I imagine the difference in surface roughness from stainless tube to ceramic coating makes very little difference to the pressure drop of the gases given the obvious turbulent flow in the system.

I can understand the heat retention argument at the headers but my question remains what impact in real terms does that have on bhp. What bang do I get for £500?
I suspect the exhaust gas temp at the tail pipe will be similar if not the same so logically the overall back pressure will be similar. Has anyone measured the difference in actual flowrates, pressure drop & temperature?

I fitted a Librands stainless steel manifold to my T16 engined +4 and the impact was very impressive in mid range torque particularly. It replaced the BL cast iron one so also saved a lot of weight & looked far nicer.
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I'm guessing ceramic coating of a mild steel manifold would help reduce the surface roughness & deliver a similar but lower result.

Stainless stress cracks and also corrodes with chlorides like road salt & not all stainless steels are the same. I agree visually you can't tell AISI304 from AISI320L

JohnV6
2022 CX Plus Four
2025 MG ZS EV aka Trigger