Please bear with me... what we have here is barely a rough draft - it's more like brainstorming out loud. If you have any questions while this approaches completion, of course feel free to email me. As time and resources allow, I will supply as many illustrations and images as possible to help out - if you have any pictures detailing some of the modifications described here, I will by all means be happy to post or link to them.
Air Filter: There isn't really much to think about as far as air filters go. There are two basic qualities you need in one: Efficient and thorough cleaning, and high flow. The stock airbox was designed with more of the former in mind than the latter, but it was thoughtfully engineered. The basic idea on a c900 is that air enters the fender area at the front of the car, travels over the wheel well, and is sucked into the airbox. The theoretical benefit is that a continuous supply of clean, fresh, cool air is available at all times. Unfortunately, this passage is commonly obstructed by the wiring for the cornering light, the cruise control pump, and the APC box. Additionally, the air intake itself is relatively small, and air must travel through a twisty path before it makes it out of the airbox assembly.
The common upgrade is an open-air element, such as the ones pioneered by K&N. The argument about the benefits of such air filters borders on the religious level. One camp insists that any benefits the open-air filter might provide are cancelled by the location of the filter (behind the hot intercooler), and that the supply of air is limited to the hot engine compartment. That makes a large degree of sense. The other camp insists that the intercooler does not substantially alter the air temperature surrounding it, and that the engine processes enough air under boost to remove the air from the area before it has a chance to seriously heat up, so its additional "processing power" is not adversely affected.
In the end, it's your choice. I run open-air K&Ns on both my '86 900S and my '90 SPG and they work well. Some individuals have actually taken the setup a step further, by removing the cruise control system and locating the filter between the fender and the bulkhead using custom-made piping. A very good solution, if you can live without cruise control. I plan on moving this direction on the SPG, as it already lacks cruise control (and soon an APC box as well!) so space is not a concern.
Air Mass Meter: The AMM is one of the major obstructions in the intake path on all LH-powered Saabs. It is smaller in internal diameter than much of the intake, and in addition has a pair of screens - one at its opening and one at its exit - further restricting airflow. There is a fair amount of mythology regarding these screens, with some folks claiming they need to be there and some folks claiming they don't. Those that claim they should be there offer up two basic arguments: One, they are designed to keep debris and greasy fingers away from the delicate wires inside and Two, they are designed to smooth airflow across those wires to ensure an accurate reading. Are either of these valid? Who knows? Saabs aren't the only cars with such a device, and this argument most certainly transcends manufacturers. In the end, experimentation is probably the only real answer.
If your car happens to feature LH 2.2, you have another point of interest on your AMM - a small adjustment screw which may be hidden behind a metal anti-tamper seal. The adjustment screw is designed to calibrate the AMM for a baseline reading, so the computer has a known reference point. Later cars recalibrate themselves automatically, so they lack this adjustment. The effect of the adjustment can be measured by connecting an ohm meter across pins #3 and #6 on the AMM's connector. A stock baseline should be about 380 ohms. Decreasing this value will lean out the mixture, increasing it will have the opposite effect. Adjustments are often useful, if not required, when making changes to other fuel-related systems in the car. Do note that such adjustments should not be made haphazardly, and should be made only with feedback from proper measuring equipment - ideally the aforementioned air:fuel meter and EGT meter.
Turbo: Without going into too much detail, here is a basic rundown of how a turbo works:
A turbo gets its energy from the exhaust stream. Engine exhaust is routed through a turbine (impeller) before being released into the exhaust pipe. The exhaust impeller sits on a common shaft with a compressor wheel, and the two are separated by seals. As exhaust passes through the turbine housing, the turbine (and thus the compressor) are caused to spin. When the compressor has achieved a sufficiently high speed, it pressurizes the intake air by up to 1 bar (about 15psi), depending on the specific model Saab. The end result is the engine receiving more air than it could normally draw in, and with a similarly increased amount of fuel higher engine output is realized. Effectively, a turbo increases the engine's displacement.
