Subaru 2002 Impreza Service Manual page 139

TURBOCHARGER SYSTEM
Intake (Induction)
F:
REGULATION OF SUPERCHARGING PRESSURE
1. BASIC FUNCTION OF THE WASTEGATE VALVE
When the engine speed increases as the throttle valve opens, the amount of exhaust gas in-
creases. This increases the speed of the turbine (approx. 20,000 to 150,000 rpm), the supercharg-
ing pressure and the engine output.
If the resultant supercharging pressure is extremely high, however, it may cause knocking and an
excessively high thermal load on such engine components as pistons. In the worst case, the en-
gine may be damaged or broken. To prevent this, the wastegate valve and its controller are pro-
vided. By sensing the supercharging pressure, the wastegate valve controller controls the opera-
tion of the wastegate valve to maintain the supercharging pressure below a predetermined level.
NF0347
(1 ) Wastegate valve
(2) Turbocharger
(3) Wastegate valve controller
While the supercharging pressure is lower than the predetermined level, the wastegate valve is
closed so that entire exhaust gas is directed to the turbine.
NF0348
IN-7