shep@datacube.UUCP (03/09/85)
Wow, back on USENET for the first time in a year! All Saab owners who are
looking for the latest information on their Saab's should be subscribing to
"The Saab Club" and their newsletter. Posted here is the unedited version of
the work I did last summer. Funny how cold, New England winters their ultra-low
density altitudes make one forget about intercoolers! Well, spring is on the
way...
For information on "The Saab Club" contact:
The Saab Club
Jeff Delahorne
5805 Oneida St.
Duluth, MN 55804
Plethora of Saab APC learning follows:
Jeff,
I've done a considerable amount of research on the Saab APC system and
thought that club members might be interested in my findings. The simple modif-
ications presented herein take my 83' from 0 to 60 in under 8.5 seconds, does
not in any way impair drivability, and hasn't toasted the engine yet! I expect
further "free" gains once I fit an intercooler and some intelligent enrichment
devices. A terse APC functionality description follows:
APC is a control system which modulates the wastegate position as a func-
tion of engine knocking and intake manifold pressure. The control system is a
"closed loop" servo which will, hopefully, maintain a maximum boost pressure
for a particular load condition. Understanding the component parts is essen-
tial:
The solenoid valve is the control element in the system. Driven by the APC
control unit the solenoid valve regulates the pneumatic pressure that deflects
the wastegate open. The solenoid valve has just two positions; open (non-
energized) and closed (energized). In the open (non-energized) state boost is
minimal, in the other extreem, boost is excessive. To obtain intermediate con-
trol, a technique know as Pulse Width Modulation (PWM) is used. The solenoid is
turned on and off at a rapid rate by a "train" of pulses from the APC control
unit. The ratio of "on" time to "off" time of this pulse train is known as the
"Duty Cycle".
In it's non-energized state the wastegate actuator is exposed to the full
pressure of the intake manifold, thus the wastegate is most easily opened.
This condition is easily simulated by just disconnecting the the electrical
connector to the solenoid valve. Un-actuated, the turbo system should run at
it's 0.30 Bar "basic setting". If you suspect any APC trouble, this is an easy
first measure to try. If, with the solenoid valve un-energized, you do not
solidly maintain the 0.30 Bar basic boost setting under load, you have troubles
in, arrggg, wastegate-land. Check those hoses for leaks. Unless you're in
Denver on a 100 degree day with urine in your gas tank; 0.30 Bars boost is NOT
going to detonate!
When the solenoid valve is in the energized state, very little pressure is
available for deflecting the wastegate open. This condition happens with some
APC electronics failures. So much pressure is available that you will quickly
boost yourself into actuating the over-boost pressure switch that cuts out the
fuel pump.
The knock detector and pressure transducer are the feedback elements for
the APC system. At the onset of engine knocking, or if boost becomes excessive
prior to knocking, APC will reduce the solenoid valve's duty cycle.
The knock detector is simply a strain-gauge like device that develops an
analogue signal proportional to the amount of vibration atop the engine.
The pressure transducer is a passive device which varies resistance with
the pressure applied relative to atmospheric pressure.
The APC control unit receives signals from, among other things, the knock
detector and the pressure transducer. It then generates the pulse width modu-
lated signal to drive the solenoid valve. It is an analogue computer of sorts;
and while it is fine for the casual driver, it poses serious performance limi-
tations to those interested in pushing the engine. Among other things, it can
not remember detonation threshold and the factory "boost retard from detona-
tion" is excessive.
Under optimum conditions (i.e. sea level and cold!) and with high octane
fuel (92 r.m. or better); the boost will tend to limit because of the pressure
transducer sensing excessive pressure, rather than engine knock. Intercooled
engines will act this way most of the time. To exploit this wonderful cool,
dense air, we need to raise the manifold pressure at which APC limits boost
without altering the APC knock/back-off functionality. This is readily done by
an adjustment at the APC control unit. Those wishing not to cut the control
unit calibration seal can perform the same thing by placing a 100 ohm, 1/4 watt
resistor in parallel with the pressure transducer. This raises the limit point
from 0.80 Bar to about 1.40 Bar. In most applications knocking will occur well
before this amount of boost, and will be limited as usual. Up until the mix-
ture begins to detonate, you will be treated with an elevated boost level, and
the associated performance gains.
This new higher operating pressure is in excess of the factory setting of
over-boost pressure switch. This switch cuts in around 0.90 Bar and shuts off
the current to the fuel pump (about 5 degrees into the red zone). The violent
reverse torque on the drivetrain that results from the abrupt fuel cut-off is
arguably worse than any short-term overboost condition. I've changed my over-
boost protection from and active to passive so that it doesn't stop fuel flow
when I need it most. Instead of cutting out the fuel pump, my overboost switch
simply activates a klaxon in the cabin. Beware: Lacking the active protection,
boost pressures can be dangerously high, for as long as you want. While short-
term (< 1 minute) operation above 1.00 Bar is great, you would be foolhardy to
run foot-to-the-floor for extended periods of time.
The performance gains are wonderful. I'm getting 0-60 in 8.4 seconds,
WITHOUT intercooler, at sea-level, on cold mornings, with high octane fuel and
gas additive. Mind you, I'm still knock limited at about 1.20 Bar. An inter-
cooler would allow similar performance without the gas additive.
I've begun work on a microprocessor version of the APC control unit. The
current version is a solid analogue machine. Significant gains can be realized
by digital control. I'm concentrating on the ability to dynamically "learn" the
detonation threshold. The existing APC unit NEEDS to knock to know where to
limit the boost. This happens every time you ramp up into boost-land. A digi-
tal system would be able to better predict where detonation might occur and
keep you on safe side of things. Additionally, it will control mixture enrich-
ment, something I'm just starting to deal with.
I'd like to hear if other club members have tried such faff!
******Follow-up letter:
Jeff,
A few things to append to my letter on APC modifications:
My Saab is an `82, one of the first fitted with APC, NOT an `83 as men-
tioned in the letter. I have 40K miles on the car, the last 10K running with
elevated boost pressures. The vehicle has some substantial modifications, but
the APC is the most horsepower/torque significant. Non-engine related, the IPD
sway bars (front and rear) have been on the car since day one... and they're
wonderful. I often do A-B comparisons against my 900S; the decrease in chassis
roll is just as one would expect.
The best Zero to 60 times, measured yesterday at 72 degrees and sea level,
no intercooler, 93 (rm) octane fuel, no gas additive: Without APC modification:
9.5 seconds. With modified APC pressure retard: 8.2 seconds.
The modifications I'm now running with go beyond simply scaling the pres-
sure transducer error signal. I think I have a minimal component fix for the
"excessive boost-retard on knock" designed into the stock APC control unit. I
don't particularly care about these problems because I plan to replace the con-
trol unit with my own. However, in the mean time I must live with it, and my
fix for the 300 millibar boost retard. I've hand drawn some of the schematic
but it would be infinitely useful if we could get our hands on an APC print
from SAAB. Any chance?
Work continues on a digital, plug-compatible, replacement for the APC con-
trol unit. Said unit would also support some enrichment device.
I hope it remains clear that I'm striving to increase performance without
wasting the engine. Blind increase of boost, especially with the onset of deto-
nation, spells big trouble. In a nutshell, I'm looking for precise
boost/mixture control to some dynamic level which will not detonate, or exceed
certain parameters.
Look forward to hearing which intercooler I should install. Please see
what you can do about an APC schematic.
**** Ok, anybody else? -Shep Siegel
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