The Great Fan Debate

I have a 69 MGB. My question is simple... which way does the fan attach? I went to the San Juan Bautista car show last Sunday and found 2 cars with the fan like I have it shown, and two cars with the fan mounted "turned around" from the picture. The pics I took of my engine before I put in the new radiator show that the fan was mount "turned around" from my attached picture.

Before it is suggested, I understand that I probably should replace my metal fan with a plastic fan.

Please check your engines so that I can put this one to bed!

The engine turns clockwise when looking at the front of it, and the fan should pull air not push. I believe you are holding it the correct way.

(but I think if you turn it around it will still pull).

I agree with Peter. The engine is turning clockwise as viewed facing from in front. The blades are pulling cooling air as they should be.

Edit: I was wrong about the front and back positioning.. It appears that the cut outs on the blades should be toward the FRONT of the car. I suspect that they have something to do with noise.

Original post:

A fixed blade fan cannot be installed backward except for offset or other deviations in side elevation.

The fan in your hand has reliefs in the blades that should, I believe, be on the engine side when installed.

Jack

fan is backwards in the pic you have posted i just remeoved mine to replace water pump and my fan is just the opposite

I just scanned through a folder I have of engine bay pics. Guess what? 1/2 are mounted with the notch rearward and the other half forward. I now do not know which is correct, however my guess is:

1/ It does not matter as far as pulling air. It will never push air when rotating clockwise.
2/ It may be more efficient with the notches to the rear (or front), or quieter, or less cavitation or whatever.
3/ If there is a curve in the profile of the blade, common sense says it should be mounted to move the maximum amount of air when the car is sitting still because when the car is moving there is normally enough airflow through the rad to keep it cool without a fan at all (as in vintage MG racers who run no fan on the track).

Anyone got an article somewhere on this topic?

If you look at the blade just below the one Rick is holding and the one two to the right, you'll see a slight camber to them. Based on Peter's theory about "common sense", Rick should turn the fan over and mount it that way.

Regardless of which direction you install it I, personally, would NEVER use a late (6 blade) metal fan. I've seen four MGB bonnets with holes in them where one of those fans has fatigued and broken, launching a blade THOUGH the bonnet. Think about that the next time you're standing beside the car, bonnet open, revving your engine with the throttle linkage. I know of one individual who had a blade break off in just such a fashion and whiz right by his ear.

I'm not the only one who's seen this. There was a thread on Plumstead's BBS a few years back.

The photo shows the fan backward. If you look at the end of the blades, you'll notice they're very slightly curved. For the best efficiency, the concave side goes toward the engine; the convex side goes toward the radiator.

HTH!

Eric's tuning tip of the week:

Throw it away! Gain 3 to 5hp (cheapest ever HP gain?), improve engine smoothness, shorter warm-up time, peace of mind from not thinking about blade exiting through hood, will not loose its nice yellow paint scheme, will not skin your knuckles when working on running engine (don't ask), quieter running engine.... the list goes on!

Replace with electric pusher if needed - if you live "up north" - additional radiator ventilation is probably not needed until next July or August! ;)

Flying fan blades are NOT a joke. We had a real case where a man was killed (severed artery) when a fan blade let go on an engine he was trying to tune! You have been warned!

rcedward Wrote:

FWIW I don't trust a 30yo plastic one either and new ones are about as much as an electric setup. I know you didn't want to hear that but my opinion.

Anyone ever think that maybe blades are flying off because the fan was mounted backwards? The fan in the picture is wrong. The center hub "hands" should always be on the backside of the blades so that the hub is holding the blade from behind . I would almost guarantee blades that are flying off are because the air is riping it off the hub NOT pushing it up against it. Try pulling a blade off the fan and then try pushing it into it and see which way is easier to rip the blade off.
Easier way to demonstrate this is this example. Take a 12"x12" piece of plywood with a knob in the center to hold. If you had a pool of water and you push the board into the water with force(flat side to the water)the board just gets pushed into your hand but you should still be able hold it . Now pull it out of the water,it will want to rip out of your hand and you will have to tighten your grip.

Glen

And if all else fails look at the picture.

http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=28984

Glen

Interesting thought, but the ones I have seen broken are always in the hub material, not the rivets. Most fail due to fatigue. They should be balanced, and mounted to clean surfaces so that they run "true", otherwise a cyclic loading is developed which causes the fan to flex and break. Unfortunately, painting them can produce both a mechanical imbalance, and an aerodynamic one!

