^^^ Lol the internal diamater is the same but the throttle plate takes up less space at WOT which will increase the maximum poitential of air flow and it smooths out the casting imprefections giving you a more laminar flow. matching the throttle body opening to the intake piping will also help reduce turbulance at the transition from the intake piping to the throttle body. Very small increases but every thing you do on the d series counts