Also, on my NA 4cyl when I ditched the stock cast manifold(small diameter runners) and replaced it with a wrapped tubular manifold I lost some torque under 1,500 rpm and gained noticeable power above 2,500 rpm. No other changes

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment