j.kenzie@sbcglobal.net

This is just spitballing, but do any of you know of a way to use an electric clutch to replace the viscous clutch on our belt driven fans? Something like the AC clutch, but triggered by a thermal switch and relay. Seems it might be easier than an electric replacement fan and more efficient than running an electric fan from electricity produced by the belt driven alternator.

Airflite40

That sounds like a good idea, but honestly I doubt anyone on here has done it since it is so much easier to use a modern electric fan setup in place of the belt driven fan.

Plus, it would be too much of an on/off type of situation since the clutch would give you just 1 fan speed, off and on. The beauty of the viscous clutch when working properly (i'm sure you already know this) is that it starts the fan spining slowly and gets faster as the engine warms up.

Even the fans on the S4+ have variable speeds.

blown 87

MB did it on some cars.

Jim bailey - 928 International

The fan is designed never to spin over about 3,400 RPM so locking it up might just launch a blade out through the hood !

j.kenzie@sbcglobal.net

Oops! When the viscous clutch fails doesn't it lock up sometimes? Also, Jim you probably know, since you are supposed to keep the viscous clutch vertical, how you you ship these?

928drvr86.5

no, just the opposite, it fails to lock up! I find that mine runs cool/fine as long as i keep air flowing, as soon as i stop in traffic for any amount of time the temp starts to climb until the aux/ac fan comes on. On rare occasion i get the light on the dash. I am going electric with mine

ZEUS+

Benz did use it with sensor, relay, switch along with the electric fan clutch assembly. Not worth it. A conventional fan clutch can fail two ways. It can freewheel and not cool. The bearing can seize and launch a blade or the whole fan as Jim B said.

blown 87

You have worked on a few of them also, not a real good set up or idea.
It was better idea that the lexus with variable speed hydraulic unit.

Most of us old school guys who started out young have launched a "Flex Fan" into or through something at some point.

dr bob

Usually the radiator, although I did have one come off and beat itself to death in the trans cooler plumbing. That was the fiberglass kind, where the !roar! told you it was really working good.

Modern electric fans are more common and certainly more reliable than they were when the cars were first designed. Look for a thread in the last year or so that described using the Ford Contour/Merc Mystique fans and shroud as an almost direct replacement for the mechanical fan. As much as I claim to be a purist, this looks like an excellent and reliable option. There are also fan-and-shroud options from a few of our Big N suppliers that may be even better. Worth a look!

mark kibort

First of all, as Jim says, the fan only spins at 3000rpm or less, at top speed. Do you know what the power required to run the driven fan at this speed? well, its less than 1hp.
This is easy to prove. just look at a home air conditioner set of blades. They are a little bigger and require only 1/3hp to spin the fan a near the same speed.

Also, the viscous clutch does allow it to "slip" so i spins slower than engine rpm. There is some friction with the belt and varied tensions.

The electric fan can be operated at 30amps (the S4 has two at roughly 15amps each) thats electrical input. their efficiency is about 50%, and the current is replaced to the battery based on voltage drop by the alternator, which in itself is about 50% efficient.
30amps at 13volts is around 400watts input. (less than 1/2 hp) mechanically, it is probably near 1/4hp at best.

On the driven fan side, there is also the clutch which drops the efficiency of the mechanical fan, probably is equal to the efficiency of the electrical fan.

Either way, if you are not racing, or in 30 degree F temps all year long, i think the driven fan is a pretty good design.

IMHO

mk

blown 87

BTDT. I put a fan blade through not only the radiator but also through the A/C condenser on a V8 Monza once.
It was the stock one, I had a vibration after pulling out the 307 and putting a bad boy 383 in it at about 3700 RPM.
I could not find the source so I tried to just drive past it, found it that time.

I put one of those stainless steel flex fans on there and put a blade 1/4 of the way through the hood after about six months of hard driving.
I miss that car, a perfect sleeper.

When I first moved into the shop here I noticed a wrench shaped impression in the ceiling of the last bay.
It is flat and you can tell it is about a 19MM combination wrench, it hit with a LOT of force.
No idea how that one happened, but I bet it had something to do with a fan.

Maybe I can get a picture of it and post it if anybody wants to see it.
it is a strange thing.

mark kibort

The air flow of a fan goes up with the cube of rpm. so, if its blowing say, 15lbs of thrust at 3000rpm.
(kind of in the ball park), and requiring 1hp to spin it, at 6000rpm, the HP is now 8HP, and the thrust of the fan is now pushing about 70lbs LBS! Its probably going to fly into the radiator and bouce through the hood!

One formula for fans is: thrust = 13(diameter in feet x power in HP x eff in %) ^2/3.

thrust = rpm ^2 x constant (if you know the thrust and want to see how it relates to speed)

or

Power = 4xT/pi x d^2 (if you know the power and the diameter and want to find thrust