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Tue Jan 27, 2015 11:47 am

I've been thinking a lot about getting my exhaust temps down. Nothing wrong with it, I just want to step it down a notch.

I know that different colors radiate heat differently, and from what I recall white does not radiate heat well. So my hypothesis is that an exhaust that is ceramic coated white will insulate heat while a black ceramic coat will radiate heat.

Anyone have input on this? Only article I found was an article from Zirotec

“The Ferrari 360 was suffering from high under bonnet
temperatures,” says Ward. “This was affecting
performance as heat from the exhaust was increasing
the air temperature inside the airbox. Applying the
coating reduced under bonnet temperatures by 50ºC, a
significant reduction that lowered air intake
temperatures and extracted better performance from
the engine.”

Tue Jan 27, 2015 12:07 pm

A flat black surface will radiate heat better than a white surface. the thick coating also adds an insulated layer. the ceramic coatings are typically thicker than exhaust paint, so the coating insulates better than paint of the same color and shine. A light color like white will help hold the heat in better than flat black a thicker coating will help get the heat in also. however there are limits on how thick you can do a ceramic coating before thermal expansion differences between the steel pipe and the ceramic will cause separation.

exhaust wrap can probably do more to help, however there are issues with keeping too much heat in the pipes, that can damage the pipes, and the wrap also tends to hold water, so there is a greater rust potential.

Get a color of ceramic coating that you also can find in a rattle can of high temp exhaust (or BBQ) paint. sure ceramic coating's are good stuff, however they still will chip, and flake off eventually, I find myself touching up the ceramic coating on my exhaust systems periodically with a spray can of flat black BBQ paint (my ceramic coating is flat black)

good luck

Tue Jan 27, 2015 12:28 pm

I run the silver ceramic simply because I like the look of it. All the ceramics keep the heat in the exhaust and does not radiate the heat into the engine compartment like any painted exhaust will.

Tue Jan 27, 2015 1:57 pm

pink and orange just look funny.as does yeller.mI did a system with titanium I didnt think I would like the look, but it turned out to look nice.(for somebody else) Ive only used the aluminum both shiney and polished look they were both in the experamential stages.neither lasted long. well they did last longer than the factory ceramic coatine on the header when I got it from scat,that lasted a month before rust specs started showing up it wasent long befoe it was nasty.I should probably try the black as it last longer and works better. this coating has been on there for about 5 years now.header is 13 years old.

Tue Jan 27, 2015 2:00 pm

bluebus86 wrote: A flat black surface will radiate heat better than a white surface. the thick coating also adds an insulated layer. the ceramic coatings are typically thicker than exhaust paint, so the coating insulates better than paint of the same color and shine. A light color like white will help hold the heat in better than flat black a thicker coating will help get the heat in also. however there are limits on how thick you can do a ceramic coating before thermal expansion differences between the steel pipe and the ceramic will cause separation.

exhaust wrap can probably do more to help, however there are issues with keeping too much heat in the pipes, that can damage the pipes, and the wrap also tends to hold water, so there is a greater rust potential.

Get a color of ceramic coating that you also can find in a rattle can of high temp exhaust (or BBQ) paint. sure ceramic coating's are good stuff, however they still will chip, and flake off eventually, I find myself touching up the ceramic coating on my exhaust systems periodically with a spray can of flat black BBQ paint (my ceramic coating is flat black)

good luck

Yes and no. In fact....white versus black components "may" not radiate any better or worse than black components.

I work with these issues on a daily basis with coatings of all type and on all materials and have done considerable research over the past three decades.

Whether one surface "radiates" better than another (all external variables like surface airflow aside for the moment) has less to do with the color and everything to do with what material the coating is made of.
In that respect most black and white surfaces are equal especially in the high temp coating field. Aside from ceramic fillers, black will be largely carbon black pigment and white will be largely titanium, some calcium and one or more types of talc. These materials have largely the same heat banking and radiating characteristics....largely built around inner voids and porosity. There are some special ceramics made with special clay structures (similar to theaterial used in shuttle tiles....which is surprisingly cheap to aquire in powder form).....that has better/altered radiating qualities.

However...it is fact that a black surface will absorb conducted, convected and radiated heat faster and in much larger quantities than a white surface will. Along the same lines....a gloss surface in either white or black absorbs less heat while a matt surface absorbs more. Large surface texture is another order of magnitude of heat absorption.

This is also where many misinterpret what is happening ...assuming that one coating has faster heat shedding properties....while in reality they are observing the fact that the black surface is actually absorbing heat from other local sources. ...when in reality the heat "shedding" characteristics of the black and white surfaces are fairly similar.

