theory of the flex fan
- Thread starter ludwig
- Start date
A
Anonymous
Guest
The flex fan is lighter (less rotational mass), the flex fan doesn't need power from the alternator (unlike an electric), and the flex fan requires less energy to drive as it spins faster (The blades flatten out at speed causing less drag. Thus, pulling less air at higher rpm, but usually the fan is only needed when the car is moving at a slow pace and the engine is idleing).
A
Anonymous
Guest
the flexfan allows for higher air flow at higher RPM cause the fan flexes and draws in the air quicker. I suppose the small gain in performance would be because the engine doesnt get as hot, and as far as i know, a cooler engine will perform better. an electric fan only uses the alternator power..i dont think the extra energy to charge the alternator for the electric fan is as great as the energy used to spin the fan via the water pump...although i could be wrong.
A
Anonymous
Guest
demon slayer":qmer6zl0 said:
Yep.... higher rpm = less air flow from the fan.
A
Anonymous
Guest
The best flex fans are probably factory ones, all the aftermarket ones seem to be noisy as hell, the blades flatten out over time, and the blades are sharp as a razor!
From dyno tests I've seen, clutch fans have the least amount of drag (although I don't think the early cars have enough room for them). The electric fans had the least amount of HP loss, so I guess the drag of the alt charging to run the elec fan is less than the drag the fan blades create at higher RPM..
I have an electric on my Stang ('68 302), before that I had a 17" 6 blade aftermarket fan (Hayden IIRC).. on real hot days (100 deg or more) it would be okay when moving, but driving slow or stopping for prolonged periods it would run all the way over to H on the gauge..
With the electric, it may run a bit hotter driving around in the real hot days, but when driving slower or stopping it only runs to about the middle of the gauge and stays there... one thing I do like about the elec, is you can turn off the car and keep running it and it will cool down the water in the rad, then when you start the can it'll cool it off (since it mixes the cooler water back into the engine).. so you can keep it from boiling over.. Most of the time if it's running hot with an engine driven fan, and you turn it off.. it gets hotter, then boils over since it no longer has airflow..
And I can say that your water pump bearings tend to last longer with an electric, without the weight hanging from the front of it (this would probably be the disadvantage of the clutch fan, more weight on the nose)
I found a nice factory flex fan that I'm gonna use on my 250 when I put it in my '69 Ranch.. It came out of a mid 70's Maverick/Comet.. I believe it's a 17" too, 7 blade.. I think the cars that had these had A/C, could be wrong tho... that would be a good cheap upgrade if you had the room for it..
From dyno tests I've seen, clutch fans have the least amount of drag (although I don't think the early cars have enough room for them). The electric fans had the least amount of HP loss, so I guess the drag of the alt charging to run the elec fan is less than the drag the fan blades create at higher RPM..
I have an electric on my Stang ('68 302), before that I had a 17" 6 blade aftermarket fan (Hayden IIRC).. on real hot days (100 deg or more) it would be okay when moving, but driving slow or stopping for prolonged periods it would run all the way over to H on the gauge..
With the electric, it may run a bit hotter driving around in the real hot days, but when driving slower or stopping it only runs to about the middle of the gauge and stays there... one thing I do like about the elec, is you can turn off the car and keep running it and it will cool down the water in the rad, then when you start the can it'll cool it off (since it mixes the cooler water back into the engine).. so you can keep it from boiling over.. Most of the time if it's running hot with an engine driven fan, and you turn it off.. it gets hotter, then boils over since it no longer has airflow..
And I can say that your water pump bearings tend to last longer with an electric, without the weight hanging from the front of it (this would probably be the disadvantage of the clutch fan, more weight on the nose)
I found a nice factory flex fan that I'm gonna use on my 250 when I put it in my '69 Ranch.. It came out of a mid 70's Maverick/Comet.. I believe it's a 17" too, 7 blade.. I think the cars that had these had A/C, could be wrong tho... that would be a good cheap upgrade if you had the room for it..