Yes, I have seen plastic fans do this too!

You didn't think cooling fans were this complicated did you? ;)

If you think of the fan blade as wing, it is mounted correctly as shown in the photo. When the fan blades rotate, the curved (or cambered) section, which is on the engine side of the fan blade, creates an area of low pressure while the opposite side of the blade toward the radiator has a higher pressure. (this is what creates lift on an aircraft wing). Since the air will move from high to low pressure (going to the side of least resistance), the air flow over the engine will be greater witht the fan mounted as shown. In this position, more air will be drawn through the radiator. The fan blades will also tend to bend towards the engine block since the "lift" is in that direction. the attached photo illustrates the effect.

Thats strange. The hub material is 1/8" thick and it's so small. It's not like these engines are seeing 10k rpm . I though either the rivits were shearing or pulling out or the thin blades were pulling out or snapping off. There must be some other history on those fans.Either that or they had a half million miles on them.Seems strange that there are so many old cars with metal fans and they don't fly apart . Are we going to have to start magnifluxing our fans?

Glen

Although some cosider looking in the Hayne's and Bentley manuals cheating, the pics there also show the concave side to the rear, fan blades to the rear in relation to the fingers of the hub. Granted that is with 3 bladed fans, but the logic holds regardless of whether you have 3 or six blades.

Herb, the high pressure side goes on the engine side so the fan is still backwards. It doesn't matter which way you mount it. It will still blow air in the same direction when turned the same direction. The only difference is strength.

Glen

edit to add- with the concave as shown in the pic the air will just want to flow around the blade rather than the blade 'cup' the air.

Oh, I wouldn't call it an epidemic - just a heads up. Actually, I have only seen one broken MG fan.

The problem is the fan sees a LOT of cycles, and if it is flexing on every cycle, it can easily fatigue - and fatigue resistance has little to do with material thickness, in fact often thinner is better (if the material is properly selected). If you think about it - the greatest bending load on the fan is close to where it meets the hub, which is exactly where the fatigue cracks start.

However - I would advise a careful visual inspection. Look for any signs of cracking near the hub, and any bending on the hub or blades - the blades should also all be the same shape and size. Also feel the blade edges for any knicks or chips. If you can, also check the balance - you can do this by carefully putting the fan horizontally on a knife edge - making sure the edge passes through the exact center of the blade assembly. When the fan is symmetrical on the knife edge, it should balance. This was a much easier technique on the old two-bladed fans used on A-series engines!

There was a three corner tear in the inner fender well, left side, of my car which I have assumed was from a broken fan blade. Scary to think of the power required to punch a hole in that gauge steel!

Yep - the one I saw exited through a beautiful, aluminum, hood!

After all, it is spinning at, or slightly faster than, engine speed - look what happens to a con rod if it lets go!

While I agree with the posts about the metal fan, if you look at the original post, Rick says the metal fan is just for an example. He is going to change to the plastic fan.

I too have seen a metal fan that came apart. It did not make it through the bonnet, but put quite a dent in it.

A example of a fan anal MGB owner.
The one in the Green B was checked for cracks at the machine shop when the engine was rebuilt. Then it was taken to a marine propeller re-builder to get trued and balanced.
It may still break but I am not loosing any sleep worrying about it.

With all fans the curve of the blade (cup) faces to the side that the air is being moved to. In this case the water pump side.

OK, to keep some originality to the car what kind of plastic fan alternatives do we have instead of using the metal blade type?

There's the late model plastic fan, but it's expensive ($85) and I've seen some say you need a shorter water pump for it to fit. Has anyone found any other fans out there that work without modification?

Ryan Twelves Wrote:

What I haven't seen addressed in this conversation is that the notches/relieved blade area is faced forward for the purpose of not catching on the bottom of the radiator. Back when we used these cars as our everyday beaters, it was very common to see a B that needed a radiator and fan due to hitting a pothole, crossing railroad tracks and other causes that would shift the car/engine/radiator just enough to catch the blade tip on the bottom tank, bend the blade into the radiator and cut a circle into the core. There were brackets added to the motor mounts to help but they are often left off, sometimes bent to be of no use. This same twisting/flexing of the body could also cause the bonnet to release, and if the secondary catch was not in proper adjustment the bonnet could fly open back over the windscreen. Bonnets and radiators were a rare and hot commodity... I still keep spares even though I keep plenty of room between the rad and fan.