With controled heat source experiments we have done, this is the first mistaken observation we made. All test surfaces had the same amount of cooling airflow at the same temp, humidity and atmospheric pressure. The white surfaces always cooled faster. As a later control we removed cooling air and left the test surfaces to cool in the enclosure. They cooled at nearly the same rate....with the white surface cooling knly a few percent faster. Spot checks showed that this was largely because the black surface was busy absorbing all of the heat from its enclosure.

In this case.....If you are trying to pull heat out of the exhaust stream....make the metal black. Ray

Tue Jan 27, 2015 4:41 pm

^^^ what he said.

Your thinking too much. Color does not make a damn bit of difference. Paint it or coat it any color you want. Color may make a difference how fast it cools down after the engine has been shut off.

And it also doesn't make a damn bit of difference what color you paint your engine tin. The tin only directs airflow, it does not radiate heat.

Tue Jan 27, 2015 5:15 pm

Since all of the exhaust is separate from the carb intake and the cooling fan intake, nothing you do to the exhaust will make a difference like referenced with the Ferrari 360.

If all tin and cooling seals are in place, there is no "under hood temps" to be concerned about. The upper engine bay will be ambient air temp.

Tue Jan 27, 2015 6:27 pm

raygreenwood wrote: If you are trying to pull heat out of the exhaust stream....make the metal black. Ray

That's Good News to Me. I waiting on a Black Ceramic coated A-1 sidewinder with a V-Band to be delivered. :twisted:

In addition to heat dissipation, I just didn't want some big chrome exhaust system sticking out from under the car and drawing attention.

Tue Jan 27, 2015 6:38 pm

This stuff kicks butt:

http://www.rustoleum.com/product-catalog/consumer-brands/auto/specialty-paints/high-heat

I don't know about heat dissipation, but if you bake it on properly, it is really durable so far on my turbo car.

Tue Jan 27, 2015 7:05 pm

raygreenwood wrote: bluebus86 wrote: A flat black surface will radiate heat better than a white surface. the thick coating also adds an insulated layer. the ceramic coatings are typically thicker than exhaust paint, so the coating insulates better than paint of the same color and shine. A light color like white will help hold the heat in better than flat black a thicker coating will help get the heat in also. however there are limits on how thick you can do a ceramic coating before thermal expansion differences between the steel pipe and the ceramic will cause separation.

exhaust wrap can probably do more to help, however there are issues with keeping too much heat in the pipes, that can damage the pipes, and the wrap also tends to hold water, so there is a greater rust potential.

Get a color of ceramic coating that you also can find in a rattle can of high temp exhaust (or BBQ) paint. sure ceramic coating's are good stuff, however they still will chip, and flake off eventually, I find myself touching up the ceramic coating on my exhaust systems periodically with a spray can of flat black BBQ paint (my ceramic coating is flat black)

good luck

Yes and no. In fact....white versus black components "may" not radiate any better or worse than black components.

I work with these issues on a daily basis with coatings of all type and on all materials and have done considerable research over the past three decades.

Whether one surface "radiates" better than another (all external variables like surface airflow aside for the moment) has less to do with the color and everything to do with what material the coating is made of.
In that respect most black and white surfaces are equal especially in the high temp coating field. Aside from ceramic fillers, black will be largely carbon black pigment and white will be largely titanium, some calcium and one or more types of talc. These materials have largely the same heat banking and radiating characteristics....largely built around inner voids and porosity. There are some special ceramics made with special clay structures (similar to theaterial used in shuttle tiles....which is surprisingly cheap to aquire in powder form).....that has better/altered radiating qualities.

However...it is fact that a black surface will absorb conducted, convected and radiated heat faster and in much larger quantities than a white surface will. Along the same lines....a gloss surface in either white or black absorbs less heat while a matt surface absorbs more. Large surface texture is another order of magnitude of heat absorption.

This is also where many misinterpret what is happening ...assuming that one coating has faster heat shedding properties....while in reality they are observing the fact that the black surface is actually absorbing heat from other local sources. ...when in reality the heat "shedding" characteristics of the black and white surfaces are fairly similar.

With controled heat source experiments we have done, this is the first mistaken observation we made. All test surfaces had the same amount of cooling airflow at the same temp, humidity and atmospheric pressure. The white surfaces always cooled faster. As a later control we removed cooling air and left the test surfaces to cool in the enclosure. They cooled at nearly the same rate....with the white surface cooling knly a few percent faster. Spot checks showed that this was largely because the black surface was busy absorbing all of the heat from its enclosure.

