Home made fan shroud.
Just fo the heck of it, I looked up emissivity factors. For radiant heat transfer, the emissivity of black paint is .97 to .99, while the emissivity of dull aluminum is
.20 to.30 . Big Difference! The emissivity of polished aluminum is only.05 .
I found this info on page 26 of Thermal Environmental Engineering by James K. Threshold. Color is important.
People have argued about this for a while, and a few days ago I came across a similar question+answer asking why computer heat sinks were painted black. The answer basically said that at the temperatures they were operating at, the radiative cooling gained by the black paint was negligible. So I tried figuring it out for myself...
The short story is, I don't think the color really matters as far as cooling goes.
According to this page
Heat Transfer: Radiation
The emissivity of black paint is 0.9 and the formula for measuring the energy per unit area radiated away (Q) at temperature T is:
Q = 0.9 * 5.67e-8 * T^4
Just to plug in some numbers, say the entire radiator is 212 degrees F (that's 373 K)
Q = 0.9 * 5.67e-8 * 373^4 = 987 watts per square meter
My '64 Dodge's radiator core is 22x19 inches, or 0.27 square meters, so it radiates
987 * 0.27 = 266 watts of power
Figuring out how much heat energy is given off from the coolant+air going through it takes more guesses, but I think it's reasonable. This page:
Stewart Components
says 189.5 HP of heat, which is 139,377 watts. Or about 500 times as much as is radiated.
Using their examples of 190 degrees F radiator inlet temp and 180 outlet, 100 gallons per minute (6.66 qt/sec) coolant flow rate, and my Dodge taking 20 quarts of coolant, I can guess how they came up with that 189 HP number. At 6.66 qt/sec, it takes 3 seconds to pump all the coolant through the system. The radiator has to cool it all down 10 degrees (5 degrees K) in 3 seconds to keep the inlet temp at 190. 20 quarts of water is about 19 liters, and it releases 4184 joules of energy to cool 1 liter of water one degree K. So to cool all that water:
19 liters * 5 degrees K * 4184 = 397480 joules of energy
power (W) = energy/time = 397480 J/ 3 seconds = 132493 watts = 180 horsepower
Which is close to their estimate.
I'd say the black paint is to keep the brass from tarnishing and turning green, rather than to improve it's cooling ability.