by Robert Holstrom
A bleed is when the air system has an alternate route to let air escape so that the device being controlled (wastegate) will see less pressure. This will make the wastegate open fully at, say, 10 PSI instead of 7 PSI. Most bleeds are combined with the stock solenoid bleed.
The standard factory wastegate control system uses a solenoid to create a bleed. The solenoid opens and closes at different rates to create more and less bleed on the wastgate. The more the solenoid is open, the more it bleeds, and the less the wastgate is open, allowing more boost. If the solenoid is open less, the wastgate will see more air pressure, and be furthur opened, allowing less boost. The solenoid is controlled by the computer by feedback it gets from the MAP sensor. If the solenoid is stuck closed, not allowing a bleed, or if you connect the vacuum line directly to the wastgate, the boost level should be about 5 PSI max. So if you only get 5 PSI your solenoid is likely shot.
A G-valve does not let air escape, but holds it back from the wastegate until a certain presure is obtained. Then the valve opens, allowing the air pressure to open the wastegate. There is a bleed which is not the major controlling factor on the wastegate side of the G-valve that allows for the wastgate to close again after the pressure closes the G-valve. If this very small bleed was not on the wastgate side, the wastegate would remain open and you could boost no more than 5 PSI. So maybe some of you consider this small bleed on the wastegate side a bleed that controls the boost? The vacuum line never sees this small bleed.
The owner-installed bleed and stock solenoid bleed always puts some pressure on the wastegate, allowing it to open a little at low boost. This is what causes some of the turbo lag. A G-Valve does not let the wastegate open at all until is sees the set pressure so it has less lag.
Note: Much of this comes from the work of Gus Mahon
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