The soleniod valve is controlled by the engine ECU to reduce the amount of air pressure applied to the wastegate actuator, which increases boost pressure. There are other kinds of boost controllers, but that is how the OEM works. The story is longer than that.

Dump valves and bypass valves are used to release the pressure in the intake pipeing when the thottle is closed under boost. This prevents a reverse pressure surge from bouncing off the throttle plate back into the turbo. That would have two negative effects. It can create a shock in the turbo that can be destructive. It also quickly reduces the turbo spin, so when you get back on throttle again, it would take longer to spin up.

The Saabs, as probably every OEM turbo configuration, uses a bypass valve to dump this flow back into the intake system, which will cause a short term duration reverse flow out of the air cleaner as the compressed air expands.

For higher boost modified systems, a dump valve can be used. This dumps the high pressure air to atmosphere. This can make a very loud noise, which for some is the only point.

A bypass valve remains open when running a manifold vacuum. So a bypass valve should never be set up to dump to atmosphere, as unfiltered air will enter the engine. A dump valve will remain closed for negative manifold pressures. Both of these valves have a connection to the manifold. Some blow off valves connect to the high pressure side of the throttle plate. There are various designs and no hard and fast defintions.

A blow off valve, or even a bypass valve that is rigged with a weak spring, and which does not have a hose connection, can act as a pop off valve. Pop off valves are simple pressure relief valves that can relieve spikes or be the protection of last resort. These are also availble purpose build as well.

• Dump valve(long), StanL, Sun, 12 Nov 2000 14:16:54
  • Upgraded software is the only way for a T7 car..., Karl , Mon, 13 Nov 2000 02:22:54
    • "Aero-spec", at least for the 9K, I thought all the, jj, Mon, 13 Nov 2000 09:43:12
      • What about 9-3 2.0 liter?? is LPT same as full turbo? n/m, George D, Mon, 13 Nov 2000 15:05:15
      • Identical for the 9K, different for 9.5, Karl , Mon, 13 Nov 2000 13:38:11
  • Re: Dump valve(long), George D, Sun, 12 Nov 2000 15:57:02
    • Are dump valve and solenoid valve the same?, Eric B, Mon, 13 Nov 2000 17:01:16
      • no... read on, Dean, Mon, 13 Nov 2000 20:29:44 <-- Viewing This Message
        • Re: no... read on, Mathias, Tue, 14 Nov 2000 02:33:22
          • Thank you for all your input. (n/m), Eric B, Thu, 16 Nov 2000 18:49:42
          • Are there any performance differences, AdamB , Tue, 14 Nov 2000 07:00:47
            • Re: Are there any performance differences, Mathias, Tue, 14 Nov 2000 08:32:57
              • Re: Are there any performance differences, dean, Tue, 14 Nov 2000 10:27:52
                • Re: Are there any performance differences, Mathias, Wed, 15 Nov 2000 06:27:02
                  • agreed, Dean, Wed, 15 Nov 2000 09:51:17

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