me say I have yet to see a true Mark VIII two speed fan, I have seen many two speed Tbird fans that were used as replacements for Mark VIII fans because they were $200 cheaper.

Here is what I can say positively, The Mark VIII Two speed may have existed at some point but it would now be at least 20 years old and if your buying it off ebay its a used Tbird/Cougar fan and way over priced for a 20 year old fan. You can buy a new Tbird /Cougar two speed fan on Rock Auto for $60, 94 Tbird or Cougar 4.6 V8. There are no new ones being made. All Mark VIII and Tbird fans have 3 wires, Inspection plates were on early models and does not designate it as a two speed. A white plug connector on the motor DOES indicate it as a Mark VIII SINGLE speed regarless of how many wires it has. There is no CFM difference in the high speed of a Tbird fan and a Mark VIII. The fan pitch was different to reduce noise in an expensive car. It none of them produce 4500 CFM unless there is a 1500 CFM wind in front of it.

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I will also include instructions for connecting the wires, assorted connectors and shrink tubing.

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