Over the years, Saab used several different turbos, tweaking with performance and drivability. Before investing money in upgrading other components in the car, you should take a look at your turbo to determine what you've got.
Most 900s and early 9000s shipped from the factory with a Garrett AiResearch T3. This turbo is generally good for 18-20psi. Later 900s ('90 SPGs, and '91+ 900Ts) came with Mitsubishi TE-05s, a small turbo good for quick spool-up but ultimately only 16-18psi. Starting in 1990, 9000Ts came with a Garrett T25, another very small turbo, good for 16-18psi. In 1994, Saab began equipping Mitsubishi TD-04s on 9000 Aeros - though strangely Saab chose to re-designate these turbos using Garret nomenclature. Slightly larger than the Garrett T25, it's usually considered good for up to 20psi. Running a turbo outside its efficiency range can actually cause a loss in performance, as the turbo struggles to compress the increased volume of air. They quickly become overworked and run excessively hot, overcoming the intercooler's ability to radiate the heat. Poor performance and ultimately detonation can result, so be sure your turbo is up the whatever challenge you've got in store for it before increasing boost.
There are several key dimensions when sizing a turbo, and that even at this point in the game there is still quite a bit of art left in the science of turbocharging an engine. Some things to consider are:
The ratio of the impeller to its housing size as well as the shape of the impeller's fins will determine how much of the exhaust stream's energy the turbo will be able to use. A larger turbine will be able to harness more of the energy, but will also cause backpressure as, at low RPM, there is insufficient exhaust to spin it. This phenomenon is often referred to as "turbo lag." On the other side of the equation, is the sizing and shape of the compressor wheel. These dimensions almost exclusively determine how effective the turbo is at low RPM and at high RPM. Too aggressive a compressor will result in extra size and weight (introducing more turbo lag) and too conservative a compressor will result in a turbo that can't keep up with the engine.
It's important you are familiar with the technology before proceeding if you want to have a good performing, reliable Saab. Corky Bell, one of the leading authorities on modern turbo systems, has written a book on the subject, and I consider it to be recommended read. You can purchase it here. Give the friendly folks at SPG9 or Swedish Dynamics a call and talk with them about your project and what you're trying to accomplish. Turbonetics is also a good source, but I personally recommend sticking with companies that specialize in Saabs - they may have information that general-service shops will not.
APC System: The APC monitors and controls boost in all 1982.5 and later Saabs, 8v or 16v. In later years, the APC functionality was built in to other systems, such as the DI/APC computer and, later still, the Trionic computer. The APC system adjusts charge pressure based on preset limits and detection of knock, which helps keep the engine performing at maximum efficiency under most any operating condition. What this means for the tuner is that you can make little goofs in tuning the turbo system and not worry (too much) about the engine destroying itself. It also means that "more boost" is not just a function of adjusting the wastegate like many cars unless you're willing to bypass the safety that the APC system offers.
Given that you plan on retaining the APC system (which you probably should, at least at first) there is only one way to achieve higher boost levels - tweak your APC box! In designing the system, Saab allowed for adjustment to the system's presets, meaning it is mind-bogglingly easy to tweak your boost. Sadly, these tweaks are not an option for those with integreated APC systems. In those situations, you will most likely have to rely on various aftermarket sources for professionally modified computers.
You will need to remove and open your APC box in order to adjust it. Inside you'll find three potentiometers, labeled "P," "F," and "K." Although there is absolutely zero official information available on the function of the three adjustments, there is general acceptance of the following:
P - The P pot defines maximum boost. The farther you turn it, the more peak boost your car will make. Often, "P" is considered to reference maximum Pressure.
F - The F pot defines where on the RPM curve maximum boost will appear and how long it will be sustained. The farther it's turned, the higher up the rev range you'll achieve (or sustain) maximum boost. Turning the P pot without adjusting the F will result in high boost that isn't sustained very long. On the contrary, simply turning the F pot can in fact result in positive horsepower increases, as the normal "boost taper" can be reduced. The "F" pot is considered to refer to the Frequency of the APC solenoid - the device that actually performs the regulatory function.
