Just a note from personal experience. I had done headers on my 5.9 to replace the ugly log style manifolds and it helped quite a bit as far as MPG goes. I'd watch after the intake, my brute force apparently does let cooler air get to the engine and it hasn't helped my mileage at all, in fact, it is worse.
If you don't put on the metal heat shield you might do better though.
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- Kyle
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