Removing all the slag and imperfections from the intake will make a noticeable difference but you do not want to go too smooth since this will impair the fuel atomization causing fuel to puddle in the head. I like to swirl the intake with 80 grit. This will break up the fuel molecules for a better mix with the air. Fuel in a vapor state mixes very well with air but does not mix well in liquid form which can look to be a rich mixture. Neither the intake manifold or intake runner of the head should ever be smooth or polished.
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