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I don't know how hot a muffler gets but I do know that when I installed a turbo-charger on my Nissan my EGT (Exhaust Gas Temperature) reading was between 1200 degrees and 1600 F. It was at 1600 under full boost and over 100mph. The reading was taken right outside the engine on the "header". I imagine the temperature would cool significantly by the time it reached the muffler. I'd guess that it'd be no hotter than 1200 degrees at the muffler to be safe.