[TR] Push or pull

Dave1massey at cs.com Dave1massey at cs.com
Wed Oct 24 06:47:16 MDT 2007


In a message dated 10/24/2007 6:26:41 AM Central Daylight Time, 
DLylis at aol.com writes: 
> The electric has a more  *efficient* blade 
> configuration, but this is where I think  the electric is superior.  The 
> stock fan 
> is limited to the air  movement by the rotation of the engine at idle.  
> Let's 
> say 800 rpm  whereas the electric is 1800 rpm.  Will I not get better 
> results 
> from the  electric for the same diameter fan considering the 25% reduction 
> in 
> the blade  area?  Am I just plain dumb for not putting on both? 

Engineering is a series of tradeoff's.  Designing the fan for an automobile 
radiator is fraught with many variables, the two most problematic are the 
variable engine speed and variable prevailing wind (vehicle speed).  The cheapest 
route is to stick some blades on the end of the crankshaft (or water pump) and 
you're done.  The downside to this is that if you size the fan to be adequate 
at idle in hot weather when stopped it is dead weight any time you are moving 
or the engine is above idle.  Electric fans are the most expensive but give 
the best result.  They can be sized to provide sufficient cooling at worst case 
conditions and the switched off when no needed.  But electric fans cost more, 
require additional wiring and frequently require an upsized alternator.

So, the question is: "Should I use both?"  The answer is, as it frequently is 
in this business, "It depends."  It depends on your climate.  It depends on 
your driving conditions.  If you drive in hot weather in traffic that can leave 
you idling and stopped for periods at a time you may need both.  But if all 
your driving is out in the country side on weekends, you probably don't need 
both.

If you do both be sure that you offset the location to minimize overlap.  I 
mounted an electric fan on my TR6 as a pusher leaving the engine driven fan in 
place.  Since the original fan services the lower 2/3's of the radiator the 
upper 1/3 doesn't get any air movement when the car is stopped.  I mounted the 
electric fan to service the upper 2/3's so that when both fans are operating I 
get airflow across the entire face of the radiator.

Dave   


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