In a message dated 10/24/2007 6:26:41 AM Central Daylight Time,
DLylis@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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