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What Lies Beneath: The Saab APC System

During Saab-Scania’s salad days of the 1970s and 1980s, its interests were varied and far-ranging. Aerospace, automotive, defense, and even computing were all areas of
research within the realm of the quirky Swedish company. The benefits of
turbocharging – originally a means to compensate for altitude on
piston-driven aircraft – were well understood to Saab. The company became known as a bit of a pioneer in the relatively under-developed field of turbocharging cars in 1978 with the Saab 99 Turbo. The 99 saw to it that the company with fighter-jet heritage had cars with performance to match. The old Triumph-based “B-Engine” was suddenly a force to be reckoned with thanks to the addition of the little exhaust-driven turbine.

The improved “H” engine found its way into the 99 (still sold in Europe at the time) and 900 by 1981. The following year, the turbocharged models would be fitted with the revolutionary Automatic Performance Control (APC) System. Saab has always believed in making cars that were not only well-built, safe, enjoyable to drive, but also practical. The APC system made it possible for the engine of turbocharged Saabs to compensate for pre-detonation or “knock” caused by low-octane or poor quality gasoline, carbon deposits, high operating temperatures, poor tuning due to lack of maintenance and so-on. Furthermore, this allowed the compression ratio of the engine to be higher, thus increasing fuel economy by taking greater advantage the available energy in the fuel. The Saab driver could now have performance, and afford it too.

The Basics of APC Operating Principals:

Knock occurs when the temperature inside of a cylinder’s combustion chamber becomes high enough to detonate the fuel/air mixture before intended by the spark plug – near top-dead-center of the compression stroke. Since forced induction naturally raises pressures, the compression ratio of a turbocharged engine must be low enough to ensure knock does not ever occur. This does not mean that a turbocharged engine cannot happily run at higher boost pressures or compression ratios, it simply means that the conditions must be right.

Basic turbocharging principals dictate fitment of a wastegate to allow excess exhaust gasses to bypass the turbocharger, thus preventing too much boost from entering the intake of the engine. It is operated by a pressure line from the compressor side which forces a diaphragm to open the wastegate once boost levels become too high.

1) Knock Sensor
2) APC “Brain Box”
3) Pressure Transducer
4) Solenoid Valve
5) RPM Sensor

One part of the APC system’s job is to act as a regulator for the wastegate, thereby controlling engine boost. Rather than the wastegate’s line running directly from the turbocharger itself, it is re-routed to the solenoid valve (#4). This valve is controlled by the APC box (#2) and has the ability to override the function of the wastegate.

The APC box is connected to a knock sensor (#1) which can detect if pre-detonation is occurring within the engine, and also to an RPM sensor (#5) to factor in engine rotational speed versus intake manifold pressure, as determined by the pressure transducer (#3). If the APC system detects knock, it will allow the wastegate to open sooner and reduce turbo boost to alleviate the problem. Because of this ability, this is why Saab was able to maintain a higher compression ratio in their APC-fitted turbocharged cars. Before APC, the ratios for turbo “B201” engines was 7.2 to 1. This increased to 8.5 to 1 with APC and, on the 16-valve “B202” engine, 9 to 1. APC can also control fuel delivery to an extent. When boost becomes excessively high, due to a stuck wastegate or misguided modding for example, it has the ability to cut fuel to the engine.

As the name suggests, Automatic Performance Control also controls performance – with the emphasis being on performance. It is not simply a means to prevent the engine from destroying itself, but also a way to vary tuning for the sake of economy, emissions, simple marketing strategy, etc. Saab’s own SPG model 900 used a tweaked APC box (known in the Saab world as the “Red Box” to allow for a more powerful engine.) This is where things get rather complicated.

Exactly how the APC’s “brain” works has been a part of Saab lore for decades now. To this day there are conflicting and diverging schools of thought on how a would-be Saab tuner can go about tweaking the APC for more power. They more-or-less all involve three circuit board potentiometers ambiguously labeled P, F, and K as well as some playing around with transistors and a soldering iron.

