heh seem like everyone pretty busy at work, i found the answer and sharing here if anyone would like to know.
A wastegate works when you've got your foot in the throttle. A blowoff valves works when you let off the gas.
Wastegates
A wastegate basically just controls how much boost the turbo is going to run. Without any wastegate, the boost would just keep on climbing until the engine blows. What it does is it lets out the excess exhaust energy before it goes into the turbo.... If the wastegate is set at 10 pounds, once the turbocharger reaches 10 pounds, it will dump the rest of the exhaust gas out to maintain that 10 pounds of boost.
Blowoff
What about the blowoff valve? "Another name for a blowoff valve is a compressor bypass valve. What that's used for is a few things. One, when the turbo is boosting, there's pressure in the charge pipe that connects to the intake manifold through the throttle body. On a manual transmission vehicle, when you shift, you close the throttle. When the throttle closes, that pressure has nowhere to go, so ... it bounces back into the turbo, which stops the turbo blades from spinning and causes compressor surge. It tries to put the brake on the turbo.
The blowoff valve opens when the throttle closes. There's a vacuum in the intake manifold, and it uses that vacuum to open the blowoff valve to let the pressure out of the pipe, so the turbo keeps on spooling. If you prevent the compressor surge, it keeps the turbo spinning, so when you step back on the throttle, the boost is right there. It gets rid of the turbo lag when you shift, ... and it protects the turbocharger. When there's compressor surge every time you shift, that will put wear on the turbo."
:mouth_closed: source of information -
http://www.enjoythedrive.com/content/?id=22417
