Recently there has been much Talk of Replacing the Mechanical Fan with an Electric Fan (or even Twin Fans)

But what if any is the Main Advantage, as while I know that Mechanical Fan Blades can break up and when they do cause untold amounts of damage.

Would it not be better in the Scheme of things, to replace the Mechanical Fan Blades every couple of years and then renew the belt at the same time.

Would it also be possible, or even practical to make some sort of Fan Guard to go over the Mechanical Fan, just in case that happens.

 

Main advantage to me is that they are more controllable. Depending on how you are set up you can decide when they come on, how many, how fast. Potentially a faster warm up and better cooling response to stop start traffic in the heat. Again depending on how the mechanical fan is set up it may be a continuous drag on the motor.

Never heard or seen a fan blade break in normal use, but maybe I have lead a sheltered life.

People say there is a potential power advantage , which I guess should be true when the fan is off. With the fan running I guess you are trading aero drag of the fan for alternator drag with increased load.

Just my 20c, I will be interested to hear what others say.

 

The main advantages are that:
1) if you use the car as an everyday transport, at some point you will be stuck in traffic, or on a motorway hold up, etc etc. Under these circumstances, an electric fan (providing the alternator is up to the current draw) will flow MUCH more air through the rad than a mechanical fan, as the mechanical fan is tied to engine revs.
2) at speed, even if the mechanical fan viscous clutch is OK, the mechanical fan is doing work and therefore saps power which is entirely superfluous to cooling, as above about 35 MPH the air ram effect outdoes anything the fan can provide. So removing the mechanical fan, providing the thermostatic control system on the electrical fan is a good one, will save fuel/give more power. The power saving is not negligible, I seem to recall (not 100% sure - takes cover) the mechanical fan can soak up about 10 BHP (say 5% of what goes to the rear wheels. This is the main reason all modern cars have electric fans, for fuel economy reasons and cost simplification on the build - no belt, no mechanical bits etc etc
3) For us home-mechanics, it does de-clutter the engine quite a bit and give better access to the front, you lose a belt and a fan hub and bearing.


Entirely unnecessary. The black new OEM fan blade will see you out OB. But your point about renewing the belt regularly is important. So is renewing the viscous clutch unit. In terms of reliability, which underlies your question, I actually believe that the mechanical system is ultimately the more reliable. If an electric device packs up, there is not much you can do, but if the mechanical fan does, the fix is both obvious and relatively easy.
Also, controlling the electric fan is not that easy to get right. It was not until I adopted Grant's system that I became happy with mine. Ditto installing the fan properly is not that straightforward either. Also, I believe it is essential on our cars to install a manual cabin switch in addition to any automatic thermostat controlled system. That needs, to be a reliable redundant circuit, a second relay and wires to the cabin and a switch. All take some doing to do properly.


It would but it is not necessary once you have the modern black fanblade.
Greg

 

The power consumption of mechanical fans with viscous coupling has been overstated for years. Not surprisingly, the number came from a manufacturer of electric fans eager to sell his goods. This topic gets debated on all old cars board, especially with the 60s muscle car guys.

The quoted 10 HP came from a dyno run where an extremely hot engine was run up to redline, and indeed the fan appeared to consume that amount of power. The flaw in that plan was that it does not replicate an actual engine under road conditions. First, the fan was run unshrouded, which spoils the aerodynamics completely. Second, a car would be moving forward which changes the airflow entering the fan dramatically. The fan on the dyno test was in a 'stalled' condition.

Third and most importantly, had the complete dyno trace been published, it would have shown that a few seconds later the viscous clutch released as per design and the load dropped dramatically.

Actual consumption of a mechanical fan under road conditions has been documented at around 2HP maximum.

Given that electric fans need power, be sure that the alternator is capable of delivering enough, especially at idle when the fans are most needed.

 

Hi

Many Thanks,

That made for some interesting reading, as I never gave any thought at all to drag on the Alternator.