Though this author admits to having spent some time boldly tinkering with the APC box of his own 900 Turbo, this is not something for the weary or faint-of-heart. For those interested, I will direct you here but beware – more than once I’ve gleefully watched my Saab’s boost gauge eagerly run into the red, only to have all the boost suddenly dump and the fuel delivery cut out and leave me coasting at 75 mph trying to restart the car. After a while, you may just find yourself tearing the box apart again to turn the screws back to their original settings, desperately hoping to regain the driveability of a normally functioning APC.

The APC system lived on in Saab turbos relatively unaltered until 1990. For the 9000 models only, which were fitted with direct ignition, DI/APC integrated the engine’s ignition control into the mix. Therefore, the system was able to control both turbo boost as well as ignition timing to keep everything running smoothly.

The success of the APC system certainly didn’t go unnoticed. Today, nearly all turbocharged cars have a similar system in place. Personally, this has allowed my cheap self to be running low-octane gas in not only my APC equipped Saab, but my Volkswagen with a 1.8T as well. Many Volvo enthusiasts have adopted and adapted APC functionality to their old 240 and 740 turbos.

Unfortunately, and no doubt thanks to years of General Motors ownership, it’s been quite some time since we’ve seen truly great and unique innovations like this from Saab, but lest we forget, we must give a nod to Trollhattan.

5 thoughts on “What Lies Beneath: The Saab APC System

  1. >Nice work!Have to point out, however, that the famous Red Box was not standard on SPG models… in fact, it was only standard on the 1993 Commemorative Edition turbos. When the SPG first came out in '85, it had exactly the same APC box as any other '85 T16. The manufacturer's quote horsepower figure for both a standard '85 T16 and SPG was 160hp. Incidentally, the '85 SPG has the same solid front rotors as any other '85 and isn't any lower. In '85, the SPG package meant you got all the options, the ultra-cool body kit (all '85 SPGs were black, and the body kid on those built in the first couple of months was black as well!), and an anti-roll bar.It's worth noting that the '85 16-vavle turbos have the APC box under the back seat, same as the 8-valve turbos. I've played around extensively with this, and it doesn't feel much different from an '87 standard 160hp box. The 160hp boxes from '91 up, designed for the Mitsu TE05, feel totally different, despite the same part number… anyway, I digress…It wasn't until '86 that the box moved under the hood, in the driver's side (LHD…) fender well. SPG performance figures stayed at 160hp for '86, and the car didn't get any lower. However, along with the normal T16, vented front rotors were introduced.In '87 with the cosmetic facelift, the famous SPG springs finally debuted. Further, the SPG got its own APC box, which was still black but had a different part number (standard T16 of this era ended in 599, SPG in 127). 165hp! '87 also marked the debut of the Red Box, sold as the "Engine Tuning Kit" option. I'm sure the 175hp version came out in '87, and cost around US$1k. What I'm not sure of is when the 185hp version came out, but I bet it wasn't until '91 with the switchover to the Mitsubishi turbo, and once LH2.4 was well sorted out. Worth noting that LH2.2 doesn't exactly like the 3barFPR and 185hp red box without some serious tweakage… and it's even worse if you throw a Group 9 A.I.D. into the mix–ask me how I know…The 185hp version of the Engine Tuning Kit included a distributor with a modified vacuum advance, and 2.8bar fuel pressure regulator to replace the standard car's 2.5bar FPR. This option was *ridiculously* expensive back in the day, which is why we RARELY see it other than on '93 CEs.I think that begins to scratch the surface, anyway! 😉

  2. >Great read, liking the Saab coverage as of late!Fuel cut-off is separate from the APC unit, but you have me wondering, is it officially part of the APC "system?"Jeff- I can vouch that in '88 SPGs had the +5 HP over the Turbo APC. What doesn't your LH2.2 like about those mods? I have a similar setup (heavily DIY modded APC, 3 bar fpr, and AID stop mod) and it's been working fine, reliably holding 16psi. Just trying to get my hands on an eprom flasher so that I can lower the control constant to run T5 injectors. : )

