[Sloan]

I have been getting conflicting advice from the well meaning local "experts" in my area and could use some additional thoughts please.

As I continue to try and solve my overheating issues when it comes to the type of fan to run I am getting advice to run only a flex fan with a clutch. Then I'm told, no, the flex fan will not move as much air as a regular steel fan and that I should run the steel fan without the clutch? I also get other advice with these three items and different configurations.

Anyone know definitively what combination of a flex fan, non flex fan, fan clutch I should combine to give me the optimal cooling?

Thanks for your help!

Sloan

[GT182]

Personally, I'd only run a steel fan with the fan clutch, and shroud. I had a just flex fan on my GTO and it didn't work fer crap. If you really want a flex fan I have one to sell. 50.00 + shipping.

Also, overheating might be a combination of other things and not just the fan with or without a fan clutch. But I'm sure you know that. Have you checked inside the block for rust build up, or the radiator being plugged up.

[PontGuy]

I'll second that. An original 7-blade steel fan with a good HD clutch is my top choice for both cooling efficiency and quiet-running. But I have run flex fans without a clutch successfully in the past.

I would never try to rig a flex fan with a clutch, or run a steel fan without a clutch. At least not in a hi-perf Pontiac.

As GT182 has said, there is a whole lot more to running cool than the fan. If you haven't already done so spend some time reading through all the posts here.

Walt

[thadeal4real]

Originally Posted by GT182

Personally, I'd only run a steel fan with the fan clutch, and shroud.
Also, overheating might be a combination of other things and not just the fan with or without a fan clutch.

GOOD LUCK

[71 Goatman]

I am using a original steel fan with a clutch. I also purchased a Summit direct fit aluminum radiator, best purchase ever!!! No overheating worries whatsoever.

Mild 462, I can sit in traffic for hours if needed.

My .02

[Sloan]

Thank you all for the input. A follow up question as perhaps I am misundersatnding the function of a clutch. Isn't this desinged to operate more at a highway speeds where you see the benefits of a clutch fan? I had been using the clutch fan with my steel fan up until some advice I received where it was suggested I take off the clutch fan and use a spacer to make up the space I lost when removing the clutch fan. The local performance individual giving me advice told me that the clutch fan was keeping my steel fan from operating at optimal speed even at idle/slower speeds. I did not think a clutch fan would impact the rotation/speed of the fan when not at highway speeds.

Any clarification would be helpful.

Thank you,

Sloan

[Pontirag]

fan opperation is a function of rad temp. Up to a predetermined rpm the fan rotates with engine speed but beyond that rpm it slows up as the engine continues to increase in RPM. Those that have a bi matelic coil in front lock up or release the fan to rotate at a separate rpm based on the heat comming off the radiator. the fluid filled oned operate differently but provide the same results.

Fans are a funny thing. you can design them to operate efficiently at low rpm or hi rpms but usually not both. the more efficient the fan is the narrower the rpms it operated at.. Most designers design a fan to operate at low to mid speed operations where most drivers drive. the fan clutch disenguages the fan when the engine operated above that rpms because , for a given design increasing the rpm outside the range where it operates efficiently drains horse power causes turbulance/ stalling of air flow aroubd the fan and fan becomes very noisy.

those stupid flex fans are bad to begine with and are worse in every respect as you increase rpms. I have never in 35 years seen a manufacturer provide any specs on those flex fans that state how efficient they are and at what rpm range they operate most efficiently.Nor any info on how safe they are rated at for a given rpm. Plus the whole idea of one size fits all in cooling systems presumes some body in china knows nore about my engine needs than a pontiac engineer or me. They are soo noisey but some people think noise means power...what ever.

You will note that HP cooling systems, HD cooling systems and about every car made after 1972 (when cooling system demands increased exponetially) no body from the factory installed fixed or flex fans in cars. there is nothing a flex fan can do that a factory fan with clutch cant do better,

[71 T/A]

I know a local guy that swears by nonthermal clutch with fan.

[ELKHORNAOG7]

The way I understand the set up is without a clutch fan you use a spacer and a 7 blade fan that is half way in the scroud and with a clutch fan they only get the 5 blade fan which is also half way in the scroud also, I could be wrong about the fan blades but I recall the 5 blade with the clutch and 7 without it.-----BOB

[PontGuy]

The steel fan in my 69 definitely has 7 blades and is made to be used with a clutch. And it works great. The number of blades may be year dependent?

