there should be very little difference in the "rev-ability" of 2.5 and 2.1 litre bottom ends.

they both have the same stroke(79mm/2.5 crank) and rod ratio (unless you are building something out of the ordinary with custom length rods and pistons with altered pin height to suit).

cam and turbo selection would cause a greater variance on "rev-ability" than the bore size difference.

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