The boost is the PSI. More boost equals more horsepower. Safe level will be 8-12 psi depending on how good an intercooler you have. You must have enough pump and injectors to feed that psi, though. rule of thumb is every pound of boost increase adds 10% more horsepower to the motor. Detonation (pinging, knocking, valves rattling, etc) will be the sign of too much boost. Excessive detonation will also be the sign of a trashed motor, so be very careful when turning up the boost. Do it a pound at a time and get a knock detector. get a good boost gauge while you are at it, the stock one sucks.