Adjusting Boost (Saab's APC system)

[Kevin Rutledge]

These seems to be some real confusion about Saab's APC system.

Saab started putting APC (Automatic Performance Control) on turbo cars back
around 1981.  Their main intention with this system was simply to give you
as much boost as possible with the grade of fuel you were using at any one
time without damaging your engine. 

On those old 8 valve non-intercooled engines, this was usually around 10
PSI.  When Saab developed the 16 valve engine, they put an intercooler on
it. Then they started using the APC to actually limit the boost level
because these engines with the Bosch LH fuel system can actually boost a
lot higher and the average consumer would tend to land in a ditch pretty
quickly.  

The system limits boost two ways: RPMs and pressure. You actually get a
"slope" or "taper" to the boost levels. The ECU has a maximum level set and
when you get close to that level, it starts reducing boost. This can be
adjusted or defeated if you wish.

The system uses a solenoid valve, which is controlled by a small ECU, to
reduce the boost level at the wastegate. The ECU uses 3 signals to operate:
Ignition pulse (Tach signal), Knock sensor signal and a signal from a
(manifold) pressure sending unit.  It is quite easy to adjust this system
for more boost by tweaking a pot, but you may not want to. These engines
breathe very well, but the fuel system for the US market does deliver
beyond "Normal US variations"  You need more fuel for more boost.

The APC system is independent of the other systems in the car, the only
catch is that you need a (compatible) tachometer signal. (it's just a
standard signal like old points type ignitions have) I know of two people
now who have installed this system on Volvo's and it works great. It
doubled their stock boost levels with NO OTHER mods.

I happen to have a Saab 900T which consistantly delivers 20 PSI of boost.
I have no idea what this works out to in HP, but it's pretty easy to break
the tires loose in 2nd gear (which isn't a good idea with a Saab tranny).
On my APC system i defeated the pressure sender, so there is no limiting
factor other than engine knock.

For more information on APC, see http://www.teleport.com/~bertram

Kevin Rutledge
Formerly a Saab technician
Webmaster for: http://www.ipdusa.com & http://www.saabstory.com