Insulation wrapped on exhaust pipes and coatings that reduce the surface thermal emissivity both raise exhaust pipe temperature.
So what?
At these temperatures, the strength of the 321 stainless steel generally used in exhaust pipes is dropping like a rock - small temperature increases result in big strength decreases - and creep resistance drops even faster.
Oxidation is also increasing dramatically with temperature.
In short, the exhaust pipes will fail much, much sooner due to cracking or wall thinning. If you are driving a turbo, you run a double risk because of the high pressures inside the exhaust pipes upstream of the turbo that stress tubing walls more and create blow torches when cracks form.
In short, don't do it unless you pledge to inspect the pipes more often, like every time the cowl is off, and unwrap the pipes for a very careful inspection at EVERY annual.
Data below. Out of curiosity I made an Excel plot, attached. It makes the point even better than the table below. Creep strength virtually zero at 800C = 1432F
Sorry 'bout that.
Fred Moreno
2. Properties at Elevated Temperatures
Short Time Elevated Temperature Tensile Strength
| Temperature, oC |
600 |
650 |
700 |
750 |
800 |
850 |
| Tensile Strength, MPa |
390 |
329 |
280 |
230 |
190 |
140 |
Creep data
Stress to develop a creep rate of 1% in the indicated time at the indicated temperature.
| Time |
Temperature oC |
550 |
600 |
650 |
700 |
800 |
| 10 000 h |
Stress MPa |
180 |
100 |
70 |
40 |
10 |
| 100 000 h |
Stress MPa |
120 |
80 |
50 |
25 |
5 |
Creep Rupture Stress
| Time |
Temperature oC |
500 |
600 |
650 |
700 |
800 |
| 1 000 h |
Stress MPa |
270 |
180 |
140 |
70 |
30 |
| 10 000 h |
Stress MPa |
240 |
130 |
90 |
50 |
10 |
| 100 000 h |
Stress MPa |
200 |
90 |
50 |
15 |
5 |
Subject: Re: [LML] Re: Ceramic exhaust coatings
One other thought, I have heard both sides of this argument before as well. I also heard it is nearly impossible to weld / repair pipes with this coating. I found a solution that solved the problem cheaply without making the pipes more difficult to weld. I wrapped my entire exhaust system with hot rod exhaust wrap. I bought it at the local auto parts store in the 1" width. Spruce sells it as well. I tested the stuff with a torch in the hangar on my hand and was amazed how well it insulates. I have 2 layers in some very hot places on the pipes. In addition to reducing the inner cowl temps, It also tames the vibration of the pipes and reduces noise.
Cheap, relatively light and effective...
Fwiw.