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It would be tough to do a true test as there are too many variables to control. But, I have an IR thermometer that I can measure surface temps with. I suspect that there is likely a temp diffence from left to right anyway, so mounting one coated muffler, and one non-coated muffler would not be sufficient, and two different cars likely would yield different temps as well.
Ncan you tell I'm an engineer?
Since you're an engineer I'm sure you know that the color of the surface you are shooting impacts the output of you IR thermometer. With that being said make sure you're comparing apples to apples in order to make a statement concerning coated mufflers vs. stock mufflers! Different surface colors, different results...