Ok I know i want T3/T4 (with a T04E 60 trim compressor) but I am not sure what turbine size I need I would like to go with a .48 A/R sense it is smaller and would spool up faster I do not plan on running any more the 15 pounds of boost. Would a .48 A/R make 15 pounds of boost with out danger of surging or will I have to go with a .63 A/R

.48 is real small.. you wont have good top end goos spool up but you top end will suck.. i say go in the .50 range, i think .48 will be to small for a 240sx.. the .60 are good in my view..
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Those are quite low. Garrett makes turbos that small, but I'm not sure what their designation is. A T or GT25 would be in the 60s though, which would be a bit better suited to an SR20 or KA24.