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Modifying cars doesnt addup the FC, possible?

how true could it be?
and how do someone actually achieve that?
heard someone claims adding performance != adding FC :D
i know new engines are sophisticated that even they hav high engine capacity and power(eg new BMW 330Ci)
but their FC still falls under the quite economical side (that car gives a 11.8km/l)

but im talking about general car modding that usually adds more power
modding engines to give power...smoother ride blah blah...