A
Anonymous
Guest
Inliner":k8980fzv said:
i suppose that doesnt matter a whole lot...i never see more than 3,000 RPM on the highway...if u dont care so much about the extra power loss, would an thin electric fan and a smaller spacer for the flex fan work, or would the two winds screw each other up?
Mustang196t8":1foulch2 said:
YES, don't know, HELL YES!
WHen I first started mine up with the flex fan, I turned the engine off immediately because I thought there was a problem.
And the blades are extremely sharp. You have to wear gloves when working anywher eon the front of the engine.
They will pull some serious air though. WIth my stock 4 blade fan, it will barely hold a sheet of paper against the radiator at idle. The flex fan held a shop rag against the radiator with no problems at all.
Slade
A
Anonymous
Guest
I agree a flex fan (usually at least 6 blades) is WAY better than a stock 4 blader!
As for the noise, it kinda makes a whine... not like a gear drive... almost like a scream or something you could say.. My friend had the same fan I had on my Stang (6 blade Hayden flex) on his '64 Comet... he was gunnin it down the street one time (heading my direction) and you could hear that fan big time.. sounded like the damn thing was gonna blow up!
I can speak from experience on the sharpness... I put one on my sister's Buick after the stock fan decided to lose a blade (doh!), and I was wearing a pair of Mechanix gloves and the blades stilll wore a hole thru them!
As for the noise, it kinda makes a whine... not like a gear drive... almost like a scream or something you could say.. My friend had the same fan I had on my Stang (6 blade Hayden flex) on his '64 Comet... he was gunnin it down the street one time (heading my direction) and you could hear that fan big time.. sounded like the damn thing was gonna blow up!
I can speak from experience on the sharpness... I put one on my sister's Buick after the stock fan decided to lose a blade (doh!), and I was wearing a pair of Mechanix gloves and the blades stilll wore a hole thru them!
A
Anonymous
Guest
but a flex fan looks way cooler with the hood up.....
A
Anonymous
Guest
i've never heard my engine without a flexfan...so i dont even know how much quieter it would be...but it doesnt really bother me.
The flex fan is a part of the past. if your cooling system is up to par, the electric fan is the best choice.
The next choice is the clutch fan.
Why pull a ton of air through a radiator when the temperature is say 45 degrees f.
The newer mustangs all use a total electric fan. its more efficient & gives more horsepower to the wheels. Plus only cools when needed.
If you have a flex fan, you need to upgrade to the modern technology. If you need a flex fan you have a cooling system problem you need to address.
A flex fan will rob you of 10hp. this has been proven by a dyno run with my local mustang club. Go electric. william
The next choice is the clutch fan.
Why pull a ton of air through a radiator when the temperature is say 45 degrees f.
The newer mustangs all use a total electric fan. its more efficient & gives more horsepower to the wheels. Plus only cools when needed.
If you have a flex fan, you need to upgrade to the modern technology. If you need a flex fan you have a cooling system problem you need to address.
A flex fan will rob you of 10hp. this has been proven by a dyno run with my local mustang club. Go electric. william
A
Anonymous
Guest
sounds good to me...i didnt know that a flex fan took that much hp...i need all i can get. i should also get a cooler thermostat too i guess...i got the stock 190 deg. thermostat...what is a good temperature thermostat to get?
A
Anonymous
Guest
Flex fans have also been known to throw a blade at times. All the flexing will eventually fatuige the blades and crack will form. My dad has a 77 suburban and GM sent out a recall letter in 1998 for all flex fans saying, in effect, "you need to go buy yourself somthing else."
Just remember you asked..... 
(this is a long one)
For the Theory part, first Fluid Dynamics. A fan and any other "fluid mover" say a blower, pump, whatever does two things to the fluid. It creates flow AND pressurizes it. Because these both go on the same time it is possible to multiple both effects together. Typically flow is expressed in cubic feet per minute for fans/blowers. For pressure there is a bunch of different engineering units but you can use pounds (force) per square foot. So if you multiply the pressue and flow together it results in "pounds (force) X foot / min". This is called hydraulic horsepower. The top number should look familiar, it's the definition of "work". And work per unit time by definition is horsepower. So all this comes down to is that to move fluid you need horsepower. Double the flow OR double the pressure and you have doubled the horsepower needed to do it.
