[71firebird400]

Just replaced the flex fan on my car due to an annoying belt squeal that I believe was caused by the fan.

Now that I have the clutch fan the car is running quite a bit hotter- previously it would stay at 170 and never get any hotter, now it will work its way up to 210 if I'm in traffic (but seems to stop at 210).

Is there a definite way to test the clutch? I've turned the car off after it was at 210 and noted that the fan was about as hard to turn as when its cold. Also, I used a rag to catch the fan during idle at 210 and it stopped spinning rather easily.

It sounds to me like the clutch is bad, but my dad has a C3 Corvette with flex fan that runs at 180 and that fan acts the same as mine when the car is hot.

Also its very obvious that this fan is moving less air than the flex did. I'd like to get this to work, it solved my belt squeal and looks better.

[north]

Clutch fans generally came with different ratio pulleys so they would turn faster at a given engine RPM. Also it is normal that a clutch fan will let a car run hotter at lower speeds but be more effective at higher road speeds as the cooling demand increases.

There are a wide variety of factory clutches also, the amount of slip they permit at a given temp-RPM varies greatly. I find old poncho's tend to like the medium duty units.

I would say the clutch is bad or weak.

[71firebird400]

My understanding is that at a certain temperature the clutch would "lock up", I assumed that if I let the car get to 210 the clutch would be locked and the fan wouldn't spin freely- perhaps I have this all wrong in my head?

[ctgross]

Let the engine run until it's up to temp., 180-190, and with the hood up so you can see the fan, turn it off and as soon as the engine stops watch the fan and count til it stops. My old one would keep spinning until I counted to five or six before it stopped. With a new clutch I installed last month, the fan stops as soon as I kill the engine. My temps. dropped 20 deg. average.

[Rugratman]

What about the distance into the shroud, did that change?

[71firebird400]

The fan is about 2/3 in the shroud, 1/3 out- what I've been told is 1/2 in 1/2 out is perfect, so I don't think that's the issue.


I'll watch the fan next time I turn the engine off and see how long it continues spinning.

[Geoff]

There are three 'levels' of clutch fan: standard, heavy duty & extra heavy duty [ also called severe duty ]. The levels basically refer to the amount of clutch slippage. From one supplier, they quote for the STD clutch 50-60% of shaft speed when engaged, 80% for HD & 90% for SD.
If you have the std fan, you would be getting a lot of slippage. Slippage = a hot[ter] engine.
I can recommend the Thermal fan that Summits sells for $46. It is a SD fan & quality built [ Flex-a-lite ]. I have had one on my car for over 2 years & not a drop of fluid leakage from the seals. Leakage = failure or failing.

[north]

They never "lock" it is as Geoff said a matter of how much or little they slip at given temperatures.

[Old Blue 66]

Originally Posted by ctgross

Let the engine run until it's up to temp., 180-190, and with the hood up so you can see the fan, turn it off and as soon as the engine stops watch the fan and count til it stops. My old one would keep spinning until I counted to five or six before it stopped. With a new clutch I installed last month, the fan stops as soon as I kill the engine. My temps. dropped 20 deg. average.

You can also just spin the fan after the car is off. If it spins more than three times, the clutch is bad. Their not expensive. If you have any doubts, just replace it. Napa has them.

[Z Code 400]

Years ago, we started using clutch fans when we recorded a 7HP increase on one of our engines slated for an oval track application. Since that time, we have used clutch fans on most of our street cars without exception. Recently, I replaced the factory 5-blade ‘Power Flex Fan’ that came as original equipment on my non-A/C 1975 Formula with a 7 blade clutch fan. The results of this swap were interesting, so I decided to share them with you.

It has been 103°F here for the past few days. My Formula, with a 160° Robertshaw Reverse-Poppet Thermostat, consistently runs 170° to 180° all the time. I drive this car 110 miles a day to work, so I am instantly aware of any small changes that occur.

