What is a Diverter Valve?

Turbocharged NG900s and OG9-3s have a Bosche diverter valve fitted to the engine which regulates the turbo power. At idle, your valve is open from vacuum pressure from the engine. When you press the throttle, the diverter valve slowly closes. When fully closed, turbo boost is allowed to be delivered to your engine. When you lift the throttle, the boost pressure is released from the valve into your airbox through a pipe for the air to be used again.

What is a Dump Valve or Blow off Valve (BOV)?

A dump valve is exactly the same as a diverter valve, only the dump valve doesn't feed air back into the airbox through a tube, it releases it into the atmostphere (engine bay), hence the phrase atmostpheric 'dump' valve. They are much louder and a 'whoosh' or 'pssst' noise is heard on throttle release. There is no difference in performance.

Do I need a new Dump Valve/Diverter Valve?

The standard Bosche valve is good for 15PSI so it isn't essential to replace them if you have a midly tuned engine. Some stock valves become defective over time and start to 'hoot' and you will find loss of boost and sluggish throttle response. Unless you are running higher boost PSI due to engine tuning or a manual boost controller, you probably don't need to use an upgraded valve.

What cars can use the Diverter Valve?

All NG900 and OG9-3 Saab's with a turbo can swap their existing valve for a upgraded diverter valve. 

What cars can use a Dump Valve/BOV?

T5 engined cars (1994 - 1999) with a red DI cassette can use a Dump Valve. T7 Engine cars with a black DI Cassette (2000-2003) cannot.