the t25's on the later z31's are set at 4.7 psi, yet still make more hp and tq than the larger t3 units at a higher boost...
the differences between the z31's (84-89) and z32's (90-96) are too many to try and list... the main differences being the dohc vvt heads in the z32 compared to the sohc in the z31, bunch of suspension upgrades.. twin turbo's, etc etc
if you're looking to do any sort of track or auto-x racing with the car, do yourself a favor and wait to find an 88/89 turbo... they were the most updated in terms of suspension and so on (10mm wider track, different style tension rods)
as for turbo upgrades, you can do as rocker said and get someone to rebuild/upgrade the stock t3 turbo (you'll want the t3 model not the t25 for this) or go with a new unit, usually people go with a moderatly sized t3/to4e turbo with an internal wastegate... running an external wastegate will require modification to the exhaust manifolds and could prove to be a pain... a few have done it, but most make completely new manifolds... that too is on
jason's site. it seems that some people use the stock EGR port to mount a wastegate, but that only feeds off of 3 of the cylinders so it's not quite as efficient.
the supporting mods for a new turbo are plenty... starting from the ground up you'll want to go:
CM or custom 3" full exhaust including downpipe
some form of cone filter and 3" pipe to eliminate the resonator.
most people axe the stock mechanical fan for an electric unit, some go with a vertical radiator to make more room for an intercooler/piping/oil cooler (me)
the supporting mods needed for more fuel (stock injectors MAX out at 15psi on the stock turbo) such as injectors, new fuel pressure regulator set at a higher base pressure (usually around 40psi), fuel pump (255lph walbro for the 240sx is popular), some upgrade to new fuel rails at the same time (higher flow, bling factor)
with a modified motor you'll see a BIG gain by going with a 'gutted plenum' upper intake manifold, can be found at
L&P performance or a few other places, but L&P have the best flowing stockish design out there...
something to control the new fuel stuff, some people get reburns done by JWT, but they're expensive and a one time thing that can't be continuously tuned... some better options are:
romulator (using the live edit software)
nistune
SDS (the best out there, usually costs around 1k but WELL worth it)
megasquirt
a few other random ones, you could use an afc, but that's somewhat of a kludge method and is just as expensive as some of the ones mentioned above.
bov or recirculation valve (can't forget that one, it's important)
suspension upgrades: springs/struts is the big one, polyurethane bushings is big too, the gains seen from going poly everything is insane. strut tower bars, thicker anti-sway bars...
wheels/tires: not often mentioned, but sticky rubber makes a HUGE difference everywhere... ligher wheels couldn't hurt either!
this is all off of the top of my head so i probably forgot something... feel free to ask away tho
