i belive its the d16y8. yeh im really thinking of that now. how easy would it be to get 250 whp out of the gsr without boost?
it would take much more money. you'd have to get the head ported and polished. upgrade cams, springs, retainers. maybe valves/valve guides. the reason is no boost (which is called natural aspiration, or NA) needs lots of flow to really make power. with boost, your just forcing the air in there.
boost a B-series. get a real good tune. an experianced tuner shouldnt be giving your motor too much boost and you'll have a reliable 250whp car for a long time (it all depends on the quality of the tune. ignition timing helps ruin motors)
B16s are fun. You have to ween every ounce of power out of it, and it's all above 4,000 - 7,800 rpm. B16 stock internals respond very well to boost.
B18s have a little more low end. making power from around 3,000 - 7,600rpm. The B18 holds well to boost, but not as good as it's lil brother the B16 from all i've read.
estimate 2,000 for the swap (motor/trans, complete swap kit), 1,000 for turbo (if you can find a good one used, or the turbo system price will go up a lot), exhaust manifold, downpipe, oil lines, charge pipe, front mount intercooler, blow off valve, head gasket, and adjustable cam gears. Then estimate another 500 - 800 for a real good tune. around 4,000 total. lil over estimating.