Need a Physics Lesson – Ceramic Coated Exhausts

[JayTheCPA]

I can rationalize a benefit from ceramic coating insides of exhaust parts as it lessens the gas’ contraction due to cooling (heat soak) and gives more spinning power to the turbo, but I am not connecting the dots on how coating the outside (OD) of the exhaust parts can give more than a small benefit where the most common notation is that it helps reduce the air temp that the engine breathes in the first place (presuming a WAI setup).

Am reading that the more popular coating shops prefer to *not* coat the inside (ID) of the pipes which leaves me with the question of where any true benefit occurs when the manifold and pipes have to then go through heat soaking. Or is it that coating just the outside provides benefit through limiting the heat soak by way of minimizing what radiates out to the atmosphere / engine bay?

Not looking to start a discussion of why any specific shop should change their philosophy of what they recommend / will ceramic coat, just want the physical merits of how ceramic coating just the outside of an exhaust system helps the turbo diesel in terms of noticeable gains.

 

[WarWagon]

Heat has to "go somewhere". So blocking the inside or the outside via ceramic slows down heat transfer via radiation and air cooling. The pipe becomes a heatsink being wrapped or coated on the outside, but, once hot the heat has nowhere to go.

 

[Twisted Steel Performance]

The biggest reason for most shops not doing the inside area is the prep and application cannot be done well enough for the coatings to bond to the base metal. There are some parts I won't coat the inside of, a twin scroll hsg is one of them....

 

[SmithvilleD]

sctrailrider is correct that it's difficult to adequately etch/profile the internal passages to ensure coatings won't delaminate. Also you can't easily spray many internal passages so you wind up "flow coating" them, which amounts to letting surface tension pull/run the material over the surfaces. Difficult to control application thickness.

It's good to have some practical/quantitative sense of the scale of benefit(s) from the thermal barrier properties. The better barriers show an outside skin temperature difference of around 150-200 degrees F cooler than a similar pipe/part that's bare. So if you've got 1600 degree exhaust going thru the pipe, a good external barrier coating will show ~ 150-200 degrees cooler outer skin temp than the same pipe/manifold bare. Of course the barrier properties vary some between products & some are more effective at higher, or lower temps, etc.

In turbo apps, the coating's performance benefit often shows in turbo transient response (how quickly a bigger turbo lights)(measureable with enough datalogger resolution).

The biggest benefit of the common silver (Al powder) ceramic header coatings (folks are most familiar with), is probably how well it prevents corrosion. Look at the 5000 hr salt test results; it's clear corrosion protection is a big benefit as most of the other coatings (often better thermal barriers, but not better corrosion protection) show more like 300-500 hr salt test results.

The thermal barrier coating combination of ID & OD coatings (if the internal sticks well enough to remain) can be additive as the heat energy has to pass through 3 distinct/different materials, as compared to going thru 2 materials with just one coating.

 

[SmithvilleD]

Here's a link to one of the coating company's articles that shows some testing results that compare a few different coating's characteristics.

http://www.cerakotehightemp.com/resources/files/testing/HTTechReport.pdf

www.techlinecoatings.com is another source of good quality products

 

[Twisted Steel Performance]

I used several different internal heat barriers in my build, the most notable was the coating I used in the head, the coating in the runners almost stop heat soak in the head all together. pics can be seen in my album's of pics here. I am a big fan of the right coatings.

I will say that I don't do many turbine hsg here, my customers like the blankets better as do I....

 

[SmithvilleD]

One reason some shops prefer not to coat internal surfaces on a turbine (or exhaust manifold) of turbo-charged engines is the question of any coating delamination particles damaging downstream components like the turbine wheel.

I mentioned the Cermakrome or Chromex ceramic coating's corrosion resistance benefit as they're the coatings I've had hold up long-term appearance-wise. I don't question the coating company's claim that other coatings they make are better thermal barriers. I just haven't found the ones I've tried to be nearly as long-term durable.

The Cermakrome coatings offer some corrosion protection under header wrap/turbine blankets also Wraps & blankets are more effective at controlling heat; just have to be aware they can also accelerate corrosion in applications where the engine does short trips, etc., not running long enough to thoroughly bake out moisture that condenses in there during cool downs.