[Pontirag]

yes, but you want a fan with the most blades and you want one that is centered in the middle of the shroud OPENING and you want to have about 1 to 1.5 inches of clearance around the fan to shroud dimension.

you want to avoid a spacer at all cost too

it does not matter what type of fan clutch you use unless your going resto

[DPlum]

Flex fan dropped my engine temp. 20 degrees

[AG]

This summer I changed the stock 7 blade fan/Hayden severe duty clutch to a 7 blade 18" Flex-a-lite flex fan. My car has a 455 9:1 SCR and a RAIV cam with RA manifolds with a stock 3 core radiator ('72 LeMans vert). On a 90+ºF dayit would run about 185ºF around town and get up to 210ºF on the highway. I have the car tuned well, did all the divider plate clearancing, have a new PRW cast impellar water pump, 160º high flow thermostat, in short did all the typical mods to run as cool as possible. I switched to the flex fan and now under the same conditions in bearly gets to 170ºF arouns town and gets to 175ºF on the highway at 70 mph. This is just my experience and doesn't mean it will work for you. Remember, RAIV cars came from the factory with 7 blade flex fans.

[MikeNoun]

Flex fans made their way to the aftermarket back in the 70's. Their advertising campaign was great; they will lower operating temps, and unleash horsepower. Unfortunately, they made those statements based on switching from a standard 4-blade fixed fan blade, the type used on nearly all high-perf cars in the 60's, compared to their flexible blade fans. The flex fan IS superior to the old steel fixed-blade units, but not a clutch fan.

When clutch fans began appearing in the early 60's, they were designed for HD cooling, installed on cars that had a/c, sometimes they were an option on other cars. But they were expensive, and many people didn't want to spend the money. By the 1970's, nearly all cars used clutch fans because they moved a ton of air, and were quiet. If an engine was efficient enough, and had a good cooling system, the manufacturer would still use a fixed-blade fan to save costs. But as manufacturers began designing cars with smaller grille areas (even no grilles) in an attempt to obtain better gas mileage, clutch fans soon became standard on nearly every American car.

An rpm style clutch fan will nearly freewheel at high rpms, and lock at low rpms. One problem with that is that if you have sustained high rpm's, such as traveling through the mountains, towing a trailer, or you have a car with steep gears and drive on the highway a lot, the clutch fan may not provide enough cooling. A flex fan would have the same problem. The flex fan's blades would flatten out at high rpm's, and not move the air.

Enter the thermal clutch fan. The thermal clutch is based on temp and rpms. It will be in a lock state when the engine is cold, it will nearly freewheel at higher rpm's (just like the regular clutch fan), but if the temp gets beyond a predetermined point (usually 190 degrees), it will lock up and help cool the engine regardless of rpms.

Thermal clutches were used on all a/c equipped cars in the 60's and 70's. Those cars needed the most cooling. Most a/c equipped cars were heavy, and the added drain of the a/c compressor turning, and low numerical gears, the clutch fan was the answer. And I believe Chrysler, Ford, and GM knew what they were doing.

The RAIV cars came with 7-blade fixed blade units because they DO move the most air, but they also cost the most horsepower. As much as 20 hp may be lost at 5000 rpm using a fixed 7-blade fan. That was one of the reasons the aftermarket sold a lot of flex fans. They did free up hp, but then your engine ran hotter. The clutch fan was the best of both worlds. Fixed 7-blade cooling, quiet, and zero hp loss at high rpm.

[ericwy]

I think either flex-fan or clutch-fan can work. They solve the fan needs in different ways.
Flex-fan blades flatten out at high rpm, Clutch slips at high rpm.

It seems that the worst case scenario is the "sitting/idling in traffic" on a hot day with AC. Engine rpm is low, fan speed low, thus low air flow thru the radiator. The standard radiator/fan cooling system has more than enough capacity to cool the engine. But it cant do squat without enough air flow. Less air = less heat transfer.

So you either:
1. increase the air flow - choices are flex, clutch, or electric.
2. increase the heat transfer capacity of the cooling system - choices are radiator, pump, or coolant.

