TheSamba.com :: View topic - I see an air-cooled engine and I want it painted......black?

Sun Oct 26, 2008 11:53 pm

Can somebody please provide an explanation behind this belief that engine components painted black provide more effective cooling?

Mon Oct 27, 2008 12:37 am

just google it, its a science question and there is hours of read out there on the net
http://www.newton.dep.anl.gov/askasci/phy00/phy00156.htm

Mon Oct 27, 2008 12:57 am

If it did the factory would of painted them.

Ever see a 3 year old engine that was painted? They look like crap.

Mon Oct 27, 2008 1:06 am

Glenn wrote: If it did the factory would of painted them.

Ever see a 3 year old engine that was painted? They look like crap.

Just because the factory did not do something does not mean it does not work.

Mass production means a lot of things that work but cost money dont get done.
(i'm an not saying painting his engine is the best idea)

The factory did not put a 2L engine in a bug either :wink:

Mon Oct 27, 2008 4:21 am

The color black will generally radiate more infra red energy away from the engine. However, paint can also be insulating and negate any gains that the color black will have given you. Anodizing aluminum Black will help radiation without hurting the thermal conductively significantly.

With no air flow there is a significant advantage to the color black, However, once you add any significant air flow, the small bit of extra cooling is insignificant by comparison.

And yes, I've actually done thermal testing that proved this many times.

Polished surfaces will reduce radiation, and are not good for transmitting heat.

I suspect that the reason why many painted cases look like crap after a short while is that the magnesium was not properly prepared and treated before painting. But again, I suspect that most paint will actually have an insulating effect that will raise the engine temperature slightly.

Scott Novak

Mon Oct 27, 2008 5:57 am

Check Robert's Blog: http://bobhooversblog.blogspot.com/2007/07/crankcase-painting.html

Primary reason to paint the magnesium case is to avert corrosion - not for looks.

Flat black has best heat radiance of any color.

Do NOT use BBQ grill or high temp paint - it is an insulator = BAD

Use enamel mixed 50/50 with Naptha or gasoline, or use flat black spraypaint.

If you bake the case in Bob's makeshift cardboard box and tinfoil oven, it will come out bulletproof.

Mine did.

Mon Oct 27, 2008 7:08 am

The problem with painting the case is that it blocks the heat flow radiating from the motor and will raise your oil temps.
Most paints contain metal oxides as pigments and fillers. These metal oxides are insulators and will block the heat flow.
About the only thing that you could apply to magnesium that you can buy over the counter is a chromate conversion coating, These contain metal salts instead of oxides and allow the heat to pass. This is what VW put on the cases in the later years.
Anodizing magnesium will completly block the heat flow and after a short time the anodizing will start to crack and spiderweb due to the heat expansion of the case.

Mon Oct 27, 2008 7:31 am

Have cake and eat it too...
We have always just DUSTED it with thin standard black.
Till it "looks" black to you.
Then stop!

Mon Oct 27, 2008 10:14 am

It's going to be interesting to see how my engine enamel stands up to use, I painted the block Chevy-red ^^

Mon Oct 27, 2008 7:55 pm

cyleriggs wrote: Can somebody please provide an explanation behind this belief that engine components painted black provide more effective cooling?

Most of this black paint theory does not apply to a Beetle. IF your engine deck lid is closed to visible light, then it does not matter which colour you paint the engine or other components, as they do not have a colour in the dark. However, if you've got an open engine bay, such as a dune buggy, then it becomes a factor.

VW painted engine parts black because it was the cheapest method to make things look presentable and also hide dirt or other marks on a new Beetle and also the inevitable stains in a used engine.

Some vehicles eg. wartime Kubels, etc, had green or brown engine components...

Mon Oct 27, 2008 9:50 pm

Quote: IF your engine deck lid is closed to visible light, then it does not matter which colour you paint the engine or other components, as they do not have a colour in the dark.

That's not correct at all. Black radiates heat the best even if it's in the dark and in a vacum (ie space). The light shining on it has nothing to do with it.

Mon Oct 27, 2008 9:56 pm

What about the black paint they use on Harley engines... same theory and all that...

