latest on boost saga

alancordeiro at alancordeiro at
Wed Dec 17 18:04:09 PST 2008


The waste gate frequency valve adjusts the position of the wastegate to 
provide "optimal" boost pressure to the intake manifold.
The way the boost is controlled is by adjusting the percentage of exhaust 
gasses that run through the turbo turbine section, and the balance that 
bypasses the turbine (goes through the wastegate).

In the beginning, the wastegate is closed, all exhaust goes through the 
turbine. As the volume of exhaust gasses builds up (due to higher intake air 
volume, greater throttle opening + higher RPM, not forgetting combustion) 
the turbine is driven faster, and builds up higher boost in the intake 
manifold. The air pressure from the intake is fed to the lower chamber of 
the wastegate diagraphm. The spring will "let go" at around 1.4 bar. To keep 
the exhaust gasses bottled in the turbine section, the ECU controls the 
wastegate frequency valve (control method commonly knows as pulse width 
modulation) to ADD in air pressure on the top of the wastegate, helping the 
spring hold the exhaust in the business stream of the turbo.

At some point the ECU decides to allow the boost to ease off...usually at 
5000 RPM, and it toggles the WGFV more off than on, decreasing boost.
This may also be a strategy used to keep the turbo from over spinning (and 
disintegrating) at higher altitudes when the air is thinner and the turbo 
needs to work harder to deliver the same "optimal" boost.

The ECU will also dump boost earlier if the throttle is not pressed down 
flat to the floor....boost is kept lower for 90% throttle than for WOT.

Hope this makes sense,

(1991 200q that had an instrument lamp (1.2W) across the WGFV for a while)

> help me learn, what is the purpose of the whole WGFV system? to allow the 
> computer to lower boost at high altitude?
> from my understanding, that's the only time the computer lowers boost. it 
> does not lower boost during detonation, correct? only pulls out timing.
> Regards,
> Robby

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