K - The K pot defines how sensitive the APC box is to knock - how much signal from the knock sensor is required before the system will start tapering boost. It is generally best to leave this potentiometer alone. Not surprisingly, "K" refers to Knock.
Two sites are highly recommended when making adjustments to your APC system. First, check out John Bertram's Volvo/APC site. John has retrofitted a Saab APC system to his Volvo 740, and described in great detail how and why the APC works. Secondly, check out the Tweak Your APC site. The author has very accurately detailed several ways to adjust your APC system to provide greater performance, and given information on factory modified boxes for reference.
Intercooler: When running more boost, more heat is created. It's that simple. Compressing air is an exothermic reaction, meaning that heat is actually produced as the air is compressed. The stock intercooler is designed to cope with stock boost levels, and is not particularly good for keeping the intake charge cool when running more. The 900 intercooler is especially poorly designed for high boost levels due to its size and placement (behind the bumper and headlight), though the 9000's front-mount design is substantially better. If you plan on going for high horsepower numbers, it will be imperative to look at upgrading the stock unit.
With the 900, you are severely limited by space and mount points. Several people have taken to the task of upgrading intercoolers, but currently there is no benchmark. All serious upgrades currently require the removal of the A/C condenser to make enough space - you'll have to decide for yourself whether going faster or staying cool is more important. With a 9000, there is adequate room up front for a thicker core with just a little work, though removing the A/C condenser would certainly improve airflow.
Below is an example of a custom front-mount intercooler on an SPG. The intercooler core is from a Porsche 944 Turbo and the radiator custom built. Cooling fans for the radiator are 6" diameter units from a motorcycle. The A/C system has been removed completely, allowing unblocked airflow to the intercooler and radiator:
Beyond this, several people have adapted 9000 intercoolers to a 900. I'll put some pictures together in the near future. Basic installation consists of removing the A/C system and installing the 9000 core in its place, then tilting back the radiator to allow the plumbing to pass by. In general, this is a very clean installation and highly effective - especially considering the wide availability of 9000 intercooler cores.
Bypass/Blowoff Valve: Before getting into this subject, there are a couple explanations that need to be made. A bypass valve is a re-circulating design, and a blowoff valve is a "dump" design. Both types operate under the same principles - when the throttle plate closes, vacuum is created in the intake manifold, which in turn causes the valve to open. Pressurized air coming from the turbo is then re-routed away from the closed throttlebody through the now-opened valve. Both valves are designed to keep the intake charge moving, thus preventing it from stopping or reversing direction and slowing, stopping, and possibly damaging the turbo. With a bypass valve, the intake charge is directed back to the low (intake) side of the turbo to be re-processed, whereas a blowoff valve dumps the intake charge straight back to the atmosphere.
Both types have their merit. Bypass valves can actually improve performance, as fast-moving, pressurized air released back into the intake helps keep the turbo spinning between shifts. Blowoff valves, on the other hand, can flow substantially more air. Generally speaking, bypass valves are good for no more than 18psi of boost; anything beyond that requires use of a blowoff. Do note that a blowoff valve can be fitted in low-boost applications but, from a performance perspective, a bypass valve very likely a better choice whenever possible.
Since the introduction of the 16vT, Saab incorporated a turbo bypass. The stock bypass valve is barely adequate for its task of handling 12psi - they fail every few years due to ruptured diaphragms. Saab did introduce a strengthened unit, which helps in dealing with stock boost, but it is still woefully inadequate for handling increased boost levels - at best you'll end up replacing it with a good degree of regularity, at worst you'll damage your turbo.
