[CMonakar]

"when you put a boost controller to a car, i know you can change the pressure level of the turbo, that means more Psi right? so this means you can change the boost of the turbo right?"

Correct. The boost controller does not act on the turbo directly though. It regulates the operation of the wastegate, a valve in the exhaust manifold between the motor and the turbo. The more the valve is open the less exhaust pressure is forced through the turbo turbine, the slower the turbine spins, which in turn causes the compressor to spin slower and generate less boost.

"so if i change the boost, this means the HP will increase dramatically? or just a litle bit? how many hp will a EVO8 have if a change the boost like 10%?"

This depends primarily on the size of the turbo compressor. Larger turbos generally produce more hp per psi. Factory turbo systems are balanced to provide good low and top end. As you increase the boost above the factory peak you will move away from the efficiency sweet-spot that the turbo was designed to operate in. As a result, the turbo will begin generating more heat than it does at factory boost levels, reducing the density of the air, in turn reducing HP, and increasing the risk of knock. You will still make more power, but the increase per pound of boost will decrease the further you move from the peak efficiency range of the turbo.

Aside from increasing boost, you may be able to improve your car's performance by using an electronic boost controller to increase the rate that boost rises to the factory limit. This is typically referred to as GAIN. When you step on the gas the wastegate starts out completely closed. As the turbo starts spinning up to speed the wastegate will start regulating boost by progressively opening. Most aftermarket systems use larger solenoids than the oem units to control the wastegate. As a result, they are able to open later without overshooting the target boost value. The benefit of this is that the turbo spools up faster and throws you back in the seat with more force while not significantly increasing the strain on the motor.

Hope this helps.