"chrome won't get you home but black will get you back"

Mon Oct 27, 2008 10:15 pm

michael1968 wrote: Quote: IF your engine deck lid is closed to visible light, then it does not matter which colour you paint the engine or other components, as they do not have a colour in the dark.

That's not correct at all. Black radiates heat the best even if it's in the dark and in a vacum (ie space). The light shining on it has nothing to do with it.

Space is not actually dark, so black objects radiate heat in space. However, black paint on an engine case hidden in a closed VW engine bay will not radiate heat any more than any other colour...

Mon Oct 27, 2008 11:03 pm

You mistake the fact that just because something is black that it is a good conductor of heat.
The color black is the best color to either absorb or adsorb heat.
But most paint binders are insulators and will block heat flow. So you will insulate the case by painting it black.

Mon Oct 27, 2008 11:23 pm

Hey, Neil68 you're right, the objects colour doesn't change it's ability to radiate heat if it's in the dark. You learn something new everyday.

The engine case is exposed to the ground (and some light) though.

I still wouldn't bother to paint it, but if you had to black would be the best (same insulation but better emissivity than the other paint colours)
[/quote]

Tue Oct 28, 2008 5:56 am

The color thing???
Be that what it may....

However, those of you that were around and read every hop-up VW
write-up that was available back in the early 70's,
might recall several testing articles that were published back then..
and proven.

Their view on it back then was that it microscopically increased the surface area
up to 15% if done right.

Done right, was "a dusting" of standard flat black on a clean surface.
Black was specified by these guys and they said that if you apply too
much, it becomes a blocker coating instead.

So this adds another viewpoint.
These are my mental notes, and my proof went out with my Archie's...
and Mick Jagger...

Tue Oct 28, 2008 7:59 am

The magnesium or Aluminum case materials are very conductive for heat. They will move a lot of heat untill they start to oxidize. The oxidization does increase the surface area by more than 100% but it is a insulator and conducts heat poorly. So the oil gets hotter. The Idea is to keep the metal from oxidizing in the first place. That is why chromate is used extensivly in aerospace to coat the magnesium. Even tho it is a silver or yellow color, it conducts heat well and prevents the metal from oxidizing. If it was a black color it would conduct heat better but it is not.
Im not saying that the black paint will hurt the heat flow. It really depends on what the baint is made out of. And you just dont know.

Tue Oct 28, 2008 1:48 pm

Jimmy111 wrote: The magnesium or Aluminum case materials are very conductive for heat. They will move a lot of heat untill they start to oxidize. The oxidization does increase the surface area by more than 100% but it is a insulator and conducts heat poorly.
Anodized aluminum does conduct heat very well actually. I think close to 90% of the conductivity of bare aluminum if I remember correctly. Also a very thin layer of anodizing (aluminum oxide) won't significantly affect the heat conduction. You can find the data in the catalog of any heat sink manufacturer.

The color black does NOT affect heat conduction. It radiates infra read radiation better.

Scott Novak

Tue Oct 28, 2008 3:45 pm

This paint color thing is a BS myth. You can paint the engine sheet metal any color you want. It will not make it run any hotter. DO NOT paint the magnesium case or the aluminum heads. These metals radiate heat much better raw.

Tue Oct 28, 2008 4:21 pm

vwracerdave wrote: This paint color thing is a BS myth. You can paint the engine sheet metal any color you want. It will not make it run any hotter. DO NOT paint the magnesium case or the aluminum heads. These metals radiate heat much better raw.
It's not a myth that black radiates more heat than a lighter color. However, painting your sheet metal won't generally have have any significant effect on the temperature of your cylinder heads.

The exception is painting the sheet metal white, where it is near the exhaust, may reduce the heat absorption from radiated heat from the exhaust and might have some effect on reducing your engine temperature. Likewise, painting your exhaust system white may reduce heat radiated into the engine.

Black anodizing your aluminum cylinder heads would indeed make them radiate heat better. But because cylinder heads have a significant amount of air blowing over them, they are not cooled significantly by radiation. They are cooled almost entirely by direct conduction to air.

Even though black paint may radiate more energy, any gains in radiation may be offset by the insulating effect of paint.

Scott Novak