With that in mind, it is absolutely imperative that when you increase the amount of boost that you're running, you also upgrade your bypass valve. With the introduction of the Saab Viggen, a new Bosch bypass valve has been designed which is rated to 20psi. For keeping costs low, and maintaining a stock look, this is a great solution as it is reliable and easy to install. This unit is available from any place that stocks Saab parts (including SPG9 and Swedish Dynamics) and should cost well under $60.
If you want something a little more stylish, Bailey Motorsports and Stratmosphere both offer piston-type bypass valves that will install in place of the factory unit. Piston based bypass valves are more reliable than the stock variety as they do not rely on a rubber diaphragm for containing and releasing boost. They are a bit pricey, but will offer improved performance and substantially increased lifespan.
Beyond the "drop in" type replacements, there are numerous companies offering several styles of bypass valves. Using one of these other types will very likely require substantial modifications to your intake tract, including welding and custom piping. HKS makes an excellent racing-quality bypass valve, appropriate called "HKS Racing Bypass Valve" and offers an 18psi rating and an piston adjustable to different pressures.
If you plan on running more than 20psi, it is advisable to move to a blowoff valve. Do note that on all Saabs running K-Jetronic or LH-Jetronic injection, blowoff valves will cause minor havoc with the mixture, as air that has been measured by the mass air sensor will be released back into the atmosphere. This will cause a momentary rich mixture, and as such may be illegal on emissions-controlled vehicles. Additionally, they can theoretically damage the exhaust manifold, turbo, or catalytic converter over time due to unburnt fuel combusting after leaving the engine.
Blowoff valves come in two basic flavors - single stage and dual stage. Single stage valves offer one adjustment, allowing you to set the vacuum level at which they'll open; dual stage valves allow you two stages of vacuum level. The dual-stage design allows the valve to respond quickly to shifts while at low engine speed, and still provide adequate relief once boost levels and engine speed have increased to maximum levels. There are no less than half a dozen manufacturers of blowoff valves these days - HKS, Blitz, and Apex'i are among the top-rated brands. Installation of a blowoff valve will likely require modifying the stock intercooler-to-throttlebody pipe to accept a mounting flange, though certain models may be adaptable to the existing throttlebody port. Personally, I would stay away from these latter types as, due to their compact design will not perform as well as a larger valve might.
Finally, if you plan on installing a blowoff valve, there is one quirk of Saab's PCV (positive crankcase ventilation) system that may cause you problems. PCV is required by US law, and Saab's implementation uses the low-side of the turbo as a source for vacuum. As a result, oil-contaminated air is circulated through the large majority of the intake system. By incorporating a blowoff valve, you may end up releasing this oily air into your engine compartment with every shift, creating quite a mess. As such, it may be in your best interest to install a filter in the middle of the PCV system or at the exit of the blowoff. There is more on this below, under Intake.
Intake: The overall intake system on Saabs are fairly well designed. Unlike many turbocharged cars, the intake piping and manifold runners are reasonably large, and able to flow a good amount of air. There are a few things worth considering when optimizing your intake system:
First, take a good look at the crankcase evaporative system. Whereas on 8v cars the evap system dumps into the air filter (relatively good design), the 16v cars have the ventilation line dumping back into the intake system, before the turbo. The former design ensures that air entering the intake tract is clean, but the latter design actually send oil and small (very smalI) amounts of debris through the turbo. The engine is more than capable of dealing with such small amounts of "foreign" material, but removing it is certainly a worthwhile endeavor. K&N makes a small filter which can be installed inline to clean this air, and I'd recommend adding one of these. If you're planning on running a blowoff valve, I'd absolutely include one of these little filter in your upgrade plan - without it, that oily air will be blown all over your engine compartment, creating quite a mess!
Next, take a look at how the cooling system interfaces with your intake system. Hot coolant is actually run through the throttlebody to assist in de-icing it when outside temperatures are cold. If you live in warmer climates, or simply do not run your car in the winter, bypassing this preheater setup is highly recommended - every last Fahrenheit degree you can remove from the intake system, the better. Removing the system is as easy as finding a small enough brass fitting, and using it to couple the two coolant hoses together. Stash it away safely, and tie it to something stationary to prevent it from flopping around.
