thats a neat engine to have in a R31... anyway. the stock turbo can go to 1 bar max. which is about 14.87psi or somewhere abouts. the cheapest way to do it is to buy a bleed valve of some sort, the turbosmart ones are good. but if you wanted to do it properly i'd go for a electronic boost controller. more accurate and better for adjusting the boost. might cost a bit more but you can do adjustments inside the car. with a front mount you could run 12psi to 14psi all day every day.