[Stripes]

I'm considering replacing my 19.5" , 7 blade gm 9796372 engine cooling fan (2.25 PITCH) with electric fan/s to get more cfm at idle speeds. Does anyone know how many cfm the stock fan makes at 700-850 rpm? I need that to ensure I am moving forward...

The addition of the AC is causing heating at idle when stopped idling( 230F.) Cruise temps around 190f.

[JLMounce]

Somebody should definitely check my math on this, but I believe at 750rpm, your fan should be moving around 1150 cfm. At 1700 rpm cruising around town it would be just shy of 2650 cfm. It might be less though, my understanding is adding fan blades reduces noise at the expense of flow. How much flow however, I don't know.

Somebody with more schooling will hopefully chime in on this

[Steve C.]

A fwiw tid bit when shopping for electric fans...........

Years ago there was no common industry standard for rating electric automotive fans, I presume the same situation today. Most companies use a cfm rating, often expressed in free-flow and not when placed behind a radiator. You can never know for certain about different cfm ratings or how a company has rated their fan since they can be tested by different criteria and methods. In comparison between brands using cfm ratings alone is difficult, typically not apples-to-apples.


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[70GS455]

Instead of replacing the mechanical fan, one option is to follow what some late model trucks use. That is to add an electric fan inside the shroud, along witht the mechanical fan, or add a pusher fan in front of the radiator.

Sent from my SM-T817V using Tapatalk

[RocktimusPryme]

Originally Posted by JLMounce

Somebody should definitely check my math on this, but I believe at 750rpm, your fan should be moving around 1150 cfm. At 1700 rpm cruising around town it would be just shy of 2650 cfm. It might be less though, my understanding is adding fan blades reduces noise at the expense of flow. How much flow however, I don't know.

Somebody with more schooling will hopefully chime in on this

Wow I always thought it was more than that. Though I admit that's not based on any factual evidence. Its just that people always say that clutch fans move a ton of air.

So at idle, which is where most muscle cars have cooling problems, a moderate electrical fan likely moves more air than a big clutch fan.


Whenever I look at electric setups and see that it only moves 2000 CFM I felt it was dinky. Might have to reprise that thought.

[Stripes]

Rock, you may still be correct. I see a LOT of people add electric fans and run HOTTER than the mechanical stock style fan. Why? Likely because of what Steve mentioned, a lack of transparency on the actual electric fan performance.

So what to do? IF a mechanical only moves around 1200 CFM at idle, nearly any electric fan should end any temp spikes at idle, but they do not. SO, now we have to determine which of the truth stretchers are stretching thier stats the least! I considered selecting the fan that draws the most amps, but that may be selecting the least efficient fan motor, and those stats could be false anyway. I need a chart to show me how many cfm go thru the radiator at varying speeds via ram induction from driving. Then I could slow down to the point the enging would begin to get hot and consider that a minimal cfm to target.

The Flex-A-Lite Direct Fit Electric Fans 105398 claims to move 4,600 CFM. Unless they have VERY long noses, this well exceeds a stock fan at idle. Has anyone run this set up and on what?

[Steve C.]

Not Pontiac specific but some on line conversations suggest a mechanical fan with a good shroud can pull 4 or 5000 CFM easy.

Interesting read on post #4 here.....

https://www.impalas.net/forums/2-gen...ctric-fan.html


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[JLMounce]

Originally Posted by RocktimusPryme

Wow I always thought it was more than that. Though I admit that's not based on any factual evidence. Its just that people always say that clutch fans move a ton of air.

So at idle, which is where most muscle cars have cooling problems, a moderate electrical fan likely moves more air than a big clutch fan.


Whenever I look at electric setups and see that it only moves 2000 CFM I felt it was dinky. Might have to reprise that thought.

I mean, it's possible my math is off, this isn't anywhere near my area of study and I'm just using a basic formula for an axial fan.

19.5" fan = 1.625 ft diameter
2.25" pitch, so each revolution should moves 3.65625 inch of air in a column
* 750 rpm = 2742.1875 / 12 = 228.51 ft per minute column of air.
3.1416 * 1.625 to get our area * 228.51 ft per minute in the column = 1166.57 cfm

that's just a free flow rating and doesn't take into effect shrouding, anything in front of the fan or if the fan is a flex type etc. So as Steve C has pointed out, this figure could be all together meaningless. My formula could also be wrong.

[Stripes]

Ok, but my factory 7 blade and severe duty clutch does great(190) without AC on our 469, but with AC at idle for 5 minutes, temps take to 230F and climbing! I need cfm more air at idle. I already have the small pulley on the water pump, cold case radiator, shroud, and all the panels. I need more idle air CFM.

