you dont.. they are the same TB except the GSR TB has a bigger opening than a d series .. i had a gsr tb on my Z6 back in the day and port matched it to the intake mani .. to get the right flow you will have to port match the D series TB to the gsr intake manifold ( lots of dremel tool work) .. and there you go . d series = 52 mm GSR = 57mm <--- someone correct me but i think im close enough for him to get the point