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Re: Questions about PSI.....
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Posted by Mark J (more from Mark J) on Mon, 18 Mar 2002 17:40:40 Share Post by Email
In Reply to: Questions about PSI....., Gehn, Mon, 18 Mar 2002 15:59:50
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Indeed, the wastegate on the turbo is set to around 5-6 psi from the factory. This is often referred to as "base boost". What allows the higher pressures is the APC system, which variably bleeds pressure from the wastegate line to allow pressure in the turbo housing (intake) to exceed pressure in the wastegate (up to a preset amount). If the APC system senses a problem in the engine (detonation for example) it will close the wastegate bleed, which will thereby open the wastegate and return the system to lower, safer boost. Neat system (when it works) because it allows relatively high boost without the usual risks associated with it.

In stock form, base boost is about half of peak boost, so a 900T with a wastegate set to 5.5 PSI will peak at about 11 PSI with APC. The APC can be tweaked however, to allow much higher peak pressures than stock, hence the numbers you see on the "performance" board.

- Mark
1985 SPG
1984 900T
1978 99T

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