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Old 10-21-2019, 08:33 AM   #1
aussie merc
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Default factory fan cfm

just a curiosity question dose anyone know whats the cfm at what revs for the factory 6 blade mercury fan [ crank driven ]
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Old 10-21-2019, 09:57 AM   #2
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Default Re: factory fan cfm

I can't tell you the CFM, but I can tell you that whatever it is, it is not enough at idle on a hot day. Two problems: The fan turns at crank speed which is substantially lower than belt driven fans, because belt driven fans get "geared up" by having a smaller pulley than the crank pulley that drives them. The other problem is that the crank mounted fans are low and not centered on the radiator. Some cooling is lost because more air is sucked up from under the radiator, rather than through the radiator.
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Old 10-21-2019, 12:17 PM   #3
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Default Re: factory fan cfm

The crank fans for 39 through 41 models that had them were actually relatively well centered. The crank hole was damn near right in the middle of the radiator and the radiator was dropped down especially for them. The ones with the generator fan were less so. Trucks also used 6-blade fans on some applications but most of the older belt drive cars were 4-blade till the 3-blade 8BA type came along.

The cubic feet of movement does depend a lot on rpm. More blades will move more air but take more power to turn and also make more noise. An engineer could come up with some flow dynamic calculations but I don't know if it even matters as long as it pulls some air at idle. Once a car is moving at a good speed, there is plenty of flow going on there.
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Old 10-21-2019, 01:58 PM   #4
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Default Re: factory fan cfm

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Originally Posted by drolston View Post
The fan turns at crank speed which is substantially lower than belt driven fans, because belt driven fans get "geared up" by having a smaller pulley than the crank pulley that drives them.

OOOPS....there goes that theory! This flathead fan, 3-BLADES no less, is belt-driven, ain't "geared-up" in the least, and looking closely at the crank and fan pullies, it almost appears that the fan pully is ever so slightly larger in diameter which would effectively slow it down to even LESS than crank speed. DD


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Old 10-21-2019, 02:36 PM   #5
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Default Re: factory fan cfm

On our 33 ,once on holiday, had a generator fail,(wire fell off inside) and as we were on the motorway,I stopped and fitted the spare ,it was a hot day and to save time I left the fan off, it ran about 10 deg hotter at 50mph .so stopped later and fitted the fan ,that dropped the temp back to normal
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Old 10-21-2019, 03:13 PM   #6
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Default Re: factory fan cfm

CFM is proportional to fan speed given everything else is equal. I would doubt actual CFM figures are available, because to measure CFM, would require a scientific test, meaning a ducted scenario which is obviously not the case in the vehicle.
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Old 10-21-2019, 06:58 PM   #7
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Default Re: factory fan cfm

Quote:
Originally Posted by V8COOPMAN View Post
OOOPS....there goes that theory! This flathead fan, 3-BLADES no less, is belt-driven, ain't "geared-up" in the least, and looking closely at the crank and fan pullies, it almost appears that the fan pully is ever so slightly larger in diameter which would effectively slow it down to even LESS than crank speed. DD


Is this drawing to scale, or just a "generic" rendering?
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Old 10-21-2019, 08:42 PM   #8
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Default Re: factory fan cfm

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Is this drawing to scale, or just a "generic" rendering?

Going by the two included pics, I'd say that the drawing is TO SCALE! DD







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Old 10-21-2019, 08:59 PM   #9
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Default Re: factory fan cfm

Pretty sure the 32 - 38 generator mounted fan was geared up with a smaller diameter pulley than the crank pulley. Have to concede that 42 - 48 are not.

I vaguely remember a discussion on this forum regarding why Ford left the simple crank mounted fan for the dual belt bracket contraption we got after the '41. My recollection is that the reason was cooling. Anybody got the straight story on this change?
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Old 10-22-2019, 03:25 PM   #10
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Default Re: factory fan cfm

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Pretty sure the 32 - 38 generator mounted fan was geared up with a smaller diameter pulley than the crank pulley. Have to concede that 42 - 48 are not.

