All turbocharger installations incorporate some form of boost limitation; otherwise, boost would rise with load until the engine destroyed itself.
Turbocharger geometry, sometimes abetted by inlet restrictions and designed-in exhaust backpressure, limit boost on constant-speed engines. Automotive and light truck engines have surplus turbocharging capability for boost at part throttle. These applications employ a wastegate—a kind of flap valve—that automatically opens at a preset level of boost to shunt exhaust gas around the turbine.
Most wastegates are controlled by a diaphragm, open to the atmosphere on one side and to manifold pressure on the other (Fig. 9-7). The Ford unit shown also incorporates a relief valve. Normally the wastegate opens at 10.7 psi; should it fail to do so, the relief valve opens at 14 psi and, because it is quite noisy, alerts the driver to the overboost condition.
Other wastegates are spring-loaded and usually include an adjustment, appropriately known as the “horsepower screw.” As usually configured, tightening the screw increases available boost. Therefore, exercise restraint.
Test wastegate operation by loading the engine while monitoring rpm and manifold pressure. If the installation does not include a boost gauge, connect a 0–20 psi pressure gauge at any point downstream of the compressor. The diaphragm-sensing line (on units so-equipped) serves as a convenient gauge point. Note, however, that the gauge must be connected with a tee fitting to keep the wastegate functional. High gear acceleration from 2500 rpm or so should generate sufficient load to open the gate.
The control philosophy discussed in the preceding paragraphs implies that the turbocharger is used for power enhancement. When mid-range torque is the object, the wastegate opens early, at the engine speed corresponding to peak torque output. Most of these applications employ computer-controlled wastegates, whose response is conditioned by manifold pressure, engine rpm, coolant temperature, and other variables. No generalized test procedure has been developed for these devices.