Essentially i need to overdrive the mechanical fan at idle and then reduce it at speed. I'm thinking like a variable clutch pulley, small diameter at idle, grows as rpm rises...

[Mike Fowke]

For comparison: I have two Spal 12" fans rated at 1,640 cfm each, a custom made shroud, and a Cold Case aluminum radiator that's about the same size as that Flex-A-Lite set up. I also have a 180 degree high flow thermostat and a Derale controller. The stock cast water pump impeller has been clearanced to the backing plate. I don't have AC. I haven't seen anything north of 180 degrees since going with this set up, and we live in similar climates.

[JLMounce]

Originally Posted by Stripes

Ok, but my factory 7 blade and severe duty clutch does great(190) without AC on our 469, but with AC at idle for 5 minutes, temps take to 230F and climbing! I need cfm more air at idle. I already have the small pulley on the water pump, cold case radiator, shroud, and all the panels. I need more idle air CFM.

Essentially i need to overdrive the mechanical fan at idle and then reduce it at speed. I'm thinking like a variable clutch pulley, small diameter at idle, grows as rpm rises...

Some thoughts.

You added another heat exchanger either in front of or behind the radiator, adding a restriction. This likely inhibits the fan's ability to pull and move air.

You've also added another relatively hefty load device. The load change at idle could change the fueling and timing requirements which can have an effect on keeping the engine cool.

A solution is still probably a fan that can pull/push more air but you might be able to aid things by addressing any tuning difference that may have occurred now that the engine is seeing a higher idle load at times.

This is really where some of these injection systems start to shine because they have rpm bumps for ac and since fueling is based on load, it keeps things in check when the AC is and isn't running. Much harder to do with a carburetor.

[i82much]

i have a 10:1 IA2 505, e-heads, stock size aluminum radiator in a non-HD core support in my 65 GTO. Flex-a-lite dual 12 inch fans, draws 22 amps total, seems to keep the car pretty cool. You don't really need the CFM when the car is moving, it is idle where you are sitting still and no air is moving through the radiator that causes you problems. So for my part I like the electric fans. Took some doing to get them set up right but now I am pretty happy.

Now, for a towing application pulling 4000 RPM up a steep hill, different story. A big mechanical fan is going to blow away an electrical fan under those circumstances.

[FrankieT/A]

Originally Posted by Stripes

The Flex-A-Lite Direct Fit Electric Fans 105398 claims to move 4,600 CFM. Unless they have VERY long noses, this well exceeds a stock fan at idle. Has anyone run this set up and on what?

I just ordered this fan about a week and a half ago. My size is on backorder and is supposed to ship mid September. The reason I ordered this particular fan is based solely on CFM (4600 cfm)and physical size. I'll report back on fit and results.

[RocktimusPryme]

Originally Posted by JLMounce

I mean, it's possible my math is off, this isn't anywhere near my area of study and I'm just using a basic formula for an axial fan.

19.5" fan = 1.625 ft diameter
2.25" pitch, so each revolution should moves 3.65625 inch of air in a column
* 750 rpm = 2742.1875 / 12 = 228.51 ft per minute column of air.
3.1416 * 1.625 to get our area * 228.51 ft per minute in the column = 1166.57 cfm

that's just a free flow rating and doesn't take into effect shrouding, anything in front of the fan or if the fan is a flex type etc. So as Steve C has pointed out, this figure could be all together meaningless. My formula could also be wrong.

Certainly seems legit.

I have a cold case, a 19.5" factory fan with a shroud. Mine is dead on the 180* thermostat when moving. But I went though a slow drive through the other day, ended up at 225, and was still going up.

I bought a couple of water pump gaskets. Going to pop off the pump and check the clearance to the spacer plate. The builder put the pump on so I dont know what it looks like.

I also bought a shorter HD clutch to try and put the fan more correctly into the shroud. 1/2 in 1/2 out. RIght now its basically all the way inside.

But if none of those things work, I dont know what else to do but go electric. Ive avoided it because of the cost, and not wanting to deal with the wiring. I hate being scared of being caught in stop n go traffic. I also want to be able to keep it cool in the staging lanes without constantly turning it off.

[i82much]

another nice thing with electric is that you can install override switch(es) so you can manually turn on the fans whenever you want. so, say you know hit traffic on a hot day. turn the fans on as soon as you run into traffic, instead of waiting for your mechanical fan clutch to engage.

[JLMounce]

Originally Posted by i82much

another nice thing with electric is that you can install override switch(es) so you can manually turn on the fans whenever you want. so, say you know hit traffic on a hot day. turn the fans on as soon as you run into traffic, instead of waiting for your mechanical fan clutch to engage.

