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OK, I'm new to Saabs, turbos and this site so please forgive my lack of knowledge. I am running into terminology that I need a quick explanation of.
Can anyone give me a one sentence explanation of what the following items do:-
- actuator
- dump-valve
- "forge re-circulating"
Thanks in advance
 

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G'day sport.

Actuator: Thing that operates the waste gate on the turbo to control the boost pressure.

Dump valve: Valve that dumps the inlet pressure when you suddenly shut the throttle. Drive with the window open and you will hear a "psssh" sound as you close the throttle. They come in two sorts, atmospheric dump valves that dump the excess air in the intake system to atmosphere. These are the noisy things used in WRC cars and by strange people in modified Subarus and the like. Recirculating dump valves dump the excess air back into the system somewhere (not sure exactly). Forge is the name of a manufacturer of such equipment.
 

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actuator - turbo waste-gate control device.

dump-valve - closed throttle pressure release to atmosphere.

forge recirculating- closed throttle pressure release to the low pressure side of the turbo. Forge is actualy a brand name, not work done to a metal.

I'll leave the details to someone with more prolixity, like Adrian, as it's too early in the morning for me to think about.
 

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Depends on what you're after, really.

Atmospheric will give you a louder 'whoosh' noise if you want to impress the Scooby brigade, but will also confuse your electronics if you have Trionic 7 or above.

Recirculating should, in theory, give quicker spool-up because, as Robert says, it 'dumps' the excess pressure back on the low-pressure side of the turbo.

hth

James
 

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On a pre-trionic Saab you need a recirculating valve, as an atmospheric one will mess up the metering.

On a trionic Saab you have the choice between the two. The standard type fitted is recirculating, so if you fit an atmospheric on you will need a blanking plate (nice alloy ones are avaliable!)

As far as I can tell there are two main differences. The recirc type is quieter (is that a pro or a con? You decide!) and in theory gives you faster spool up times on the turbo, as the high pressure is "dumped" into the intake system before the turbine compressor. This means when you back of the throttle, e.g. to change gear, and then accellerate again, this pressure wave will help spin the turbo up again.

The atmos type makes a great/annoying* noise

* Delete as applicable
 
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