When I swapped to the 18" 7 blade fan and massive 6.2 liter diesel clutch, the fan seemed to move a lot of air, especially at idle. As the coolant temperature increased to operating ranges, the fan would engage and disengage, much like an electric fan does. I was very pleased because the clutch fan was much, much quieter than my factory 5-blade 'Power Flex' fan.

When I drove the car, I immediately noticed my coolant temperatures had increased to 190°-215° during cruise. This car will not go over 180° if you leave it idling in the drive through at Burger King on a +100°F day. The coolant temp was steady and would not go above the 215° mark, but it was much higher than the car normally runs. Later that evening. The outside temperatures dropped to around 85°F and I drove the car the 35 mile trip from the shop back to my home. My coolant temperature stayed at 210° the entire trip, running at a constant 70-75mph.

Initially, I thought this could be caused by running without the ram air system in place (which is out being refinished) and the pressurized air entering the engine compartment through the hood vents was impairing flow through the radiator. But, the system cooled perfectly with the solid fan in place. This prompted me to examine the operating characteristics of both the Pontiac ‘Power Flex’ fan and the fan clutch.

According to the 1975 Pontiac Shop Manual, the ‘Power Flex’ fan is a variable-pitch fan used in police and taxi applications where maximum cooling is critical. The fan pitch reduces by 50% at 5000 rpm, when the ram effect is sufficient to force air across the radiator surfaces.

The 1986 GM Truck Shop Manual describes the fan clutch as a viscous coupling that is de-clutched below 150° to 170° radiator surface temperature. When radiator surface temps reach this point, the clutch engages and the fan is limited to a maximum speed of 2200 rpm, regardless of engine rpm.

Although I think there is a definite performance increase to be had with a clutch fan, it just doesn’t do the job of cooling that the factory (and noisy) 5-blade flex fan does.

Looks like the solid factory fan is going back on the car. The blades stop dead (like a solid fan) when the engine is shut off. The probelm is, the fan just doesn't turn enough rpm to provide adequate airflow.

Here are some photos of the instrument panel in my 1975 Formula on a trip to the Mojave Desert (Lucerne Valley, CA.) on December 29, 2007. The temperatures were around 85°F at the time and I maintained a steady 100mph to 125mph the entire trip as traffic conditions would permit. My temperature gauge runs in this exact position, even on +100°F days and I have verfied the factory gauge with a remote-mounted Stewart Warner 360° gauge.

With the 7 blade clutch fan (brand new GM HD clutch) my car runs 210°F on a 65° night at 70mph, with less then 1/3 of the blades outside the shroud...Robert

P.S. The server won't let me upload the images. E-mail me if you want to see where my temperature guage runs...

[Z Code 400]

By the way, on our oval track cars, the power increase was worth the increases in coolant temperatures. Those engines ran hot anyways, especially with the radiator fin surfaces full of clay. However, when we started running asphalt cars, we quickly found the clutch fan wasn't cutting it.

At Mesa Marin Raceway in Bakersfiled, California which was a high banked 1/2 mile oval track (17° corners and 7° straightaways) we removed the fan from our car once and it would overheat even at 90+mph. The car simply had to have a fan to cool properly.

We used the Flex-a-Lite 18" 7 Blade Fixed-Pitch Race Fan with an 'hourglass' style billet aluminum spacer that features short attaching bolts for both the fan and the water pump, as the long spacers and corresponding bolts were prohibited.

No more cooling problems...Robert

[Z Code 400]

Originally Posted by 71firebird400

Also its very obvious that this fan is moving less air than the flex did. I'd like to get this to work, it solved my belt squeal and looks better.

Very old circle track trick:

Dayco Diesel Engine Grade Belts. Good to over 8500+ rpm with solid fans...Robert

[Z Code 400]

I picked up a Ford 7-blade flex fan (18.0" diameter) from an older Lincoln. Now this thing moves a lot of air, but it's very quiet....No belt squeal with good belts, even shifting at 5500...Robert

[Z Code 400]

Of interesting note.....

