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Old 08-04-2010, 12:06 PM   #1
snoopykissedlucy
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Default 5 blade vs 7 blade mechanical fan

All engine things being equal, is a 7 blade fan better than a 5 blade fan?

I assume that an extra 2 blades and 1 inch diameter would be a good thing...........but someone once said that a mechanical fan is basically a solid wall at highway speeds, not allowing the rushing air to come through. I figure a 7 blade is a bigger solid wall. Also, I figure the extra weight can't be a good thing.

Any thoughts/opinions?(yes, we all agree an aluminum/electric fan combo is the way to go, but I'm trying to keep this low buck)

Lastly, is it true half the blade should stick out of the shroud?


PS- I have a 1981, stock fan shroud is 19.5 inches across, currently running a 5 blade 17.5 inch mechanical fan(no electric). I am installing solid motor mounts, so I'm not worried about side to side movement.
Click the image to open in full size.

Last edited by snoopykissedlucy; 08-04-2010 at 12:22 PM.
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Old 08-04-2010, 01:18 PM   #2
Easy Mike
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My advice is to stick with the stock fan and clutch. Your car should also have the electric auxiliary fan.

Yep. The tips of the fan blades are a little bit outside the shroud which is probably why the CAUTION: FAN sticker is on there.

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Old 08-04-2010, 03:22 PM   #3
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Yes the fan blades should stick out of the aft end of the shroud by half the blade width. The 7 blade fan seems to work o.k. for A/C cars that run hotter. If you get a 7 blade fan get the one that has offset blades. The offset blade fans run quieter and are still balanced.

Also be sure to have the front hood seal in place as well as the air dam under the car. Both make a big difference in the vehicle's ability to cool by directing as much air as possible through the radiator.
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Old 08-04-2010, 07:37 PM   #4
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The Caution Fan sticker was the first thing I removed after I bought the car!

Ok, the cheapest 7 blade, 18.5 inch fan I found was at www.mamotorworks.com for $80.
BUT........I also just found this 6 blade, 18 inch fan in my local Autozone for $20.

Needless to say, I'm going to try the $20 fan..........anyone ever try this?
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Old 08-04-2010, 08:08 PM   #5
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It's not just the number of blades, it's the pitch and shape of the blades. Some Vettes have pretty odd shaped blades...it must be for a specific reason or the engineers wouldn't have spent their time designing it.

There are also some cars that have asymmetric blades. Why that I don't know but again, there must be an advantage to it for certain cars.
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Old 08-04-2010, 08:21 PM   #6
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Very good point.........I just noticed the cheap fan is just riveted onto the flat blades.

The stock 7 blade fan has the U and V shapes built into the fan blades........hmmmm, is that to quench the noise?
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Old 08-04-2010, 08:34 PM   #7
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While universal fans can work on many cars, the fan your car came equipped with was designed specifically to a certain set of priorities...to keep the engine cool within certain temperature parameters, cost (to satisfy the accountants), ease of manufacture, whether the car has a/c, longevity, weight, etc. That's not to say a fan with more blades won't work and work well, as automotive engineering has made a great deal of advances since our cars were built.

What the U and V in the blades are there for is unknown to me...there has to be either an engineering or a cost/manufacturing reason for it. It may be as a noise canceling device as you mentioned.
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Old 08-04-2010, 08:44 PM   #8
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It looks like they might be strengthening ribs that allows the fan blades to maintain pitch.
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Old 08-04-2010, 08:50 PM   #9
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Whoa, I'm not an engineer but check this out written by an engineer:
http://www.madsci.org/posts/archives...6517.Eg.r.html
Query:


--------------------------------------------------------------------------------

Re: Why are radiator fan blades non-symmetrical?
Date: Wed Jul 1 10:22:22 1998
Posted By: Justin Roux, Other (pls. specify below), Grad, professional engineer plus research, Intergraph UK Ltd
Area of science: Engineering
ID: 897534113.Eg
--------------------------------------------------------------------------------
Message:


Radiator fan blades in cars are sometimes asymmetrical, that is very true,
but it has nothing to do with enhancing the performance of the fan itself.
Quite simply, anything less than a maximum length blade is going to impede
the performance of the fan.

However, as anyone who has driven a car with a snapped fan belt can tell
you,
once the car is moving, the radiator will remain reasonably cool for some
time without the fan due to the passage of air through the engine and grill
created by the car's motion. We can therefore see that the purpose of the
fan
is chiefly to keep the radiator cool while the car is stationary or moving
slowly - the car with the snapped fan-belt turns it's engine off when
stationary or it overheats in a few moments. Usually the fan points
forwards
and draws air through the radiator to simulate the air currents of the car
in
motion. There is an added benefit of circulating air around the engine
cavity.

So, why do we see one blade out of five cut short, offset, or with a hole
in
it? Moderations to the fan blades that will alter the fan's performance by,
say 5 to 10 percent, are of little consequence. Turn the blade with your
hand
or attempt to service the engine and you will find that you may require a
screwdriver or some other tool to access a point that would normally be
obscured. Alternatively, the shape of the fan may be determined by its
manufacture. Die-cast plastic items are notorius for modifications for easy
mould extraction, simple die design, and the like.

