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Re: very confused about saab boost control
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Posted by BillJ (more from BillJ) on Mon, 12 Mar 2001 22:55:05 Share Post by Email
In Reply to: very confused about saab boost control, jayh, Mon, 12 Mar 2001 20:33:29
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At low boost, the wastegate works like any other turbochagrer installation. This is what is referred to as base boost. The base boost level is quiet low.

When the APC decides the engine can handle more boost (it has the Air Mass Meter and knock sensor to get information from) it uses the APC (or BPC) solenoid to bleed pressure off the wastegate, fooling the wastegate into thinking the boost pressure is lower and it needs to stay open, therefore increasing the boost pressure. This is safe because the APC has feedback from the engine to know it is not detonating, even though it would be in the "danger zone" according to other boost control systems. If the engine starts to knock, APC will reduce the boost.

If, despite the APC thinking the engine needs more boost and isn't detonating (knocking), the boost pressure exceeds a preset limit, the turbo pressure switch switches off the fuel pump, causing a dramatic deceleration, but protecting the engine against sensor failure (e.g. a malfunctioning knock sensor).

Better to play around with the APC (and still give it a chance to stay in control) that play with a very crude manual boost controller that has no way of knowing when the engine is about to go bang.

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