With a clutch fan, I had noticeably increased coolant temperatures as I drove faster. This proves that airflow through the grille on my 1975 Firebird is inefficient unless the fan is creating negative pressure in the engine compartment.

My coolant temperatures have always been around 170°-180°F in California's +100°F summer temperatures. Driving home from the shop after installing the 7 blade fan and clutch, I was seeing steady 210°F coolant temperatures with only 85°F ambient temperatures.

With the 7 blade Lincoln Flex Fan, using a 2.50" spacer, the blades are even with the back of the factory shroud. With the 7 blade clutch fan, roughly 1/4 inch of the blades were exposed outside the shroud.

A quick check of the Hayden catalog confirmed the length of the factory clutch for a 1975 Firebird is exactly the same length as the 6.2 litre diesel clutch.

The 7-blade Lincoln flex fan is very quiet, but I do like the HP improvement with a clutch fan. I considered doing some more experiments with the clutch, but I believe I am fighting the vehicle's aerodynamics more than anything else.

I also have concerns about a clutch fan on the dyno too. Obviously, airflow through the radiator is minimal at road speeds (on my car) without the fan's negative pressure contribution. This would be even worse on the dyno, even with the big supplemental fans they employ during testing.

It is possible that running the car (as I am currently) with the ram air system removed, is allowing air to enter the engine compartment through the hood scoops and impair cooling slightly.

Since I am increasing my SCR on my 350 Pontiac from stock (7.80:1) to 9.25:1 with the addition of 1966 389/421 heads and swapping in one of my popular custom hydraulic flat tappets (.466"/.466" - 220°/226° @ .050" - 111.5° LSA) the HP and heat production of this engine are bound to go way, way up. So, I am already starting to address these issues ahead of time.

I have plans to switch to an aluminum radiator and I might just try experimentation with the Hayden Severe Duty Clutch Fan for the 6.2 Litre Diesel, which is designed for the 3.73:1 to 4.11:1 axle ratios and will operate at higher rpm.

In consideration of the pulley diameter, my non A/C 1975 Formula has a mauch larger pulley than the 6.2, so I should be able to keep fan clutch rpm at a reasonable level, probably lower than it would be on the 6.2 in stock form.

I will continue to post the results of my experiments...Robert

[Z Code 400]

Here are some shots of the severe-duty clutch and 7 blade fan on my 1975 Formula...

[Z Code 400]

Today I decided to conduct some testing to see which setup would yield the best overall cooling performance. My 1975 Formula is a totally stock, 62,000 mile, non air-conditioned car. It is equipped with a Modine 13fpi 3 row radiator, factory shroud and 160°F Robertshaw Balanced Sleeve Thermostat.

Today, I removed the 18” (7) Blade Lincoln Flex Fan and factory Pontiac 2-1/4” spacer and installed an 18.25” (7) Blade GM Fan from a 1981 Monte Carlo.

I used an Autozone ‘Torq-Flo’ Severe-Duty Fan Clutch, part #922799, which is intended for a 1984 ¾ Ton Suburban 4X4, 6.2 Litre Diesel, with 3.73:1 and higher (numerical) axle ratio. I made a custom 5/8” x ¾” bronze pilot adapter to mate the 6.2’s ¾” pilot to the Formula’s 5/8” water pump shaft.

I am running the big (factory) non A/C water pump pulley.

This fan clutch says ‘Made in China’ on both the product and the box. However, it is surprisingly well made. The pilot measured on size, as did the bolt holes. However, the usual ¼” mounting holes (this clutch has both GM and Ford patterns without slots) are machined to a full 5/16” on the hub.

When installed, I had approximately ½” to ¾” of the fan blade’s width extending out the rear face of the fan shroud. See attached photos for reference.

The initial test was conducted at 12:00 noon today in my driveway with the car in direct sunlight and the hood open. Ambient temperature was 92°F with 25% humidity.

