Vacuum Canisters: Basic Information
In normally aspirated engines, manifold pressure is always below atmospheric pressure, providing a reliable source of vacuum. But manifold pressure is sometimes positive (i.e. above atmospheric) in turbocharged engines so is not reliable.
There are two main functions of the vacuum in the N54:
- The first is to power the brake booster
- The second is to operate the turbo wastegates
The wastegate is connected to a diaphram that is moved by the differential pressure between the atmosphere (nominally 14.7 psi) at one surface and whatever pressure is applied to the other surface. If the DME decides the wastegate needs to be closed to generate boost, it electrically operates the boost solenoids which are just valves between the vacuum source and the wastegate diaphram.
I suspect that vacuum cannisters are present to provide a buffer so that vacuum levels don’t vary too much when loads are suddenly applied. Similar perhaps to including storage tanks on air compressors.
I have read learned multiple sources that removing the canisters did not create problems with the car. People mostly removed them to improve rattle with the turbochargers, likely dur to a canister leak, if not from the cannisters themselves, then from some of the vacuum lines to and/or from them, which were probably replaced or at least shortened when the cannisters were removed. I suppose some more subtle explanation is possible also.
Vacuum Canisters: Location
The location of those canisters in this picture
Vacuum Canisters: What the look like
Vacuum Canisters: DELETE BEFORE AND AFTER
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