 

Hi Greg

Many Thanks,

That has told me everything that I need to know, so I will stay with my Mechanical Fan but inspect it a lot more often, to check for any Stress Cracking.

 

I've heard that said several times by guys with flat batteries after trying to run headlights and electric fans at the same time in heavy traffic.

 

Hi Mikey

Many Thanks

And very well explained if I may say so, with lots of interesting facts to help to back up the Theory.

So on balance, I think I'd rather stay with the Mechanical Fan.

 

Cheers Mikey

Another very good reason, at least for me to decide to stay with the Mechanical Fan, although I'll be checking for Stress Cracks more often from now on.

 

I would agree with Greg : better fuel economy, better airflow, more space - those are big pluses for electric fun.
The only plus for mechanical fan imho is that it is already there and for old cars it is a bit of their DNA... which we are trying to preserve.

 

When I bought my 94 XJS, the PO had fitted twin electric fans. As mentioned above, with the V12's low idle speed, I found it was draining the battery in traffic, the alternator could not keep up with demand. Add in running AC blowers and the electrical demand is at the highest when the output is the lowest.

Electric fans are not all equal too in terms of the amount of air they move. The ones I had didn't seem to be particularly good, the mechanical fan moves a lot more air. I can feel the difference at idle, there is a steady flow of hot air out the right side of the car at idle, and I didn't feel that with the electric fans.

Regarding fan hp, a friend works for the railroad and gave me a cab ride on one of his trips. Modern locomotives are highly computerized and the amount of information available about the state of the systems is amazing. Grinding up a steep grade in full throttle the computer showed it was generating almost 4500 hp, and the rad fan power consumption was 8 hp. The power consumption of a small mechanical fan in an XJS, is nothing compared to a locomotive!

 

Your first point is dead right. I have wired the OEM extra air valve thingy, in the back of the A bank airbox, that is normally used to raise revs when the aircon is on, so it comes on with the main electric fan. Thus raising the revs to about 950/1000 at tickover, thus making my 115 amp alt easily keep the fan going and the volts adequate to not discharge the battery.


Also 100% true about the fan's capabilities. You need one that will flow serious amounts of air. My 17 inch Pacet blows a gale, better than the mechanical at tickover, but plenty of them only push pathetic amounts of air. Buying the highest CFPM you can find is essential.
Greg

 

Isn't that tough on the transmission when sitting in traffic in D with the higher revs? Harder on the brakes too in creeping traffic.

 

Wow- interesting data!

Thanks.

 

It gets put into neutral in that case. But remember, the extra air solenoid unit is there (OEM) to prevent stalling when stationary in traffic and the aircon compressor kicks in. So in my case, the extra load on the engine is coming from turning the alternator, rather than the compressor, so probably not a great deal of difference.
Greg

 

Nah.

A small rise in temp, maybe, but the TH400 is one very tough trans, and we ALL Change our trans fluid every 2 years, don't we????.

 

Thanks Grant, most reassuring.
Greg

 

I certainly would not replace a perfectly good mechanical fan with electric. I did replace mine because the viscous clutch was bad and my electric fans cost less than a new clutch.

All electric fans are not equal (the reason I purchased OEM for fans not aftermarket). The shrouding around the fan is important and should be circular as close to the blade tips as possible without touching, this reduced washoff which is where the air flies off the tip of the blade, Like new Jets have winglets.

Blade profile is important and some of the latest have V cuts in the trailing edge which cuts down on noise, we can thank the Owl for this one.

So if you do need to replace the fans look at OEM units, the manufacturers spend lots of $$$ to design these.

 

Very true, I would think that electric fan design is also matched with radiator design. Installing low-flow electric fan on high-flow-designed radiator may not work well.
But overall I would prefer electric fan. My ford Taurus had electric fan, and even on a hottest day in stop-and-go traffic the fan would only kick-in for a short period and then go off. And when in motion - no fan at all, natural air flow did the job. I liked that.

 

The earlier XJS's were fitted with a metal fan instead of plastic. Anyone know what year the change took place?

Mark