Rick
That is the fist time anyone has explained the purpose of the relief notches. That makes sense.

Thanks everyone for the input. Here is what I have gleaned:

1. My fan (as originally pictured in this post) is backwards.
2. Metal fan blades break.
3. A plastic fan is not available for a 69 MGB.

I decided to order a new metal fan from Brian at O'Conner Classics. It will be in my hot little hands tomorrow at noon.

I am impressed by the logic displayed by all of you. Having all of you as answer guys is one of the reasons I bought my MG after a 30 year hiatus (had a 73' MGB in high school).

Yup.. and it also shows you that we can be wrong, (I now think I was on my first response post), and admit it. But it is through these reasoned debates that we all learn, which is why we come here.

I am also very pleased that no-one hijacked this thread or or made any jokes or off-coloured comments throughout. It was an enjoyable educational experience.

So, what have we decided....? We're all going to remove the mechanical fans, horsepower numbers will rise and we'll all put in new, aftemarket electric fans?

I'm in. By the way; had a friend stop by today to do a couple of things on his '66 and noticed the 3-blader in his car has the "fan blade curves" towards the engine. Just though I'd throw that in.

Simon his fan is on correct. My Money's CS prop had a cupped shape and it was facing towards the engine. The shape is more efficient at moving air due to the extra lift.

Have any of us ever had our own fan let go?

Functionally, I like an electric fan... you can always paint the blades yellow! I'll have to go ask the local Fluidyne rep if their testing shows a preference for pullers or pushers.
.
Frankly, I think the mounting location in most cars (including later B's) is the result of packaging considerations more than effectiveness.

The fan's purpose is to move sufficient air through the rad when the car is stationary or moving slowly. Thats it.

So why put added strain on the water pump shaft and waste that 3-5hp by making a fan turn all the time?
Mike

PS You can increase the effectiveness of your rad by closing off the gap above it with foam and blocking other areas where air can escape without passing through it.

To follow up on the electric fan option, I'm going to remove my OEM fan and replace it with a Hayden electric fan.

I have a question: What diameter fan should I install on my 1969 MGB radiator?

I replaced the mechanical fan in my 77 Spit with the largest Hayden I could get in there.

It was too small. It made everything worse. Now I have both and all cooling issues are gone.

Look at the later cars. They have two large motors I.E, a fairly large CFM. You need that much CFM. Engineering 101.

A Hayden fan is a bad idea if you live in a warm area. Guys from Montana and Maine can get away with it. I can tell you Ohio was too hot.

Whatever the HP rating you save with the fan, cannot be detected it in the human dyno, guaranteed.

Larry

The man who knows has spoken:

From: John Twist [mailto:johntwist@universitymotorsltd.com]
Sent: Monday, October 29, 2007 12:44 PM
To: Rick Pronge
Subject: Re: Correct Fan Direction


Dear Rick,

The fan is backwards in the picture that you attached.

John
Safety Fast!
John H Twist
University Motors Ltd
6490 Fulton Street East
Ada, MI 49301
616 682 0800
616 682 0801 Fax
www.universitymotorsltd.com

The one who knows has spoken:

From: John Twist [mailto:johntwist@universitymotorsltd.com]
Sent: Monday, October 29, 2007 12:44 PM
To: Rick Pronge
Subject: Re: Correct Fan Direction


Dear Rick,

The fan is backwards in the picture that you attached.

John
Safety Fast!
John H Twist
University Motors Ltd
6490 Fulton Street East
Ada, MI 49301
616 682 0800
616 682 0801 Fax
www.universitymotorsltd.com

I would not argue with that authority for sure.
So for everyone to remember, cutouts go forward towards the rad. Would be so much easier if the manufacturer just put an 'F' stamped on the correct side of the cowl.

All these responses and NO ONE has personally had a fan break. Two people have seen the results of a broken blade, but have not seen why or where it broke. Hmm... Its starting to taste like an exaggerated urban legend!

The later plastic fan can be installed in a pre-18V engine by using the later (matching) short water pump and pulley. Without the water pump and pulley, the fan is too deep and contacts the radiator. The early pulley won't fit on the late water pump either - it contacts th epump body and the belts don't line up.

B-racer Wrote:

Looks like John Twist says the main body of the fan faces out and the blades are in between the fan body and motor. That would also mean the Moss pic is correct. Am I wrong?????????