lets not forget heat.
the hotter the gasses are the less dense they are and the less dense the easier they are to move.
thats why you see guys putting blankets on their headers.
also the reason why some of the Ford F- and E- series exhaust systems are heat shielded.
that does 2 things, 1 lowers the ammount of energy radiated to sourrounding objects by the hot pipe, and 2 keeps the pipes hot so the gasses stay hot so they flow better.
