I may have not previously looked in the right place for information relating to CFM rating for stock OEM viscous coupled fan assembly (running in OEM shroud) - specifically relating to a big block installation.
When 'sitting on the fence' some years back re. installing a twin electric fan assembly it seemed that a typical installation Vette related would 'pull' perhaps 2400 cfm. Looking earlier today at Spall's website it seems that the highest output 15" single fan 'pulls' 3000 or so cfm (but perhaps with a 20 amp current drain?).
I seem to remember some years back being unable to find any GM info on CFM flow of stock viscous coiupled fan - does anyone on here have knowledge of this please.

 

Not sure you can compare them. At idle and low and high vehicle speeds the electric fans are at their maximum flow. The clutch fan will not reach it's maximum flow until the engine reaches it's maximum rpm.

 

Yep. Electrics have basically one speed, and can be easily measured.
Fans with clutches: the speed always varies with rpm, and the clutches always have a certain amount of slip, so they are always less than engine rpm. And the slip varies with temp. There are 3 or 4 different clutches that have different amounts of slip. See Hayden for viscous clutch info.

 

If a single electric fan or dual-fan assembly provides sufficient flow when you need it (at idle, parked), why would you do anything else?

There are zero clutch-driven fans that will give you 3000 CFM at idle. You can control the speed of an electric fan directly if that's too much, you don't have that control with a clutch fan.

 

This bit is some of the most valuable bullet-pointed troubleshooting I've seen:

 

Mechanical fans get such a bad rap these days. And yet all the HD trucks run mechanical. I wonder why?

If you google a little, people have measured car mechanical fans as pulling 5000cfm, and large HD truck ones near 10,000cfm.
Engine Masters tested one without a clutch (?why?) and it sucked up 30 HP when mounted solid.

You will never find an electric fan that can pull that much air.
You will never find an electric motor on a fan that has that much power to it.
No mfgr mounts their metal fans solid.

The thing is mechanical fans freewheel almost all the time, and typically only use a few HP, like 4. The clutch only locks up when you need it.
They rarely spin over say 3000 rpm.
The viscous clutches do go bad eventually and are a little hard to troubleshoot.

They are simple. They rarely break. They are not susceptible to corroded connections, etc. What is not to like? Are you electric fan guys really worried about the 4 HP? Try spinning a big alternator. HP is HP. Moving 3000 CFM requires the same HP no matter where it comes from.

 

Corvettes haven't had clutch driven fans since 1982. NASCAR doesn't use them. Every transverse engine car in the world doesn't use them. Sure, some trucks still use them. Why? The same reason GM does anything. To save money. I'll concede that clutch driven fans are simple and reliable during the warranty period, and are good enough for truck applications. I'll also concede that a LOT of electric fan installations are done wrong, and that gives them a bad name. But just like with Q-Jet rebuilds and poly bushings, the information is out there to do it correctly.

Am I worried about 4 HP? Not really. I am worried about having a spinning mass in my engine compartment while I'm tuning the car, and I'm annoyed that the C3 design requires you to remove the radiator to remove the fan shroud. I don't drag race, and I don't have AC, but if I did either, I could directly control when the fans turn on and off, how fast they run, leave them on for a bit when the engine is shut down, and even disable the alternator at WOT. Can you do that with a clutch fan?

I've pulled the radiator to remove the fan shroud twice. Never again. I now have two surplus fan shrouds, and two surplus clutch fans.

Bonus anecdote: The harmonic damper in my 80 (with electric fans) recently failed. To R&R the damper, I had to remove the air cleaner and the one fan belt. To steal the damper off of my 79, I had to remove the hood, radiator, fan shroud, clutch fan assembly, and the fan belts.

 

Agreed there are 2 sides to every story.
You can add a lot of computer control to an electric fan that you never could with the clutch type, etc. etc.
That's what makes this hobby fun.
I even saw one guy use PWM controllers for his digital fans and made them multiple speed.
It's all cool if that is what you want to do.
And agreed C3 fan shrouds are a PITA.
My point is neither is better than the other.... just different pluses & minuses.
The old analog and mechanical stuff is kind of like carburetors, a lot of people have never even learned how to tune/repair them.
It's kind of like an almost lost art. I think that is the part I enjoy. I have zero desire to make mine all "modern". But some do.

 

Appreciate your knowledge re. fans and cooling Leigh - have to say I'm with you re. too much 'modern' stuff......too many people use the 'modern' stuff simply because it is 'modern'. Me? I prefer to be selective about what 'modern' stuff I use.
I'm going to ask a similar question relating to brakes shortly.......

 

Yup, as the old saying goes, there's more than one way to skin a cat.
that said, I agree. I too am selective on what I wish to modernise and what not to.
I run C5 electric fans, controlled by my ECM and a trienary switch on my air-con through duel relays.
my Air-con is very much modernised for my comfort in the tropical Queensland heat.
yet I won't change other things like my steering.
It's all personal preference.

 

LOL
Yeah in your situation you better have top-performing A/C!

Everyone has kind of a "theme" for their car.

In my case I am keeping the OEM LS6 Holley Carb. My dyno guy said it had the flatest AFR fuel curve he had ever seen on a Holley! But this was an OEM GM one, not a generic aftermarket, and it had already been tweaked, by GM, for an LS6.
I see no reason to change over to EFI.
But I do have an AFR meter, and I will fine-tune that Holley even more to make that 454 as "efficient" as it can be! 72 emissions specs be danged.
It will never be great, but it will be much better than a richly calibrated aftermarket double-pumper which is optimized for performance only, where cruising is just an afterthought.
But once tuned, I will pack the AFR gauge away so as not to spoil the vintage image.

The same way with my suspension, I will modify the stock components, for as much performance as possible, but unless you measure bar & spring diameters it will be difficult to notice. I have a Tremec O.D., but may leave the 4spd console shift plate intact.

That's my general "theme". Looks stock, but packs a punch! May run low 12s, even high11s, and hopefully get 15mpg+ highway cruising! I got that much in my similarly tuned LT-1 with no O.D. I bet I can beat that.

So as long as I can make the stock fan/shroud work well, it stays!

Then I envision a 3000 mile cross country trip in my future! Bucket list. Check!

Fan Info:
I will put in a new fan clutch when I build it. And 7 blade fan.
If my A/C BBC runs warm on test runs I will just put in a more HD fan clutch. (There are like 4 versions)
Pretty sure I can do that without pulling the rad & shroud.
Best part is, it only kicks in when it is needed.

Story:
Local guys 67 427 coupe was running warm. No radiator seals, etc.
Replaced the 50 yr old fan clutch and problem solved!

 

It's been fun making both cars appear more stock, inside and out. Just don't pop the hood.

A C5 electric fan assembly is a solid choice for an LS6. The 79 will likely get dual Spals, though. They work great in my 80, and the budget DeWitts radiator comes bundled with it.

But again, the electronics portion is part of the hobby, for me. YMMV, and as long as you are getting the cooling you need, either solution works.