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..."optimum" pipe diameter depends upon "where & how" you want that optimization to occur & do:
1) If you want the optimization to enhance torque and fuel economy, then you want smaller pipe diameter to coincide with engine speed where maximum torque is desired, roughly 3000-4000 rpm.
2) Conversely, if you want the optization to ehance HP (and FE be damned) then you want larger pipe diameter to coincide with engine speed where maximum HP is achieved, typically just beyond OEM HP-peak but below engine redline RPM.
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