Shroud or not to shroud

[Barrie Robinson <barrie@look.ca>]

I have been told that I must shroud my electric puller fan to get 
engine/engine bay temp down but I am having trouble understanding the 
physics of this.  I know shrouding is always done on new cars so one would 
suspect that they know what they are doing.

If one considers a fan stuck in front of a rad it seems reasonable to 
suppose it pulls air in through the rad in a profile almost exactly as its 
fan blade coverage.   Thus the rad not covered by the fan just has air 
pushed over it by normal car motion.  In other words how can the electric 
fan, humping away pulling air through the rad core directly in front of it, 
effect the flow of the non-covered area?   What must happen is that the 
shroud forces air to be pulled through the non-fan blade covered areas at a 
FASTER rate than if they were left alone.   This points to the need for a 
sufficiently powerful electric fan.  So I suppose I should make a 
shroud.  Does anyone know how to make a shroud for a D&D Fabrications heavy 
duty rad and a Perma-Cool 19114 14" fan?

Regards

Barrie Robinson
barrie@look.ca 
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