In this case.....If you are trying to pull heat out of the exhaust stream....make the metal black. Ray

So what in the quotation you cite is the "No" part of the "Yes and No"???????? :?: :?:

I was under the impression the poster was trying to keep heat in the pipe?

I did my pipes flat black ceramic to reject heat under the car. Coating guy told me there flat black coating radiates best of the finishes they offer, been doing many other parts flat black with rattle cans for same reason, heat rejection, even some engine parts can benefit from this treatment (BBQ paint or Exhaust paint).

Tue Jan 27, 2015 7:20 pm

hmm, so I cannot make the pipes cooler while the engine is running by means of changing color.

Shields and wrap achieve that, I guess.

Tue Jan 27, 2015 7:38 pm

Brian wrote: hmm, so I cannot make the pipes cooler while the engine is running by means of changing color.

Shields and wrap achieve that, I guess.

Wrap will not make the pipe cooler, it will keep the heat in and make the pipe hotter. A thin layer of coating, of a flat sheen, black will likely be the best combination to keep the pipe cool. air flow over the pipe can make a difference also. which pipe is are you concerned with?

Flat Black is also nice cause it hides dirt and grime well.

Tue Jan 27, 2015 8:13 pm

bluebus86 wrote:
Wrap will not make the pipe cooler, it will keep the heat in and make the pipe hotter.

That's what I'm getting at. Not actually getting the pipes cooler by eliminating heat, just moving the heat.

My apron gets hot, not too hot though. I just want to see if there is something I can do about it. I think I'm going to put a shield in.

Tue Jan 27, 2015 8:27 pm

Wrapping the pipes will shield the apron from the heat.

But it will hold moisture on the pipe and rust the pipe away at an accelerated rate.

Tue Jan 27, 2015 8:35 pm

I've seen the horror pictures

Tue Jan 27, 2015 8:49 pm

Eaallred wrote: Wrapping the pipes will shield the apron from the heat.

But it will hold moisture on the pipe and rust the pipe away at an accelerated rate.

Accelerated like X5.

My brother had a Jet Hot coated exhaust manifold, downpipe and exhaust pipe on an auto-x Scirocco. We had to adjust the shift linkage during an event and and right after runs. By the time we got the car lifted and on jackstands, the exhaust was cool enough to not burn hands and arms. It was all coated inside and out and performed wonderfully. A good ceramic coating will keep heat in the exhaust to enhance flow and keep temps under control.

I had a header on my last Rabbit that was a good quality header, I wrapped it, and less than a year later, it was trashed.

Tue Jan 27, 2015 10:27 pm

Hmmm... Couldn't resist this one.

Color can make a difference in an object's radiation, hence heat loss. But not any color you can see. Most of us see things in the region below 1 micron. (Typically the 0.4-0.7 micron region is called "visible light".)

For visible color to make much of a difference for thermal emission, the surface has to be pretty toasty - more toasty than anything you want on your car... (The sun's peak emission is in the visible, tungsten bulbs peak outside the visible. 6000K in the curve is ~solar emission, tungsten bulbs would be about 3000K. Things glow red in the range of 1000K or so.) Not very meaningful, for what we typically work with around here.

It's strangely difficult to find a good rendition of the Planck function (which governs radiative transfer) on the internet, similar to what we have on decorative wall charts in my industry. Here's one:

Objects can absorb solar radiation in the visible - hence black objects get hotter than white ones in the sun. This is because solar light has a lot of visible content, where "color" & light/dark surfaces matter. But this has nothing to do with their radiation, which is happening at wavelengths you can't see. White objects can be pretty black in the infrared, and vice-versa.

And the "color" isn't necessarily the only thing. The surface finish makes a difference, as has been noted above. "Flat" surfaces can be composed of lots of little "cavities", that trap light - or emit heat - more efficiently than shiny ones... (Subject to lots of other factors.) Why you don't feel heat radiating from shiny clean metal surfaces, until you touch them: they typically have pretty low emissivity in the infrared, compared to most other things. (Interesting experiments can be found here, with your toaster.)

And then the insulation properties of the coatings are another whole dimension in themselves... Everything discussed above is much more straightforward than insulation properties.

And, of course... This is all most meaningful if we all lived and worked in a vacuum, so air conduction and convection, etc., wouldn't matter. But we don't live in a vacuum, so those properties matter.

Anyway... Don't worry about any color you can see, as long as it isn't shiny bare metal. (And for shiny bare metal, it's only the part of the spectrum that you don't see that matters, of course.)

