Thermostatic Clutch Fan
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Converting to a Thermostatically Controlled Clutch Fan

After fixing the radiator on my 1958 Buick Special, I wanted to improve the cooling ability a bit more, quiet down the fan noise, and see if I could gain a few horsepower in the process. How? By converting from the original 4 blade "fixed" style fan to a modern thermostatically controlled clutch fan with more blades - in this case I used a 5 blade fan, but I could also swap to a 7 blade fan for even better cooling if needed. Many folks ready this will be thinking to themselves "Why do this? It's added stuff to break and it's more complicated. I like my old cars nice and simple." That's fine if you want a perfect 100 point restoration, but I'm not into perfect restorations much; instead I'm very much into functional and easy changes to make my cars better - and this change certainly qualifies.

Why Should You Do This?

First, some background. A fixed fan always spins at the same speed as the water pump - needed or not. This is simple to do by just bolting the fan to the front of the water pump hub, but it has a few drawbacks.

  •  A fixed fan delays and slows engine warm-up because it's pulling cold air over the engine and radiator as the engine is still not up to temperature and in reality, needs no cooling yet.
  •  A fixed fan also makes lots of noise. Especially when driving on the highway, it tends to beat at the air instead of adjust to any differences.
  •  A fixed fan has no ability to "give" under sudden acceleration or to not "engage" when not needed. Your engine does not always need to have additional cooling - cool nights, cruising down the highway at part throttle and various other situations don't really require the fan to be engaged and spinning 100% of the time. The added drag of having to spin the fan when not needed takes horsepower away that could be used to push the car forward just to thrash at the air.

Converting to a clutch fan addresses each of these points with a very simple and well-proven device - the 100% mechanical thermostatic clutch unit that mounts between the water pump and the fan.

  •  The thermostatic portion allows faster warm-ups and better cold drivability as a result. The clutch allows the fan to "freewheel" (not spin at engine speed) until the engine is up to temperature and the fan is really needed. This means less airflow through the radiator and faster warm-up times. A small bimetallic spring similar to the one in a choke coil is mounted on the front of the clutch where it is exposed to heat from the radiator. When it gets hot enough, it "locks up" the clutch and the fan spins at engine speed to provide full cooling. When it cools off enough, it "unlocks" and allows the fan to "freewheel" once again.
  •  The clutch portion allows the fan to be quieter because it can "freewheel" and adjust it's speed to match that of the incoming airflow through the radiator. It also does this at idle so that the fan, although it is spinning, is spinning at a greatly reduced speed compared to a fixed fan, thus making much less noise.
  •  The clutch portion allow the fan to "give" in cases of sudden acceleration (so it accelerates more slowly) and the thermostatic portion allows the fan to "freewheel" when not needed. This allows a large amount of "freewheeling" at higher engine speeds such as when you are out on the highway and the fan really isn't needed, and the thermostatic portion allows the clutch to freewheel unless the engine is above a certain temperature and the fan is not needed right now because the engine is not warm or is not warm enough to need more airflow through the radiator.

A Word About Electric Fans...

Some folks will preach that an electric fan is the ultimate swap, but I don't believe so in most older or "modified" cars. An electric fan simply cannot move anywhere near as much air as a good mechanical engine driven fan - and that's important to keep your hot rod cool. An electric fan is expensive and takes wiring, controls, and other such stuff to run. A mechanical thermostatic clutch fan bolts on and requires no maintenance, no wires, and costs about $70 to $100 for everything you need for the conversion. It will also likely last for 100,000 miles or more without problems. A mechanical fan takes less horsepower overall to turn. This may seem counter-intuitive, but an electric fan takes power from the electrical system which is in turn taking horsepower used to turn the alternator - and it can add up for high amperage things like a fan motor. You also "lose" energy in the conversion from mechanical power to electrical and from electrical power back to mechanical power again. Dyno tests from various magazines have confirmed that the lowest horsepower loss and the best cooling (basically the ideal combination) is using a clutch style fan. Who would have thought that something from the 1970's would be so perfect?

