[Half-Inch Stud]

bobzdar, moor-daddy, I stand by my definition of heat radiation as light.
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Radiation: The rate at which an object radiates energy has been found to be proportional to the fourth power of its absolute temperature T, and the surface of the object.

DQ/Dt = esAT4
where s is a universal constant called the Stefan-Boltzmann constant which has a value of 6.67x10-8 Wm-2K-4; and e is the emissivity, a characteristic of the material, it takes values between 0 and 1.

Any object not only emits energy by radiation, but it also absorbs energy radiated by other bodies. The net radiant heat flow from the object at T1 to an environment at T2 is then given by the equation

DQ/Dt = esA(T14-T24)

Prevost law of exchange states, that in a state of thermal equilibrium the amount of energy radiated per second from an object, is equal to the energy absorbed by it, in the form of radiation from surrounding objects.

Thus a good emitter is a good absorber.

An ideal absorber, called a black body, absorbs all the energy incident on it. Its emissivity is equal to unity, e = 1. The human body is a very good approximation to a black body, e = 0.95.

Radiative heat transfer can take place through a vacuum, unlike conduction and convection processes. It is found experimentally that the wavelength corresponding to maximum energy radiation decreases with temperature.

This relation is known as Wien displacement law

l at max energy = B/T

where proportionality constant B is equal to 2.9x10-3Km. Earth’s surface (~300K) emits mainly infrared radiation.
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HIS

[Half-Inch Stud]

Review:

Ya want hotter: install a higher thermostat.
Ya want cooler: paint the block, compartment and hood BLACK. Or up the cooling system.
Which one of these makes more power? hotter.

You want more power per bang: review the conditions inside the combustion chamber.
We all run our engines up to the onset of ping. Then we back off somewhere:
Octane, Timing, pedal, compression, mixture, cam dynamics, coatings, gear match, oh and external engine temperaure. A cooler engine allows more timing advance - (How much more? ) because it has lower CYL pressures.

Some may turn up the radio or turn down their hearing aid, yet ping still robs power (new topic huh) HIS

[gearbanger]

I personally had a 389 that ran a little warm. It was GM blue. I pulled it out and painted it black for a change, and I never noticed any difference in temperature. It may have changed a degree or two, but I guarantee it didn't change 20 degrees.

Does anyone else have any information to support the theory that a black engine will run cooler?

[Nickv]

Interesting discussion.

My take. Very simple and to the point.

In addition to black being a good absorber of heat, it is also a good radiator of heat. That means black objects don't store heat as well as their white counterparts. They give off more heat easily to the surrounding area. Which in turn helps the object that is painted black cool down. This is why infrared telescopes are painted black and the reason the SR71 Blackbird is painted black.

And that is why my engine in my 67 is painted black. Does it help? Maybe. I do know that my 67 which is a low 11 second street car never runs hotter than 185 in the Summertime with stock radiator and a flex fan, ever.

[Northern Goat]

WOW. Over 2000 views and 100 replies in 5 days. I don't know if I was more confused before or after I posted this question. I guess there is a reason this didn't make sense to me.

[Dragncar]

If it was true all big time NASCAR engines would be black,the could not afford to be any other color.

[bobzdar]

Unpainted should shed the most heat due to one less layer insulating the engine. The reason you don't see shiny heatsinks is a surface area issue, not one of color. A shiny surface has a lot less surface area than a rough surface, so it willl not work anywhere near as well as a heatsink. Most passive (read: no fan or anything blowing on it) heatsinks used in computers on the parts that put out the most heat (cpu and gpu) are unpainted. The less important ones may be annodized but are not always black, though that's probably the cheapest color.

[Pitch 400]

I have to go with Dragncar. Nascar racers will spend $1,000's to find 5 HP. That's how competive they are. As far as a SR-71 is concerned, what color would you like it to be, yellow, pink,or green. Those would be some real searius colors for a 90'000 foot spy plane! Black has allways been the stealth color of choice- SR-71, B-1 bomber, B-2 bomber, F-117 fighter. Those planes used special black paint because of its low radar signature. AKA, less heat loss for less radar reflectivity. So that pretty blows the idea of black paint transferring more BTUs. Pitch

[Motor Daddy]

HIS, a wise man once said, and I quote

"It is never polite to wrap complex math around a simple concept" HIS 2005

Let me make this as simple as possible as to my understanding.

Radiation is the vehicle that transports the heat, not the heat itself! The heat itself is ENERGY being transported!

[oxfordsteve]

I like the idea of making it simple. My collegue next door tells her 9th grade physical science students that conduction of heat occurs between materials in direct contact with each other while convection of heat occurs through air flowing around the object and radiation occurs when an object transfers energy across space to another object.

My field of expertise is Biology so I will not pretend to be an expert on physics and I certainly do not know as much about thermal transfer as some members do. Sometimes physics seems to defy common sense but here is my tiny bit of common sense with my small amount of physical science knowledge:

Engine conducts majority of heat transfer to coolant through conduction, most of the little left through convection due to airflow around. Radiation only becomes significant after engine is shut off and sits there with no coolant flow or airflow. In that case if it was considered a "perfect black body" then it might be a slightly better emitter than other colors while transferring heat. (I have serious doubts about the black body status as a coat of rustoleum may not qualify it but I yield due to lack of specific expertise in that area.)

I just don't see that any radiation from the surface of the block significantly affects cooling compared to say flow of coolant regardless of color.

If I am wrong I am sorry, I am not speaking as an expert, just offering an opinion. I am open minded if someone can offer definitive proof otherwise.

[Half-Inch Stud]

motor-daddy, ya got me - I was not being polite.

After 6 pages of interesting energy balance discussion, Just seems we can now focus on the combustion chamber:

"Boy that black engine paint made my combustion chambers 3degrees cooler to the touch compared to my PMD Blue, I wonder on the HP and ping? any help??" HIS

[Motor Daddy]

Do I believe that painting an engine black would show a visible decrease in temperatures, or somehow increase HP? Not for a second!

Since the power production of an engine is due to the release of heat from gasoline (in this case), it seems logical that the more heat conservation in the engine, the more heat that goes into making power. That's all fine and dandy, but a million other variables come into play. Increased temperatures also cause metal to expand, which means tighter clearances, which means more friction, and heat lost to it, (lost power). Like Jim pointed out earlier, heated intake air is less dense, which causes lower VE (lost power). Higher cylinder temps cause the air/fuel mix to spontaniously combust BTDC, causing too high of a pressure to early while the piston is still on the compression stroke after the spark has ignited a seperate part of the air/fuel mix (detonation, lost power).

Does an engine produce more power when hotter?
Does an engine that is painted black run cooler? If it does, does that mean it looses power?

I would never paint my Pontiac engine black. I like the looks of a Pontiac engine painted Pontiac blue, and would be willing to sacrafice some HP to keep it that way!