This works with AC as well. If you look at instructions for kits like vintage air and you look at how the OEM's handle it, regardless of whether the engine is in need of cooling, if the AC is running, the fan needs to be running. The AC kits usually come with an override as such.

With a fixed fan it's not an issue. It can become an issue with a clutch type fan though as you mention.

[Stripes]

Originally Posted by Mike Fowke

For comparison: I have two Spal 12" fans rated at 1,640 cfm each, a custom made shroud, and a Cold Case aluminum radiator that's about the same size as that Flex-A-Lite set up. I also have a 180 degree high flow thermostat and a Derale controller. The stock cast water pump impeller has been clearanced to the backing plate. I don't have AC. I haven't seen anything north of 180 degrees since going with this set up, and we live in similar climates.

I had no issues before the heat load of the AC was added. And its still ok when the car is moving... Can your car idle endlessly without overheating in 100F weather? Can the fans keep up?

[Stripes]

Originally Posted by RocktimusPryme

Certainly seems legit.

I have a cold case, a 19.5" factory fan with a shroud. Mine is dead on the 180* thermostat when moving. But I went though a slow drive through the other day, ended up at 225, and was still going up.

I bought a couple of water pump gaskets. Going to pop off the pump and check the clearance to the spacer plate. The builder put the pump on so I dont know what it looks like.

I also bought a shorter HD clutch to try and put the fan more correctly into the shroud. 1/2 in 1/2 out. RIght now its basically all the way inside.

But if none of those things work, I dont know what else to do but go electric. Ive avoided it because of the cost, and not wanting to deal with the wiring. I hate being scared of being caught in stop n go traffic. I also want to be able to keep it cool in the staging lanes without constantly turning it off.

i'm right there with you. I need more air and if it takes a helicopter mounted to the back of the radiator, I'm in if it can keep up. Biggest fear is mounting a pair or one and finding out it does LESS than the mechanical at idle...

[Formulabruce]

Few points.
GM DESIGNED the Shroud to be efficient and use the Least horsepower.
The "cone" shape ( Note No sharp corners) increases the volume of air at
Rpm velocity.. placement of the fan in the shroud is critical.
In 1970 Pontiac added a top air deflector to AC Cars on Firebirds and others to help pull only
Out front air into the radiator and condenser. Making sure your shroud is tight at front and can't draw any warm air
From engine compartment.
I always recommend at 160 or 170 "Static " T~Stat.
Oddly enough the boys from Road Kill did a video on how much power is lost
For different fan set ups. The STOCK set up with clutch fan used the LEAST Hp.
For big engines, its also a better set up than electric fans with minimal to flat fan shrouds
Air and fluid does NOT like sharp edges or corners.
New fuel runs hotter than fuel in the 70's due to more calories, but is less dense, so we burn more.
If you need more cooling than a stock set up with clean radiator I suppose a electric fan out front of the consensor
Might be an option. I would try T stat first ,and be certain your gauge is accurate. Back in the 70's at the Dealer
I worked we had some customers bring in cars for too hot issues, and their gauge was off. Pontiac said that
Plus or minus 5% is acceptable as the gauges are just a general glance at systems.
Of course a IR gun pointed at the T stat housing tells all.
My second class for ASE was cooling and heating. The older instructor has a huge photo on his office
Wall which showed GM engineers standing around a car with a shroud that had probably 25 vacuum gauges
Hooked to it in various spots. Pic was from 1960 I was told. Later in the 60s the fan shroud was tested in small wind tunnels.
~~~rant off.. good luck with cooling!
PS. the fan/hp test video from roadkill was moved to the hotrod pay site for the non belivers.

[PunchT37]

Originally Posted by Stripes

Ok, but my factory 7 blade and severe duty clutch does great(190) without AC on our 469, but with AC at idle for 5 minutes, temps take to 230F and climbing! I need cfm more air at idle. I already have the small pulley on the water pump, cold case radiator, shroud, and all the panels. I need more idle air CFM.

Essentially i need to overdrive the mechanical fan at idle and then reduce it at speed. I'm thinking like a variable clutch pulley, small diameter at idle, grows as rpm rises...

My new engine did the same thing with a 180 stat. Edel heads, flowkooler pump and cold case. 19.5, 7 blade fan. I put a 160 stat in. Problem has calmed down. My ac has 134 in it. I believe that stuff gives off more heat than 12. Still, ac add more heat than none. But, those cars had ac when new. Mine runs 170 to 175 with ac off, and 185 to 190 with ac on. I am in south Louisiana. So...