The clutch was engaged on startup with a considerable amount of fan noise. The fast idle was set to 1800 for this part of the testing. After about 3 minutes, the fan de-clutched and cooling fan rpm dropped to a very slow spin. Coolant temperatures began climbing up from 140°F to a full 180°F without the fan clutch engaging. There was no perceivable airflow coming from the fan or at the core surface of the radiator.

As the coolant temperature approached 195°F, the fan engaged with an engine speed drop of 200 rpm. This fan is LOUD beyond description when the clutch is engaged. Airflow through the radiator is phenomenal. I noticed what initially felt like a ‘shudder’ or ‘vibration’ when the clutch engaged, but it could not be felt on the engine itself. However, you could feel it on the fan shroud and it would seem that this is a turbulence issue from the air being whipped around by the aggressive pitch of the 7 blade fan.

Coolant temperatures began to drop steadily and eventually reached a low of 185°F. The fan did not de-clutch again during the 15 minute test period. I blipped the throttle several times to 4000 to 5000 rpm and experienced no belt squeal whatsoever.

This evening, I left the house around 5:00pm for my workplace, roughly 55 miles away. Weather conditions at my home were 88°F, 34% humidity with a barometer reading of 29.83 and winds from north at 11 mph. My trip takes me on a due east course, so the crosswind did not contribute to the test.

Coolant temperatures stayed around 170°F to 180°F during 55 mile trip. My speed ranged from 55 to 85 mph with an average speed of 63.4 mph. I fully opened the secondaries twice during passing maneuvers.

When I arrived at work, at approximately 5:52pm, the weather conditions were 87°F 29% humidity with a barometer reading of 29.85 and winds from North at 13 mph. I parked the Formula directly behind another car (facing north) to block the 13 mph winds as much as possible.

I allowed the car to sit and idle for a full 10 minutes and the coolant temperature stayed steady at 185°F.

This concludes this portion of the test. I hope this information is helpful...Robert

[Z Code 400]

Of interesting note, the clutch fan reduced engine speed a full 200 rpm when fully engaged, but when letting the alternator adjustment loose at 1800 rpm with the 7 blade Lincoln flex fan (effectively stopping the fan from turning) there was no engine rpm increase. The amount of airflow between both fans at 1800 rpm is very similar to the hand, but the flex fan is noticeably quieter.

More to come...Robert

[71firebird400]

Thanks a lot, Robert. I hadn't checked this thread out for a while and am now seeing all the info you posted.


Unfortunately I haven't had time to try anything different on my car, I've been too busy putting in a new trans and the car's been on jack stands for about a week and a half. Hopefully I'll get a new clutch in the near future and will be able to tell if that was the issue after all.

If it still runs hot I may just put the old flex fan back on; that thing moved a LOT of air!

[RandyW]

My 1975 Formula is a totally stock, 62,000 mile, non air-conditioned car. It is equipped with a Modine 13fpi 3 row radiator, factory shroud and 160°F Robertshaw Balanced Sleeve Thermostat.

I also have a '75 Formula and found the same basic thing. About 2 weeks ago I swapped the stock 350 for a '68 400 WS, mildly ported stock heads, 488/491-233/238 cam.
My temps were 210-215 all the time, could see 195 at 40 mph but not above or below that. I have 3.42 gears and a t-350. I regapped the plugs to .045 since I have hei, that dropped a few degrees off, put a new clutch on the factory 7 blade fan, (I have a/c). Not much different. yesterday I installed a 19" Haden steel flex fan with 2" spacer leaving about 1/3 blade exposure. Drove it all over the place 180-190 all the time. recovers from a hot start in 30 seconds, use to take 5 minutes, I'm sold!

[Z Code 400]

Randy,

This severe duty clutch (so far) seems to be working very well. I am sure running with the ram air system removed is pressurizing the engine compartment to some extent, which may also hamper airflow through the radiator...Robert

P.S. You got a 19" fan to fit in that shroud?????