Funny you say that - the only other time I've wrapped an exhaust, I did it lower down the exhaust towards the muffler and it rusted out (despite the SoCal climate). I feel relatively safe wrapping the manifold though, if any moisture gets in there it will dry out quickly. I have not heard many people say that wrapping the exhaust exhaust increases longevity though.

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Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission (my only daily driver)

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