I agree that ceramic coating will make a difference. I've recently discovered Zircotec's ThermoHold solution. It's applied with robotic plasma-spray robotic system, baked at 550°C (1,022
°F) and is suitable for applications of up to 1,400
°C (2,550
°F). This results in a thicker 0.3mm (0.012″) textured coating that allegedly has 2x the heat retention of the smooth-finish ceramic coatings that I'm familiar with. I've ordered some sample parts, but I'm not sure I want to ship my headers to the UK. Does anyone have experience with it?
I disagree that stainless steel will transmit even more heat. If you look at the table below, carbon steel has a thermal conductivity (TC) that is 2.8x higher than 304 or 316 stainless steel. The 2.8x won't be fully realized in an exhaust system because stainless steel's TC increases as temperature increases whereas carbon steel's TC decreases. That said the cross over point is around 1,000
°C (1,832
°F) which is pretty much the top end of a turbo application, so stainless steel should have a lower TC across your entire EGT range.
Note that stainless steel's TC is lower than both Inconel and Titanium (I have seen titanium as low as 14.2), but those materials are lighter and more durable. I also assume that their TC performance doesn't drop off the way that stainless steel does.
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If you're serious about heat mitigation, you fabricate in stainless steel and then ceramic coat it. I didn't use stainless steel for looks, I did it because it's significantly more durable and it transmits less heat. I haven't decided which ceramic coating I'll use.
If you decide to go stainless, you'll need to choose which type. Here's what my fabricator recommends:
*304 Stainless Steel: 16 for street and 18 for race
*321 Stainless Steel: 18 for street and 20 for race
The logic is that if you're racing, you'll likely trade off long-term durability to save weight. 321 will cost 30-40% more than 304, but it allows you to go up a gauge (i.e., lighter tube) for similar durability.