  3. >Ian – I guess that's an area of debate as to how technical you want to get concerning what is exactly part of the APC system. I've always believed that the information the system reads gives the car the ability to cut its own fuel supply… somehow… be that the RPM sensor, boost readings, what-have-you. I've never been quite sure how (or if) the F.I. ECU has the ability to "talk" to APC and vice-versa. Jeff – any thoughts?Also, Jeff – As for the "Red Box" on SPGs thing – After I wrote the article, I kind of second-guessed myself on if I was remembering correctly which cars were equipped with it. Consider it lazy editing on my part – sorry. I've been know to do that from time-to-time. Hope it doesn't make me come off as not knowing my stuff…

  4. >Ian – just a note, but it is possible to bypass the obnoxious "headbanger's ball" overpressure cutoff switch. It's a brown and white wire that runs with the loom under the passenger footwell. Although I believe the overpressure switch kicks on and cuts current to the fuel pump at slightly over 17psi – and if you're on the stock 2.5bar FPR and 21lb/hr injectors, even a 16v Turbo is WAY lean in the AFR range that that point. I did the 900Aero mods on my '88 SPG's box and i was pegging my aftermarket 20psi boost gauge pretty hard – and i went back to the stock settings after a few days, with the audible knock. I ended up going with a Swedish Dynamics Red Series APC box, which is a great plug-and-play box for about $130 on ebay.Also, it will cause the fragile, poorly designed 5-speed gearbox to blow the %@#$ up, even if you're borderline obsessive about not beating on it and changing the trans fluid twice as often as necessary. If you get a 900 Turbo it should be a 90 or newer as they had a beefier cluster bearing. I suppose that's an issue for another RWP post.Anyway, fantastic article! Makes me miss my SPG even more.

  5. >Fuel cut-off has nothing to do with the APC system–APC doesn't even know that fuel cut-off exists… similarly, the Jetronic doesn't know that APC exists!Up to '89, with either K-jet or LH2.2, fuel cutoff was handled by an overpressure switch behind the kick panel. There's a tee off the hose which also feeds the pressure transducer which IS part of the APC system. The old style overpressure switch is not only easily defeat-able, it's also adjustable! In '89 with the switch to LH2.4 (which, by the way, was fantastic, and everyone who says LH2.4 sucks and eats through ECUs hasn't simply replaced their '89 or '90 ECU with a '91/up unit, which work perfectly forever, it seems), the overpressure function became more complicated. While still not part of APC in any way, now it was simply a function of how much air is coming through the AMM–if too much air is coming through, cut the fuel pump momentarily. No more nice overpressure switch to tinker with. This can only be defeated/tweaked with EPROM flashes.I've got T5 injectors and a Jak Stoll LH2.2 chip in my '85, but I have nasty hot start issues. Zero vacuum leaks, all sensors brand new, everything adjusted per Bentley. Once it's running, it's fantastic, though driveability (on-off throttle backlash) and fuel economy are notably worse than the stock injector and chips, even after a lot of tweaking… I've even dialed in the AMM using an official old Saab tool called the "pulserelations meter". Prior to the T5 injectors and JS chip, with the red box, 3bar, and 010-dizzy vac advance, I was getting really inconsistent boost and piles of knock, with timing adjusted to spec. Same result in my buddy's '87 T16 convertible when we tried the same setup there, to see if there was something borked with my engine. I'm probably going to take all this stuff out, as the new laws up here in BC say if you're nipped going more than 40km/h over, you lose your car for a week, and it all gets ridiculously expensive… I've had my supra-legal fun and am more or less done risking it… with the present setup, as soon as your toenail grows you're doing 110mph… the Group 9 A.I.D is awesome as it allows you to set more initial advance, so the car is awesome off boost, and an absolute monster in the midrange. Kinda dies off around 4750rpm when the timing should ideally be re-advanced, which is where Brad at KC Saabs distributor (with the mechanical advance guts from an 8V dizzy) comes into play… Actually, what I'd really like todo is just throw T5 in it…Don't worry about the red box thing, that's a super common misconception, and you clearly know your stuff!The superior pinion bearing and housing actually came in in '89. '89 and '90 have the best gearbox components… while the ancient "chillcast" cases from 99s are the best exteriors… Then in '91, the syncros were redesigned to feel nicer but it reduced durability.'89 really is my favourite year. You've got LH2.4 which rocks once it's got a '91/up ECU in it, the best gearbox, and no ABS.

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