in hot rod magazine i found an article about turbos and stuff and i came across a formula for hp. it seemed to work for my stock z6. try it

Displacement in cubic inches X max rpm / 1728 / 2 X boost in bar X approx effiency = Cubic Feet Minute (of air)
CFM x .07 = LBS of air
LBS x 10 = approx HP

most 4 valve heads = .85 to .90 effiency
most 2 valve v8 heads = .7 to .8 effiency

example---> 98 x 7200 / 1728 / 2 x 1(or 0psi) x .85 = 173.5
173.5 x .07 = 12.15
12.15 x 10 = 121.5 HP

 

Tuning would come in play...