Noob here with some questions. (Wastegate, Blow off)

ChipsWithDips · 09-10-2011, 05:05 PM

General wastegate operation:
Wastegates are a valve that allow exhaust gas to bypass the turbine.
Without a wastegate, boost pressure would continually rise until your motor pops or until the overall system just can't flow any more.
When a wastegate is fully closed, all air is flowing through the turbine, your turbo will be spooling up.
A wastegate never directly "restricts" the airflow path into the turbine at all, it only allows a secondary path of less resistance for air to bypass your turbine.

Wastegate sizing:
If a wastegate valve is too small, it will not be able to divert enough air away from the turbine, and your boost pressure will continue to rise as rpms increase. This is known as boost creep.
As long as your wastegate is large enough to avoid boost creep at your desired power levels, there is no reason to go to a larger valve.

Wastegate signal:
The wastegate actuator typically gets it's "signal" from a vacuum line(well, not really vacuum when you are in boost, but that's what they call them) that connects to the intake manifold, though you could technically tap into any point after the compressor.
Tapping in closer to the compressor will result in overall less boost since there is some pressure drop through your intake piping, intercooler, throttlebody. For example if you ran a vacuum line straight from your compressor outlet to the wastegate, the wastegate might "see" 8psi while the pressure in the intake manifold is only 6psi.

Boost control:
Without a boost controller your wastegate spring pressure is the main thing that will determine your psi.
Boost controllers work by intercepting the boost signal before it gets to the wastegate actuator. They can make the wastegate see less pressure than is really in your intake, keeping the wastegate closed longer. They cannot make the wastegate see more pressure than is really there, which is why they can only raise boost and not lower it.
If you plan to run a boost controller, you can get a wastegate spring that is lower than your desired psi. If not then just pick the spring psi you want to run.

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09-10-2011, 05:05 PM	#15 (permalink)
ChipsWithDips Enthusiast Member Join Date: Mar 2011 Location: Austin, TX Posts: 279 Drives: '11 Gunmetal 370Z MT Rep Power: 15	General wastegate operation: Wastegates are a valve that allow exhaust gas to bypass the turbine. Without a wastegate, boost pressure would continually rise until your motor pops or until the overall system just can't flow any more. When a wastegate is fully closed, all air is flowing through the turbine, your turbo will be spooling up. A wastegate never directly "restricts" the airflow path into the turbine at all, it only allows a secondary path of less resistance for air to bypass your turbine. Wastegate sizing: If a wastegate valve is too small, it will not be able to divert enough air away from the turbine, and your boost pressure will continue to rise as rpms increase. This is known as boost creep. As long as your wastegate is large enough to avoid boost creep at your desired power levels, there is no reason to go to a larger valve. Wastegate signal: The wastegate actuator typically gets it's "signal" from a vacuum line(well, not really vacuum when you are in boost, but that's what they call them) that connects to the intake manifold, though you could technically tap into any point after the compressor. Tapping in closer to the compressor will result in overall less boost since there is some pressure drop through your intake piping, intercooler, throttlebody. For example if you ran a vacuum line straight from your compressor outlet to the wastegate, the wastegate might "see" 8psi while the pressure in the intake manifold is only 6psi. Boost control: Without a boost controller your wastegate spring pressure is the main thing that will determine your psi. Boost controllers work by intercepting the boost signal before it gets to the wastegate actuator. They can make the wastegate see less pressure than is really in your intake, keeping the wastegate closed longer. They cannot make the wastegate see more pressure than is really there, which is why they can only raise boost and not lower it. If you plan to run a boost controller, you can get a wastegate spring that is lower than your desired psi. If not then just pick the spring psi you want to run.