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Radiator fans: mechanical and electric car cooling

by Daniel Stern (part 5 of a series)

The fan moves air across the radiator (and A/C condenser, transmission fluid cooler, power steering fluid cooler…) when the car's not moving fast enough for the natural airflow to cool it.

Once you're under way at a reasonable speed, the car's forward motion pushes enough air across the radiator without the fan. Still, in most older cars, the fan was always running when the engine was on, because it was spun by a simple belt-and-pulley system.

Engine-driven fans come in three types: solid, clutch, and flex.

Solid fans

A solid fan is the most basic type: four to seven metal or plastic blades, criss-crossed around a central hub, bolted onto the water pump pulley. The speed of the pulley, driven by a fan belt, is the speed of the fan; when the engine runs faster, the fan runs faster. When set up correctly, it pulls enough air across the radiator to keep the engine cool (and the air conditioner working, if the car's equipped).

Its simplicity means this kind of fan is the least likely to break, but it is seldom optimal. It runs whenever the engine's running, so it delays engine warmup; and it's also spinning (and making noise, and costing power and fuel) unnecessarily when the car's moving down the road fast enough to ram plenty of air through the radiator.

For various reasons related to airflow, solid fans use much more power and fuel in dyno tests than when the car is actually being run and wind is rushing past the blades. Those magazine articles bellowing about "10 whole entire horsepower to spin the fan" at the equivalent of 65 mph are, though, grossly overstated.

Clutch fans

The clutch fan was a big improvement. This is just a solid fan that is bolted to the body of a clutch unit (the hub of the clutch is still bolted to the water pump pulley). The clutch can either drive the blade, or let the blade freewheel.

There are different types of fan clutch. The basic variety is a simple centrifugal clutch which lets the blade slip (freewheel) above a certain rotative speed. It's assumed that if the engine is spinning faster than a certain speed, the car's moving fast enough for the fan to be unnecessary. This is better than a solid fan, but as long as the engine's spinning below the slip speed the fan's still spinning (and making noise and costing power and fuel) whether it's needed or not.


A shroud makes sure that all of the fan's effort brings air through the radiator. Without a shroud, air is pulled from between the fan and the radiator, meaning less airflow through the radiator and A/C condenser. Shrouds started out as part of air conditioning and heavy-duty cooling packages in the early 1960s, then gradually became standard equipment by the late 1970s.

A more refined clutch is thermostatic: a bimetallic spring on the front reacts to the temperature of air at the front of the fan to engage or disengage the clutch. But even these are not all the same. There are light-duty and heavy-duty fan clutches, and within those categories they differ in such things as temperature setpoint and presence or absence of an idle-lock feature.

The idle-lock feature, usually found only in heavy-duty clutches, drives the fan whenever the engine is at idle or low speed. Without the idle-lock feature, the clutch is purely thermostatic. If the air coming off the radiator isn't hot enough to make the clutch engage, the fan will not be driven. In situations calling for assured airflow at idle and low speed (such as a car with air conditioning, or low-speed/stuck-in-traffic driving), the idle-lock feature is good to have.


The Mopar Performance fan and clutch package is a regular production light-duty 5-blade fan and a light-duty clutch without idle-lock. If the demands on your cooling system are ordinary, this package would do fine. But if you want or need more airflow across the radiator and A/C condenser, you can do well with a wrecking yard fan (7 blade in your choice of pitch and diameter) and a Chrysler p/n 4266665 clutch (heavy duty with idle-lock - discontinued). When shopping the yards for fan blades, look under the hood of whatever rear-drive American car or truck you happen across; the bolt pattern of the fan blade to the clutch is pretty universal though the industry, and as long as the vehicle you're looking at uses V-belts and not a serpentine belt-which usually means a reverse-rotation fan and clutch-you're all set. Dodge trucks through '93 are good candidates. Make sure the fan you get isn't bent!

The typical American-type fan clutch protrudes several inches forward of the fan blade. A couple of inches' space is needed from the front face of the clutch to the rear face of the radiator, all of which can add up to needing six or seven inches of space from the front of the water pump pulley to the radiator. That's not a problem with a short engine and/or a long engine bay, but let the engine bay be short and/or the engine be long front-to-back, and this type of clutch fan just won't fit.


It can be a real challenge finding a fan clutch that fits in the short space available in an A-body slant-6 application, for example, and works the way you want it to. Various Mercedes and Toyota cars have a clutch fan where the clutch body forms the central hub of the fan blade, rather than protruding in front, with a wide variety of fan blade counts and diameters, from five to eleven (!) blades made of plastic, steel, or cast aluminum (which can be given a nice polish, if that's how you roll). Happily, many of these have hub-to-pulley mount dimensions compatible with a 6- or 8-cylinder Mopar water pump hub.

This kind of fan can fit in some applications that won't take a US-type setup. But some engine bays are just too short, and some engines just too long, even for this reduced-length type of setup.

Flex fans

Then there is the flex fan. This may be either a one-piece plastic item that looks a lot like the solid steel fan above, or it might be a spring steel or aluminum blade riveted to each "leg" of a central metal spider hub and support.

Flex fans are noisy because of oscillation (vibration) in the flexible blades as they straddle the flex point. A given level of force (from air resistance) against the blade flattens it out. The fan's overall build (blade contour and spacing, etc.) is calculated such that the fan will tend to flatten out at a given RPM; as this threshhold is approached, turbulence at the blade edges makes noise, and the blade then begins to flatten, which reduces air resistance, which causes the blade to curl, which increases air resistance, which causes the blade to flatten, which reduces air resistance, etc. Lots of noise.

Flex fans are dangerous; all that flexing eventually adds up to material fatigue, and they have been known to break and fling blades through hoods and hapless bystanders. If you do go this route, make sure the fan you install is of good quality, doesn't have a lot of years or miles on it, and is properly selected for the RPM range of the engine and correctly installed.

Electric fans

That brings us to the electric fan. Virtually all today's cars have electric motorized fans. These work well: they can be designed and specified to run at the optimal speed (or range of speeds), and only when needed. Without a fan, the cold engine warms up faster, saving fuel and reducing emissions. Overheating due to lack of airflow across the radiator on a hot day in slow traffic is a thing of the past; the electric fan speed doesn't depend on engine speed. They can be set up to run for a few minutes after engine shutoff to cool the engine bay down, helping with hot restart problems.


Retrofitting an electric fan to an older car isn't as straightforward as finding one that'll fit, bolting it on, and hooking it up. Fans draw a lot of power. Right up through the late 1980s, most of Chrysler's alternators and charging system wiring configurations were marginally adequate for the car as originally equipped. To safely, dependably, and effectively run an electric fan in such a car, the charging system will require substantial upgrades.

Which brings up a point commonly misunderstood: even electric fans are engine-driven. All the electricity in a car must come from the alternator, which must be spun by the engine. Alternators do not sit there happily producing their rated current no matter how small the demand; the more power we need (as for example by switching on an electric fan), the harder the alternator is to turn, so the more engine power it takes to turn it.


You can demonstrate this to yourself very easily: start a car, let it run at slow/curb idle, then turn on the high beams. A modern car will bump the idle air up a little to maintain idle speed, but you'll detect the sag before that compensation happens. An older car does not have any such compensatory mechanism, and you will hear and feel the idle speed drop. So in terms of power and fuel economy the gain from an electric fan may be not nearly so large as it might seem at first-especially when the ancillary upgrades to the charging system are factored in (time, effort, and dollars).

That just about covers all the major parts of the system. Stay cool!

4: Coolant <> Back to 1: Introduction and thermostats

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