Was browsing and found this:
http://www.funkypower.co.uk/catalogue.php?make=NISSAN&model=200SX&year=S15+SILVIA+TURBO&level1=FORCED+INDUCTION&level2=CHECK+VALVE&level3=&id_product=2027
Its a GReddy Relief Valve..
What do they do, is there a stock one fitted, and if so, where, and are they...