Doug,
Since I make my living working with cooling systems for 105 and 125F ambient conditions, I can tell you one thing. a fan sitting in space like that does a really good job of making an air donut due to recirculation. you can check this out with cans of aerosol smoke (or a holloween fog machine) The air is brought through the fan, goes around the blades and comes back through the fan - not really making much of an effort to pull through the radiator. Therefor you will get almost the same cooling if you took the fan in the picture and nailed it to a wall. (I ran the Durango for 2 months without a fan - relying solely on highway air flow.) It provides marginal air flow at idle, however is not helping at all above 15-20 mph.
14 gauge is 0.078. that is extremely thick for a fan shroud. It is also the thickness of material that RangeRover uses on their frames. you need nothing thicker than 20 gauge in either aluminum or steel.
the reason why George has cooling when he is moving is because there is airflow when the truck is moving and not when its is idling.
There are many rules of thumb when it comes to radiator cooling systems:
Rule #1 - Fan diameter to radiator face diameter ratio. The Fan diameter should be no less than 85% of the width of the radiator. The bigger the better without overhanging more than 5%
Rule #2 - Fan spacing from radiator should be no less than 1/2 the diameter of the fan hub away.
Rule #3 - Use a box shroud as a minimum, Lip or venturi shroud preferred.
Rule #4 - Fan tip clearance at absolute minimum to prevent recirculation
Rule #5 - Airspeed through the radiator should be between 8 and 15 ft/sec. (it should be able to hold a half empty cereal box on the radiator face just due to air flow)
Rule #6 - Fan speed is dependent on fan profile, number of blades and diameter to get the desired air flow.
Rule #7 - Recirculation is bad.
Rule #8 - The fan is no longer helping above 25mph.
Just as a general comment, automotive systems are not designed to cool 100% of the heat that the engine is capable of producing. This is because they are sized on the average duty cycle for a road going vehicle which is 10% power at idle for 10% of the time, 90% power for acceleration at 5% of the time, etc (each manufacturer has their own duty cycle profile). The other end of the spectrum is Logging equipment and garbage compactors that expect 50% fouling of the radiator at 100% load 100% of the time. The Military falls just short of that, however has the uphill grade requirement (must cool fully loaded at top speed while traveling uphill at X% grade at 125+F ambient)
Assuming that your engine is in good tune (ignition timing and fueling can seriously increase the amount of heat generated at idle when there is no fan air flow), the radiator is not fouled - a simple box fan shroud with a 1" lip around the fan opening will do wonders. If your running a 18" fan, the hole should be between 19" and 19.5" max (assuming soft isomounts allowing between 3/8" and 5/8" movement in each direction)
The box should end at 1/3 of the fan profile, with the lip extending to the 2/3 mark. Simple adhesive backed foam works well to seal gaps between the shroud and the radiator.
Hope this helps.
Scott
