Engine Cooling - Fan Size Dia vs. CFM

BeasleyRB · #1 (**permalink**) 09-25-2007, 07:52 PM

New car(just finished my B&B), new engine (351W, mild cam, alum heads, etc), runs 200 to 205 or so on the highway at 90 degree ambient. A little cooler in the 75 degree evenings. Stop at a few lights and the temp goes over 225.. Scary. Get moving and it comes back down but not fast enough for me. Have water wetter in it. I have a 16 inch Summit fan that is rated at 2,100 cfm, puller. Heavy wire so current to motor is good. The fan is not shrouded and I am improving the "ducting" in the front end to force more air through the radiator. (That would help at speed but not at 6 stop lights in a small town all timed to hold a Cobra up.) It is fall now and somewhat driveable so it won't be driveable in the hot Oklahoma summer. I've been looking for a higher CFM 16 inch fan but noticed some 15 inch fans that move close to 2,800 CFM. Would the higher velocity 15 inch fan be more effective than the lower 16 inch one. I know cooling one of these things down is as much an art as a science but it can be done. I am hoping to short cut some pain and benefit from your experiences. Thanks for any advice.

496fe · #2 (**permalink**) 09-25-2007, 08:46 PM

CFM seems to be whatever the manufacturer decides it is. There are no standards or tests that I'm aware of. I have found that the single most important factor in cooling is HEAT TRANSFER, not water volume or flow or pressure (sorry for shouting). The two best ways to draw heat out of a radiator are by 1) increasing the air flow through the radiator and 2) increasing the area of the cooling surface. The easiest way to accomplish these is by making sure that all available air flows t-h-r-o-u-g-h the radiator, not around or worse yet, over.

I would look first to shrouding the fan and sealing the radiator to the body. I speak from experience - a 608-HP 496-inch FE BB in a B&B that has never seen 205 degrees in southern Cal in stop-and-go in the summer

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Brian
(notice the correct spelling of Brian, not like they spell it in OK

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SantaFe66 · #3 (**permalink**) 09-25-2007, 10:59 PM

Brian,

So how does the shrouding help if you're stuck in light after light? I'm amazed that your obviously "hot" engine doesn't overheat in traffic. Mine isn't extensively modified but still overheats, just like the author of this thread. Mine is a 390 with two 11" SPAL puller fans. Granted, mine isn't shrouded either but there must be more to this than just that. I've also tried water wetter; no help.

Sorry to piggyback on this thread but it sounds like we have the same problem and I find it hard to believe that just shrouding will fix it since city driving is what makes it worse where there's little air movement other than what the fans generate.

Thanks for any help you can offer both of us.

Paul

eschaider · #4 (**permalink**) 09-26-2007, 12:51 AM

Check out the Lincoln MK VIII fans. They are 18" dia and 4500-5000 CFM. Best of all they are about $35 at your local salvage yard.

Ed

496fe · #5 (**permalink**) 09-26-2007, 08:21 AM

The concept behind fans is to lower pressure on one side of the radiator so air will flow from high pressure (front of radiator) to low pressure (engine side). The shrouding makes this process much more efficient. If it is not shrouded, the air can just blow in circles in the engine compartment.

I like the Lincoln fan, although I doubt it moves as much air as claimed. Moving 4,500-5,000 CFM would require probably around 40-45 Amps

. The Spal 17-inch is enough, which pulls around 22 amps.

Brian

BeasleyRB · #6 (**permalink**) 09-26-2007, 02:53 PM

Thanks for the input. I commented originally it was as much as Art as Science to get one of these things to run at temperature now I think I have to add Magic! Seriously, I know there is science at work here but it sure seems you have to chase this issue for most cars until you get it resolved. Brian, I'm glad your engine is not overheating. I will seal the radiator well and look at fabricating a shround unless one of you B&B guys know what shroud/fan combo works with the room allowed and where I can buy one. (Brian, is yours the standard brass radiator B&B provides?)
I noticed David Kirkham said they do not shround their radiators so air will flow through them at speed and not be hampered by a fan or shroud that might slow airflow. They do seal them off however.
Thanks for the thoughts and I would welcome any other advice.

Bob

496fe · #7 (**permalink**) 09-26-2007, 04:04 PM

Bob,

I am using one of the aluminum radiators with a custom shroud. A local fabricator made it. Kirkham is correct... if you do not get caught at a light

. Many of the fan/shroud combinations have trap doors that allow air through at speed and close when stopped, giving you the best of both worlds. I think the Viper fans are like this.

Brian

eschaider · #8 (**permalink**) 10-01-2007, 02:15 PM

If you choose to use the Mark VIII fan and you get the later '98 version it does have a significant appetite for power. The starting amps can exceed 100 and if you run it on high depending on how closely packed the cooling fins are and the restriction the radiator provides you will have a steady state appetite for 38 - 50 amps. Needless to say wiring needs to be up to the job.

Ed

Barnsnake · #9 (**permalink**) 10-02-2007, 09:50 PM

Before buying another fan or fully ducting the radiator, you should check the basics.
I would verify that the thermostat opens at the expected temperature, and that it opens fully.
I would also verify the temperature gauge readings. If the gauge is reading 20 degrees high you could go nuts trying to fix it with air flow.