If you really wanted to know about the radiative emission of different surface treatments, you can buy a cheap thermal imager these days, that works in the 8-14 micron region. This is where near-ambient objects radiate most effectively. Put a variety of coatings on a hot plate, and take a pic with the IR camera. Could be fun, and less painful than the experiment with the toaster.

We just refreshed the ceramic coat on our Ghia's exhaust... Looks like silvery metal, but it isn't...

Tue Jan 27, 2015 10:59 pm

I miss my physics classes.

for reference

I'm going to paint a rainbow on my exhaust and see what happens.

Tue Jan 27, 2015 11:18 pm

I like to maintain my exhaust flow heat, even on na engines...

Tue Jan 27, 2015 11:21 pm

e&m_ghia wrote: Hmmm... Couldn't resist this one.

Color can make a difference in an object's radiation, hence heat loss. But not any color you can see. Most of us see things in the region below 1 micron. (Typically the 0.4-0.7 micron region is called "visible light".)

For visible color to make much of a difference for thermal emission, the surface has to be pretty toasty - more toasty than anything you want on your car... (The sun's peak emission is in the visible, tungsten bulbs peak outside the visible. 6000K in the curve is ~solar emission, tungsten bulbs would be about 3000K. Things glow red in the range of 1000K or so.) Not very meaningful, for what we typically work with around here.

It's strangely difficult to find a good rendition of the Planck function (which governs radiative transfer) on the internet, similar to what we have on decorative wall charts in my industry. Here's one:

Objects can absorb solar radiation in the visible - hence black objects get hotter than white ones in the sun. This is because solar light has a lot of visible content, where "color" & light/dark surfaces matter. But this has nothing to do with their radiation, which is happening at wavelengths you can't see. White objects can be pretty black in the infrared, and vice-versa.

And the "color" isn't necessarily the only thing. The surface finish makes a difference, as has been noted above. "Flat" surfaces can be composed of lots of little "cavities", that trap light - or emit heat - more efficiently than shiny ones... (Subject to lots of other factors.) Why you don't feel heat radiating from shiny clean metal surfaces, until you touch them: they typically have pretty low emissivity in the infrared, compared to most other things. (Interesting experiments can be found here, with your toaster.)

And then the insulation properties of the coatings are another whole dimension in themselves... Everything discussed above is much more straightforward than insulation properties.

And, of course... This is all most meaningful if we all lived and worked in a vacuum, so air conduction and convection, etc., wouldn't matter. But we don't live in a vacuum, so those properties matter.

Anyway... Don't worry about any color you can see, as long as it isn't shiny bare metal. (And for shiny bare metal, it's only the part of the spectrum that you don't see that matters, of course.)

If you really wanted to know about the radiative emission of different surface treatments, you can buy a cheap thermal imager these days, that works in the 8-14 micron region. This is where near-ambient objects radiate most effectively. Put a variety of coatings on a hot plate, and take a pic with the IR camera. Could be fun, and less painful than the experiment with the toaster.

We just refreshed the ceramic coat on our Ghia's exhaust... Looks like silvery metal, but it isn't...

One point you missed is that dark objects also absorb infrared wavelengths at a higher rate than light ones.....as well as UV. Not just the "visible" wavelengths.

Also your first diagram is typically used for explaining spectral "color". Whats its missing is a graph section for how much wattage or energy is behind that xkelvin of wavelength you are measuring.

The reason matt surfaces make a difference is not surface area. That is a very minor issue....small numbers because most of that surface area is shaded from direct absorption. The primary mechanism is reflectivity. A very high amount of any wavelength of light above Infrared is reflected away before it can be absorbed.....by a glossy surface regardless of color. But a glossy black compared to a glossy white....the black will still get much hotter.

Black surfaces still get hotter than white even when heated from the side opposite.....through the metal or plastic or stone...whatever your substrate. Black is black all the way through. In this case the surface volume will play into how much it can absorb.

The numbers are much lower in the case of indurect absorption versus light absorption but yes there is a bit of difference in how much pigments like carbons and metals (in the case of high temp paints) heat bank....as compared to calcium or titanium oxides. In that case its purely mineral structure.....and not color. It just happens that carbon as an element absorbs heat a little better.

When you factor in how many ways there are to transfer heat....convection, radiation, conduction.....you will realize that black and white surfaces will absorb heat even when you think they wouldnt.

Yeah....originally I would have thought that the original poster would have been trying to KEEP the heat in his exhaust.....less heating up of surrounding surfaces....higher exhaust gas velocity.......but after reading the way it was put. ....I gathered he was worried about egt.....and there is nothing paint wise you can do about that because the source is constant and hugely larger than any radiator system you coild have via a coating of any kind. Ray