I have a friend that coats his snowmobile expansion chambers and swears by it, claiming lower underhood temps and performance gains - no formal dyno runs but increased fuel delivery under similar conditions (he's had to increase his fuel to keep exhaust temps down as scavenging is more effective)

With cars, the sensitivity is going to be different than a two stroke, of course, but there are reasonably creditable companies that have tested this with positive results. The thermodynamics alone would say you would keep underhood temps down as well as retain heat in the exhaust, but header temps go up also which could give thermal growth or fatigue concerns dependant on the application. As K-Man says, the exact benefits in these specific circumstances would be interesting to quantify and measure. Seek out the advice of one of the engine tuners on this topic would be my advice.

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GT Silver CS
1980 Honda CBX
2004 Suzuki SV1000
2006 Subaru Legacy
2002 Honda Insight