They may be tuned but they aren't tuned for anything down low. This is why long tubes still make 10-15 hp over them. Remember, a properly tuned header has a proper collector, proper length primaries for the rpm range, and correct cross section or step. Stock manifolds have tiny primary lengths, making them useful for only high rpms since on the street, 30-35" length primaries are ideal. In addition, stock manifolds do not have anything close to a proper collector design since it's way too short and probably doesn't have a proper merge cone in it.
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