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Much confusion Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 4 Feb 2004 10:56:50
In Reply to: Blow Off Valve Installation, Marc, Tue, 3 Feb 2004 20:48:56 Members do not see ads below this line. - Help Keep This Site Online - Signup |
There is no Blow Off Valve on the Saab turbo, as I understand a BOV.
A BOV is a valve that vents turbo pressure to atmosphere when the turbo pressure exceeds a certain value. It's a cheap pressure regulator, and can handle the hooter valve issue (see below). But it's a crude method.
The Saab turbo has two pressure regulating methods. SteveC is referring to what is called the 'hooter valve'. This is a valve that connects the input to the output of the turbo, and is controlled by manifold pressure with a third, small hose. The Hooter valve is the black plastic valve with two fat and one thin hose.
If you're running at full boost with the throttle plate open, and then suddenly close the throttle, you've got this big column of air that has no place to go. The pressure builds up fast, and a pressure wave will hit the closed throttle and bounce back to the turbo. This will strain the connections and possibly 'stall' the turbo. One approach is simply to vent this overpressure. The hooter valve opens at this high pressure, and ports the high pressure side back to the input of the turbo. This keeps the turbo spinning, keeps the air moving. This improves throttle response when you hop back on the throttle. It's called a 'hooter' valve because it can make a hooting noise when actuated.
The other regulating mechanism is the wastegate actuator. This adjusts the turbo boost. Boost pressure feeds into the actuator, which is just a piston working against a spring. When the pressure gets high enough, the piston moves, and opens the wastegate on the turbo. This sets the limit on the turbo pressure.
Now, there is the APC (automatic performance control) solenoid, also known as the BPC (Boost Performance Control). This is the solenoid with three hoses mounted on the radiator. What the APC solenoid does is connect the wastegate actuator to the intake manifold. For full boost, the solenoid is energized, and it bleeds some of this pressure off and back into the intake side of the turbo. This means that it takes MORE intake pressure to get the wastegate actuator to move. So you get full boost. If the APC system wants to reduce boost pressure, it de-energizes the solenoid - this closes off that bleed, and all the intake manifold pressure goes to the wastegate. Therefore, less intake pressure makes for more actuator movement, and you get less maximum boost.
The major difference between what is usually called the APC solenoid and the later BPC, is that the APC was on or off. With some BPC systems, the duty cycle of the BPC can set some point between full boost and base boost.
The nice thing about the Saab turbo control method is that it is unobtrusive and efficient. It makes for very nice drivability, and the performance control means you get the most performance the engine can give, over variations in fuel quality, engine condition, etc.
A Blow Off Valve is something folks might do on a performance modded engine. I can't see it being of any value in a stock engine.
posted by 192.249....
, Wed, 4 Feb 2004 10:56:50 <-- Viewing This Message, Tue, 3 Feb 2004 22:07:58, Wed, 4 Feb 2004 08:45:32
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