The only advantages an electric fan has are in mounting and in their "instant on/instant off" ability to regulate engine temperature more directly. For mounting, they can go in front of or behind the radiator and they can be mounted in some pretty tight places where a larger mechanical fan might not fit very well. The instant on/off control means they only spin when needed, and can be used to fine-tune the engine temp and allow for it to be much hotter and still not overheat, even to the point of running the fan after the engine shuts off to prevent the cooling system from boiling over. This ability to get hotter faster and run hotter is something newer cars use a lot for emissions purposes, but that's not a big concern for anything originally built with a carburetor and certainly not the primary consideration for a performance vehicle.

The Technical Details

In the case of my 1958 Buick Special, this was a pretty easy swap. Buick didn't change it's water pump mounting that was used from the 1950's era all the way up through the Buick 455 engines in the early 1970's, so the physical mounting was easy - find a later model unit and bolt it on. Other GM engines should be similar - a fan is a fan is a fan, and most GM V8's use the same water pump hub style, so you can easily find a later model fan that mounts to your early water pump. The same goes for Fords - I'm working on the same swap for my 1964 Ranchero - I'll post pics of that swap when I do it. For a donor car, the Buick 455 engine was commonly installed in Electras in the early 1970's and other luxury cars that came equipped with AC (needs more cooling) and where fan noise was not desirable. The fan diameters were similar (in my case approx 20"), so, I was able to grab the fan and clutch assembly off of a 1971 Buick Electra that I parted out, and used them as the basis for this swap. If your fan is a different diameter, then hunt the junkyard for a similar unit. Looks for AC equipped car and trucks.

The biggest issue is to find a fan and clutch assembly that is the right diameter and will match up to your water pump hub. The first photo shows the fans diameter for comparison.  In my case I did some clever parts book hunting to find out that the same fan spacer was used from 1950-something until the first year where clutch fans were used, and that once a clutch fan was used, the fixed and clutch fans both bolted to the same water pump hubs. Yes, you really can learn all that just by staring at part numbers in a book for a while! The only hitch here is the place where the fan ends up at relative to the radiator and fan shroud - for proper cooling the fan should be positioned with approx half of the depth of the fan blades into the shroud. The dimension from the water pump hub to the radiator changed slightly with each generation of vehicle. In earlier cars like my '58, this was typically accommodated with different length fan spacers for the year and model of car. In later years, it was accommodated with different depth fan shrouds. The net result here is that you may need to use a custom length spacer to get the later model clutch fan to line up with your earlier model radiator shroud. In my case, I used a universal 1/2" spacer behind the new fan clutch assembly to get the fan to sit at the the proper location approximately halfway into the existing radiator shroud. The final photos show this quite nicely.

There is also the problem of mounting the fan clutch to the water pump hub. Most original non-clutch style fan mounted with bolts that go all the way through the fan, through the spacer, and into the water pump hub. From what I can see, they pretty much all work this way. Most clutch style fans mount to the back of the front section of the clutch assembly (so the clutch part is in front of the fan) and then the clutch/fan assembly is then mounted to the water pump hub with studs and nuts. It's a heavier assembly and there's not really enough room to get bolts in there easily because the fan clutch and far are in the way. Using studs helps align things on installation and makes it all work much, much easier. Most water pump hubs I've seen are fine thread, so buy the right studs for your needs. Beware on the overall length and installation depth on the studs. If they project out the back of the water pump hub too far, they can hit the water pump housing as the hub rotates. If they are too long in the front, you won't be able to get the nuts on or they will hit the fan clutch assembly. Finally, you need to make sure the studs project far enough to actually install the lock washers and nuts on them to hold the fan/clutch assembly in place. The final picture shows a close-up of the studs where you can see just a few threads projecting beyond the nuts once they are installed and tightened down.

After I did the mock-up with the fan clutch from the parts car, I went out and replaced the clutch with a new unit from the parts store. The fan itself was used with only some minor cleanup to remove the grease and general dirt/crud on it from living in the engine compartment of a greasy parts car. I could be really anal and sandblast and paint the fan. I will probably do that later, but for now I was focused on getting it installed and working right.


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Page last updated 01/02/2009 01:51:39 PM