What is a safe RPM limit for an engine driven fan?

 

Been to 5grand in my 68 BB.
Marshal

 

I went 6000-6100 rpms more times then I can count with the stock 7 blade clutch fan and severe duty clutch.... Never had any issues. Remember the 427/435hp cars had 6500 red lines with a stock clutch fan.

I now run an electric fan for a 15 rwhp gain though...(which I confirmed on the chasis dyno)

 

Switched to an electric fan after buying a new one and replacing it only to watch this. An easy 20 hp gain for the cost of an electric fan seems like a good deal and a better cooling choice


The best choice is none of the above, go electric, best choice and better cooling

 

on the other hand the aftermarket flex fans have been known to fly through hoods

 

Usually in pieces.

And they can penetrate surprisingly heavy old sheet metal hoods, not just Corvette Fiberglass ones.

On the topic here, the stock fan blades (inspected and verified to be in good condition) on a stock(ish) fan clutch draw a lot more air through the radiator than any electric fan setup.

One place I'd disagree with other posters, for a Corvette, or for any engine that sees a lot of high RPM operation, I'd go with a "standard duty" fan clutch, and avoid "severe duty" fan clutches. "Standard duty clutches engage at about 60 to 70 percent of shaft speed, and when disengaged, tend to be around 15 to 30 percent of shaft speed. "Severe duty" fan clutches engage to around 80 to 90 percent of shaft speed, and run a little faster when disengaged as well. On an engine running high RPM's, you want less engagement. That will reduce the horsepower lost to the fan, and it will reduce the speed of the fan blades, reducing stress there.

"Severe duty" clutches are ideal for towing trailers or hauling heavy loads, but not really the best idea for "race car" engines.

 

I had a flex fan break on my 70 340 Duster and it made quite a dent at 130 mph @ 5300 rpm. Scared the crap out of me

 

for once I am glad a common sense answer comes into play when it comes to cooling threads. and as I said before there is a reason GM still uses clutch fans today

 

Yep. My 88 P/U sent one into the hood seconds after I closed it.Cut the rad hose clean in two.

 

I still use my stock 7-blade clutch/fan. Been over 6000 rpm lots of times.No problems so far and keeps my engine cool. It does blow a massive amount of air!

 

See in red... but I concur with a lot of your statement, just not on EVERY car.

 

never thought about it or worried.
now throwing a belt, yup.
few times.
hit 7k many times.

 

never thought about it or worried.
now throwing a belt, yup.
few times.
hit 7k many times.

 

Yep. The added bonus no one seems to talk about is the A/C. It blows stone cold in bumper to bumper traffic. It would never do that before.

 

That is pretty common, especially on cars with A/C in hot summer weather. My preferred solution to that is to add a supplemental pusher electric fan to the front of the AC condenser. Run that when the compressor is running, or when engine coolant temp exceeds a threshold. I keep the mechanical clutch driven fan for cruising.

That was a stock setup on Mercedes Benz cars with mechanical fans from the mid 1970's through the 1990's or early 2000's.

On older vehicles use a coolant temp sensor switch which cuts on around 210 (maybe higher) and cuts off above thermostat temp, but 10 to 20 degrees below the cut on temperature. It's important to cut it off if the AC is off and the engine coolant temp comes down to "normal operating" temp, otherwise it runs forever after it cuts on.

 

you have a sweet setup.
done good

 

Thank you all for the feed back. I currently run dual 12” with a custom stainless shroud behind a DeWitts radiator. I don’t really like the amount of pull the fans have and felt the stock fan and shroud set up pulled much more air. Only went electric as I installed a 383 stroker that pulled 500hp with 468 tq on the dyno stand with 6200 rpm. This is a street driven 78. I don’t hit the big rpm often I’m running aluminum heads.

It runs cool as it is however last July I had a long wait in line getting into a local auto meet and it got hot. Only time.

I like the idea of a pusher fan out front for the AC. Sounds like the plan for me.

I’m going to reinstall the stock fan and shroud and see how it does.

Thanks again for all the supporting input.

 

Thank you Rick.

 

12 inch Spal fans with curved blades move ~1,300CFM each. The straight bladed "high performance" Spal fans move ~1,700CFM each. With lesser brands and "eBay specials," it goes down hill from there.

I don't have the numbers for any C3 mechanical fan assemblies, but the S10 4.3L V6 (a 350 V8 with two holes missing) mechanical fan theoretically can move over 9,000 CFM at 5,000 RPM engine speed. In actual use, they top out around 6,000 CFM at about 3,000 RPM on the tach, and by that point even a "severe duty" fan clutch almost always starts to disengage, even under heavy loads and high temperatures. I would expect that the C3 mechanical fans are in the same range.

Using a shrouded setup in front of the A/C condenser also works just fine. My 560SEL has a shrouded dual pusher fan on the A/C condenser from the factory, and it never overheats with that in combination with a mechanical fan and clutch. It seems like it would block too much air, but in actual practice it doesn't. It actually reduces aero drag slightly, and it still allows sufficient air through when the engine is running and the mechanical fan is spinning. Actually, on race cars, I usually aim for an opening in front of the radiator at most 1/4 the area of the radiator core (less, often much less for circle track cars on high speed ovals), and that's without a fan at all, but the vehicle is usually moving at all times. Most electric fans or fan sets you'd use on the condenser are bigger than that.

Oh, and I'm probably stating the obvious and preaching to the choir, but fan clutches should be replaced when the water pump is replaced. They wear out (at similar rates), and those two parts (water pump and fan clutch) are pretty much a unit replacement when either one wears out.

 

Members would be amazed at how much an add-on Pusher fan improves the efficiency of a Puller fan (without blocking grill airflow)