That, I'm afraid, is the answer. There is nothing magical about the hole or
the short/offset blade. The choice of electric or belt drives rests with
the
designer and hinges upon the size of the engine cavity, the dimensions and
orientation of the engine, and economies in production of the car.

In response to further queries from the original questioner, Justin adds:

You are referring to fans used for cooling the radiator in automobiles, yes?
Your question addressed as to why for example, in a five-bladed fan, the
blades were not each set radially at 72 degrees from the next as opposed to
the symmetry of the rake angle of the blades.

Firstly let me re-iterate that many fans (supplied new and unbroken!) have
either: one blade cut short, or a blade with a hole in to facilitate tool
access to the engine or easy manufacture. Both these features will give an
asymmetrical fan. However, since receiving the memo below I have researched
further and can give you something that I think will better meet your
expectations.

Where the number of blades = n, the intonation between the blades should be
360/n degrees for symmetry to exist. However, two blades can be separated by
a lesser angle, OR one blade made a shorter length to set up disharmony in
the aircurrents drawn through the radiator, the effect of this is appreciated
at high rpm (and therefore this is usually seen in high performance engines)
where harmonic noise would be produced leading to vibration in the engine and
auditory discomfort to the driver and those nearby. Electric fans (normally
encountered in economy cars and smaller engines) do not run at sufficient
speed nor draw air in sufficient quantities and high enough flow rates to
produce this phenomenon.

I should imagine that that is more along the lines of what you were looking
for.

Best Regards,
Justin Roux BEng (Hon) AMIMechE


--------------------------------------------------------------------------------
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Old 08-04-2010, 09:00 PM   #10
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I found this link on non-symmetrical fan blade spacing as well(done to reduce tone):
http://www.eng-tips.com/viewthread.c...=184853&page=7
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Old 08-05-2010, 03:48 AM   #11
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All high HP motors used 5 blade fans, low HP and most hydraulic lifter motors had 7 blade fans.
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Old 08-05-2010, 01:31 PM   #12
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At one time I swapped the 5-blade fan on my '83 K20 truck for a 7-blade. I never had cooling issues to begin with and you could tell that the 7-blade took more HP to drive. My advice, stay with the 5-blade unless there is a compelling reason to switch.
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Old 08-05-2010, 07:54 PM   #13
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Thanks for the great advice.........up until now, I assumed the more blades the better.

But if you think about it, 5 blades doesn't require as much engine power as 6 or 7.

I guess bottom line, use a few blades as possible but still keep the engine cool.
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Old 08-06-2010, 03:45 PM   #14
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Quote:
Originally Posted by snoopykissedlucy View Post
All engine things being equal, is a 7 blade fan better than a 5 blade fan?

I assume that an extra 2 blades and 1 inch diameter would be a good thing...........but someone once said that a mechanical fan is basically a solid wall at highway speeds, not allowing the rushing air to come through. I figure a 7 blade is a bigger solid wall. Also, I figure the extra weight can't be a good thing.

Any thoughts/opinions?(yes, we all agree an aluminum/electric fan combo is the way to go, but I'm trying to keep this low buck)

................
No, we don't all agree that that's the best combo. My flex fan setup (fan, spacer, and bolts) weighs 27 ounces. Cools my bigblock just fine. I'm guessing your electric fan weighs more than 27 ounces, and the "upgraded" alternator to drive that fan is additional weight, too.
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Old 08-06-2010, 06:20 PM   #15
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Quote:
Originally Posted by 69427 View Post
No, we don't all agree that that's the best combo. My flex fan setup (fan, spacer, and bolts) weighs 27 ounces. Cools my bigblock just fine. I'm guessing your electric fan weighs more than 27 ounces, and the "upgraded" alternator to drive that fan is additional weight, too.
I like the idea of a flex fan - seems like an easy DIY?
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Old 08-06-2010, 08:16 PM   #16
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Hey 69437, can you share some pics of your flex fan setup.............I used to be a flex fan fan but could never get it to properly cool my 350.....probably because of the spacing away from the radiator or fan shroud...

Either way, the cheapest clutch I found was $55.........so I'm all for a flex fan with no clutch.....if it works!
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Old 08-06-2010, 08:27 PM   #17
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You never mentioned an overheating problem. If that is not an issue. stick with the original fan designed for your Vette. Why are you wasting time?
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Old 08-06-2010, 08:31 PM   #18
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Lemans, when I bought the car and drove it home.....I eventually saw the white billowy smoke out the tail pipes......blown head gasket.

My experience says the engine overheated at some point..........I figured it was probably radiator related or fan related.......which is why I'm questioning the 5 blade mechanical fan that I have right now.

What are you using?
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Old 08-06-2010, 09:38 PM   #19
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Just went out and counted. I have 7 staggered blades.
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Old 08-06-2010, 09:53 PM   #20
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I look at it this way but I ain't no engineer....the 5 blade will "chop" a bigger slice and pull more air ??
Kinda like a food processer...more blades the finer the cut ??
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Old 08-06-2010, 09:53 PM
 
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