What manages the level of boost on the MK5 GTI?

[SMB]

Wasn't sure if I shoul dpost this here or in the GTI forum, but here goes.

In olden days of turbo's the boost was capped by the integral wastegate on the turbo, being activated by the boost pressure itself. With modern chipping not touching this part it obviously doesn't play such a key part in limiting the boost the car achieves. SO on the Mk5 GTI what limits boost? Is the wastegate activated by the ECU? If you adjust the wastegate linkage, does it impact performance or does the ECU override it all?

The blow off valve, as I see it just stops the turbine from stalling, with backpressure on a closed throttle, and doesn't cap the boost as such. Is this right? thanks

[edition30]

Control pressure is formed from the charge

pressure and intake pressure via the energised

charge pressure control solenoid valve N75. The

control pressure affects the vacuum unit, which

actuates the wastegate valve via a linkage. The

wastegate valve opens a bypass channel to

allow part of the exhaust gases past the turbine

into the exhaust gas system. This control feature

allows the speed of the turbine to be controlled

and the maximum charge pressure can thereby be regulated

If the control features fails, the vacuum

unit is affected directly by the charge

pressure, which imparts pressure

against the spring. The maximum

charge pressure is thereby restricted to

a basic operating charge pressure.

In order to prevent the turbocharger from braking too heavily in overrun and between gear changes, an

electric turbocharger air recirculation valve N249 is installed.

The electric overrun air recirculation control is much more durable than the pneumatic one.

In overrun, the vacuum unit is completely closed. The overrun air recirculation control is open, even

between gear changes.

During overrun, pressure is built up in the compressor housing due to the prevailing charge pressure. This

pressure build-up causes the compressor wheel to brake heavily, which leads to a reduction in the

prevailing charge pressure (turbo drop). To prevent this from happening, the turbocharger air

recirculation valve N249 is opened by an electric servomotor. It opens a bypass channel to pass

compressed air via the compressor wheel back to the suction side of the compressor circuit. This keeps

the turbine at a constant speed. When the throttle valve is opened, the turbocharger air recirculation

valve N249 is closed and charge pressure is immediately available again.

[Tobes49]

The ECU has a set boost map with a set limit either way. Boost either side of this set range will cause fault code and restricted boost (Limp Mode) Postive devation and negative devation are the faults code for boost to high and low.

The wastegate is controlled by boost pressure (might be vacuum if a diesel) in the feed to the wastegate sits a N75 valve (like Edition30 said) this is a controlled air leak that leak boost back into the inlet to make the turbo overboost. The N75 is controlled by a Pulse width modulation signal.