Now comes the Heat Transfer part. Part of the heat from the combustion chamber is transfered by convection out the radiator. Please note this is convection not conduction!!! Convection requires the movement of some type of fluid. Anyway convection heat transfer is very dependant on the velocity of the fluid(s) involved. Obviously the faster the flow the faster the heat transfer (to a point). For a car, the rule of thumb is that the convective heat transfer out of the radiator (based on water flow and air flow) is greater than the heat generated in the engine at speeds above 30 mph. A car runs cooler on the interstate than in stop and go traffic simply because of the air flow across/thru the radiator.
So basically your car needs a fan when you are driving below 30 mph and after that it is consumign hp from the engine. Obviously for a belt driven fan the blades spin faster with the RPMs of the engine and it takes more horsepower to turn the blade!!! So a flex fan is designed so that the blades flatten out at highway speeds. As the blades flatten out they move less air. So if the flow is lower then the hp required to turn the blades is lower. This draws less horsepower from the engine and more hp can go to the back wheels!!!
Sorry this is so long.
(this is a long one)For the Theory part, first Fluid Dynamics. A fan and any other "fluid mover" say a blower, pump, whatever does two things to the fluid. It creates flow AND pressurizes it. Because these both go on the same time it is possible to multiple both effects together. Typically flow is expressed in cubic feet per minute for fans/blowers. For pressure there is a bunch of different engineering units but you can use pounds (force) per square foot. So if you multiply the pressue and flow together it results in "pounds (force) X foot / min". This is called hydraulic horsepower. The top number should look familiar, it's the definition of "work". And work per unit time by definition is horsepower. So all this comes down to is that to move fluid you need horsepower. Double the flow OR double the pressure and you have doubled the horsepower needed to do it.
Now comes the Heat Transfer part. Part of the heat from the combustion chamber is transfered by convection out the radiator. Please note this is convection not conduction!!! Convection requires the movement of some type of fluid. Anyway convection heat transfer is very dependant on the velocity of the fluid(s) involved. Obviously the faster the flow the faster the heat transfer (to a point). For a car, the rule of thumb is that the convective heat transfer out of the radiator (based on water flow and air flow) is greater than the heat generated in the engine at speeds above 30 mph. A car runs cooler on the interstate than in stop and go traffic simply because of the air flow across/thru the radiator.
So basically your car needs a fan when you are driving below 30 mph and after that it is consumign hp from the engine. Obviously for a belt driven fan the blades spin faster with the RPMs of the engine and it takes more horsepower to turn the blade!!! So a flex fan is designed so that the blades flatten out at highway speeds. As the blades flatten out they move less air. So if the flow is lower then the hp required to turn the blades is lower. This draws less horsepower from the engine and more hp can go to the back wheels!!!
Sorry this is so long.
Whoa! You always get more than you thought you asked for on this forum.
Then there's that part where the fan acts more like a barrier to air flow than a facilitator once the speed of the air flowing in exceeds the speed of the air the fan is pushing.
Thanks y'all. I'm going electric.
Has anybody used a Zoic fan. They are not too expensive and they list a 1400 CFM with only 2.5" of depth, so it will fit in an older Mustang. I taped the space at about 2.66" in my stock engine.
Then there's that part where the fan acts more like a barrier to air flow than a facilitator once the speed of the air flowing in exceeds the speed of the air the fan is pushing.
Thanks y'all. I'm going electric.
Has anybody used a Zoic fan. They are not too expensive and they list a 1400 CFM with only 2.5" of depth, so it will fit in an older Mustang. I taped the space at about 2.66" in my stock engine.
Similar threads
All Small Six
New Adventures in electrics fans and inline 6’s
