I know this is probably stupid to revive this incredibly old thread, but I found this query kind of interesting.
First off, I agree with Tan 14psi = doubled hp over natural aspiration
Baluch on the other hand...thats an overgeneralization and psi/hp is relative to the original performance of an engine.
Here's the theory.
Assuming tuning is done correctly, you get a stoichiometric mix of O2 and fuel igniting in the combustion chamber. We can establish that we have a set ratio of oxygen and fuel and it should never change. So if we dump in twice as much gas then we MUST have twice as much oxygen for proper combustion.
On planet Earth, there is a constant air pressure, 14.7 psi at sea level (this is the base air pressure in the combustion for all non turbo vehicles). So without turbo on a car, people can honestly say they are running their cars slightly over 14psi (relative to the vacuum of space anyways).
We indirectly measure amount of gas particles with pressure, so if we turbo our cars to lets say 14.7 psi (on top of the 14.7 psi the Earth already gives us [totalling 29.4psi]) then we effectively have twice as much oxygen before ignition. Therefore the amt of fuel being pumped in is doubled, then at ignition we should theoretically get doubled the explosive power. So yeah, as Tan said if you do a 14 psi boost then you've nearly doubled the amount of oxygen in your turboed engine for double the power.
Of course you lose potential power the higher you go because you encounter more resistance in the change of inertia of the internal components.
So if your NA car makes about 100 hp
-Boosting it to 14.7 psi will theoretically give you 200hp
-Boosting it to 7.35 psi will theoretically give you 150hp and so on
[ (HP of NA car) x (1 + (boost psi/14.7)) = HP at boost psi ]
As for Baluch, his numbers are close enough but not for all engines. If you want an easy psi to hp conversion just divide your NA hp by 14.7 and that's it.
-If your Suby is 147 hp NATURALLY ASPIRATED stock then, sure, 1psi boost = 10 hp (147/14.7)
-But for a car that is 240 hp NA stock, a turbo would yield an equivalency of 1psi = 16.3hp
To put it more accurately for Baluch, every 1 psi will get you approx 6.8% extra hp over a naturally aspirated setup (again, theoretical).
Cheers