Here's what I did :
1. I run a 160F stat, factory 7 blade 19" fan, factory shroud, & 4 core rad.
Heavy duty Hayden (not severe duty- too much parasitic loss) clutch with the 5/8" inch longer shaft (Cadillac model) puts the fan 5/8" deeper into the shroud.

2. 70% water + 30% antifreeze. Water has a higher heat holding capacity than antifreeze which means it can transfer more heat for the same volume which means the heat transfer capacity of the cooling system is increased.

My engine's cooling behavior is excellent under normal conditions (170F). Worse case scenario is when outside temps creep up above 95F and AC is on.
Sitting at a stop light last week it was 106F the coolant temperature creeped up to the 220F mark. But, once the light turned green it dropped down to about 210F.

[Region Warrior]

I've used both clutch and flex since 1975 without problems.
Both have to be 19" 7 blade correctly mounted into shroud.

Clutch type will free wheel at highway speeds giving some extra power.
Flex makes more cooling from idle and up but may cost some power at high speed.
Niether make a dif in et-n-speed in my 68 FB(runs consistant 12.0's @ 114.)
But the 19" flex lowers top end 1/4 mile temps more.

[george kujanski]

My wife had a '73 Mustang 302, A/C, with factory stock flex fan. Never had an issue.

My 2001 Yukon has a clutch fan, fully inserted into the shroud.

From what I understand, a thermal clutch never really 'locks up', It slips less when the air thru the rad is hotter, thereby increasing airflow. It's designed to slip, to save energy like at higher RPM when there is more airflow thru the rad, and fan-induced airflow is not necessary.

I have a thermal clutch on my GTO, I am going to install a shroud, and make sure the fan is fully into the shroud. A while back one of our members did a test with various fan vs. shroud positions...the fully inserted version moved the most air.

George

[GT182]

Something else to ponder.....

How good is your temperature guage? Is it reading temps correctly and is new, old, or original and never calibrated after all these years? Also your sending unit in the intake...... old, new, clean, dirty or otherwise?

Lastnight I pulled my sending unit.... thermocouple type with a Sunpro temp gauge. It was a bit on the dirty side, but no scale on it. Cleaned it off with carb cleaner and reinstalled it. After putting the coolant back in I ran the car for 20 minutes at 1200 rpms. Temp stayed at 160 the whole time. Trying a 160 for now but will put the 180 stat back in soon. Took the car out on the street and around the block. It went up to 180, until I had to sit at a long stoplight, then it went to 195 but no more. Once moving it dropped back to 170 and stayed there..... top speed was 40 to 45mph. Hey, at least it's not getting so hot that the car marks it's territory like it did a couple of weeks ago. LOL

Saturday night we're having an outlaw cruise night on the Kirkwood Highway in Newark, DE. That will be the big test to see what the temps will be in stop and go traffic with the forecast of cooler temps here.

[hillpc]

From what I understand, a thermal clutch never really 'locks up', It slips less when the air thru the rad is hotter, thereby increasing airflow. It's designed to slip, to save energy like at higher RPM when there is more airflow thru the rad, and fan-induced airflow is not necessary.

---------------
Righto; my 75 Buick (LeSabre) service manual says the (thermal) clutch fan is supposed to engage at 185 degrees air temp hitting the fan. (Put a thermometer between radiator and fan.) To see if it's working, you're supposed to partially block the grille airflow, run at a 2000 rpm idle, turn on the A/C, and listen for the increased fan roar and watch for higher fan blade speed as the air temp climbs past 185. I couldn't hear anything change from startup to over 200 degrees air temp, nor could I make out any difference in speed visually (looks like a blur to me anyway). Since I'm driving the car to Arizona (2000 miles one way) and back a week from now, I don't want to take any coooling chances, so I installed a solid 7 blade steel fan I had sitting on a shelf. Feels like a good bit more wind blowing now when I stand next to the engine compartment at idle. And man, does it make some noise (a howl of sorts) as the rpm's get up right before a shift. Probably waste some fuel at highway speeds but at least I know I'll be OK at idle with the A/C on.

[Pontirag]

factory fan with a thermoclutch. any thing else is people seeking absolution.