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== Principles of operation ==
Without a boost controller, air pressure is fed from the charge air (compressed side) of the turbocharger directly to the wastegate actuator via a vacuum hose. This air pressure can come from anywhere on the intake after the turbo, including after the [[throttle body]], though that is less common. This air pressure pushes against the force of a [[spring (device)|spring]] located in the wastegate actuator to allow the wastegate to open and re-direct [[exhaust gas]] so that it does not reach the turbine wheel. In this simple configuration, the spring's springrate and preload determine how much boost pressure the system will achieve. Springs are classified by the boost pressure they typically achieve, such as a "7 [[Pound-force per square inch|psi]] spring" that will allow the turbocharger to reach [[Mechanical equilibrium|equilibrium]] at approximately {{convert|7|psi|bar|abbr=on}}.
One primary problem of this system is the wastegate will start to open well before the actual desired boost pressure is achieved. This negatively affects the threshold of boost onset and also increases turbocharger lag. For instance, a spring rated at 7 psi may allow the wastegate to begin to (but not fully) open at as little as {{convert|3.5|psi|bar|abbr=on}}.
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