Hmmm...I'm not so sure about that, either. There's a 1934 Ford document floating-around regarding a change in fan pulley diameter from 4" to 4-5/8" to REDUCE fan speed because of excessive fan noise. You also have to remember that fans don't like excessive speed because they have a tendency to throw blades.....just ask "19Fordy". The two pulleys on this 21-stud appear to be near identical in diameter measurements. DD


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Old 10-22-2019, 04:49 PM   #11
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Default Re: factory fan cfm

I'd like to toss another "wrench" in to all of this. If for no other reason than perhaps someone's curiosity.
I'd recently completed a full (authentic) restoration on a '40 Ford convertible with a factory installed mercury engine. The fan, at first glance, appeared like any other 1940 Ford six blade fan. On closer inspection, most would notice that all six of the blades were "long". This is a fairly rare piece and was available and listed as "extra cooling...".

I know little about the thermal dynamics in regard to our old Fords.
I do know that the engine pans as well as radiator air deflector pans, were there for an engineered reason.
Somewhere around here I have a copy of a drawing from the Benson Ford Research Center that depicts air flow under the hood ('40 Ford) WITH all pans intact.
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Old 10-22-2019, 06:46 PM   #12
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Default Re: factory fan cfm

you ain't wrong that its not that effective at idle everything i learnt in trade school not to do they did how times have changed 1 fan is that low 1/2 the blades hang below the core 2 its 2" away from the core so it just drags air in from the sides no shroud certanly dosent help the cause either just trying to work out what cfm fan will get me close to having some temp control
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Old 10-22-2019, 07:38 PM   #13
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Default Re: factory fan cfm

I did quite a bit of fan design and testing during my career, but it was over 20 years ago. Do you mind if I ask what exactly you are trying to achieve?
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Old 10-23-2019, 02:28 PM   #14
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Default Re: factory fan cfm

cooling is only effective if you can get the balance between air flow and coolant flow correct . since an electric fan runs at a set speed there is no increase in cfm as engine speed/load increases so you need to start towards the high end of air flow requirements and guess what all fans are not equal .Fans of the same size can have a difference of over 100% in airflow
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Old 10-23-2019, 02:37 PM   #15
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Default Re: factory fan cfm

and heres why the concern with cooling just heard the weather report next few days can expect temps of 35 deg C or higher not bad for spring cant wait for summer
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Old 10-23-2019, 03:32 PM   #16
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Default Re: factory fan cfm

A big part of the air flow through the radiator is ram air with any speed. The fan component is more of an issue at low speeds and idle.
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Old 10-23-2019, 04:53 PM   #17
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Default Re: factory fan cfm

Yes, lots of variables at work simultaneously. You can expect a one degree increase in coolant temp for every degree increase in air inlet temp to radiator everything else being equal. Engine cooling systems for military vehicles are typically designed around max engine load (most heat rejected into coolant) at lowest vehicle speed. This is referred to as a drawbar pull test. For passenger vehicles this would equate to climbing a constant grade at Gross Combination Weight Rating with AC system engaged. Not a typical scenario for most. At idle, engine reject heat is minimal, but airflow through the radiator is then entirely dependent on the system cooling fan(s). There is also the issue of removing the hot air out from the engine compartment as quickly and efficiently as possible.
More blades is better, but there are diminishing returns. You could add auxillary electric fan(s) and stage them based on coolant temp. You could also use a thermostatic clutch attached to the fan to reduce unnecessary noise, power draw.
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Old 10-23-2019, 10:48 PM   #18
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Default Re: factory fan cfm

there are several options but its a 39 crank mounted fan twin diameter 6 blade mounted to the drive hub by vulcanized rubber single belt, crab dissy yeh all the good stuff so most options just went out the window best bet shroud plate and electric fan/s just trying to make sure weve got enough airflow as we have a couple of long uphill drags and steep passes we use
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