Which surface? Do the bottom of the primaries and collectors reach that temperature? Not trying to argue I'm just wondering if you mean the entire header surface or primaries.

 

The primaries are the main area of concern, but if you wrap them, the exhaust gas temperature is going to stay very high all the way to the point where the wrap stops. The wrap acts as an insulator. The tube metal temps will be very close to the exhaust gas temp.

 

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