With the simple fixes out of the way, you're left with things that will cost substantial amounts of money - porting and polishing the intake tract. As with all mass-manufactured cars, the throttlebody and intake manifold are a compromise between performance and cost. While generally well-designed, their actual individual quality is nothing outstanding. Cleaning them up certainly isn't going to net you huge horsepower increases, but the smoother the path the air must follow, the better the system will perform.
Once removed from the car, the throttlebody can be cleaned up by grinding away the brass fittings which protrude to its inside surface. Be cautious not to dislodge the ports - they can become less securely fitted over the years. If one does free itself from the throttlebody, it can be replaced using an epoxy like JB Weld. If you've got money burning a hole in your pocket, don't overlook the possibility of having a machine shop bore out the throttlebody and re-size the throttle plate. SPG9 will be happy to undertake such an operation.
The intake manifold can also be cleaned up. Owners of earlier cars (those from 1987 or earlier) may notice that the intake ports on the manifold are of alternating sizes. Although some may simply grind these away, saving them may be worthwhile - the idea is that turbulence is created in the cylinder, resulting in "swirling" air. This can help excite the mixture and improve combustion - it's a technology Honda pioneeed in the '70s and has fallen into wide use by nearly every major manufacturer. Later cars achieve the swirling affect by slightly offsetting the lobes on the intake cam and letting the valves initiate the swirl.
Ported or not, the intake manifold can be smoothed out, or polished. Its interior surface is quite rough, and with the right tools (polishing kits are available from most performance automotive shops) and some patience benefits may be achieved. It's worthwhile to note that while a smooth intake manifold is desirable, a mirror finish is not. A textured surface free of major obstacles is ideal, as some turbulence in the intake charge helps ensure a proper mixing of air and fuel. Many have had excellent luck with a process called Extrude Hone - check out their website for information on the process. It's pricey, and the results are probably not tangible, but a properly polished intake manifold is a significant piece of a much larger puzzle.
Fuel System: The fuel system on 16v Saabs is a completely standard system. All of them (save certain Euro-spec cars that were never imported to the US) use Bosch LH Jetronic, earlier ones using v2.2 and later ones using v2.4. The system was never really state of the art even in its time, but is reasonably flexible. LH incorporates a hot-wire measurement system and an oxygen sensor feedback system ("closed loop"). The hot-wire (called the Air Mass Meter, or AMM) measures incoming air to provide the management system with a an accurate idea of how much air is incoming. That information is cross-referenced on a two-dimensional matrix which results in a quantity of fuel to be supplied. The oxygen sensor tests the exhaust gases and helps the main computer fine-tune the mixture.
The computer itself isn't tunable by the average individual. Mostly, it's a black box and you simply cannot tamper with it. Getting more power from the system more often than not requires tricking it by swapping components or modifying existing ones. Doing so can become tricky and somewhat tiresome, but in the end your choice is to live with it, or replace it wholesale with an aftermarket system.
If you choose to live with the system, there are some steps you can take to help improve performance. Before doing so, however, I strongly recommend reading Charles O. Probst's book on Bosch fuel injection - it'll help familiarize you with the basic components of the system, and help you understand what you're dealing with (or up against, depending on how you look at it). It is available for purchase here. Once you've got that down, here are some tips to help get the most out of the stock system:
First - the fuel pressure regulator (FPR). The regulator's job is to react to increased manifold pressure, and ensure that fuel pressure at the injectors is always a fixed amount over that pressure. For example, at 1bar (14.7psi) of boost, the stock 2.5bar regulator will provide 3.5bar of fuel pressure at the injector. This ensures that the fuel will have enough force to enter a pressurized manifold, and also that the fuel atomizes quickly and thoroughly once injected. By upgrading the fuel pressure regulator you can fool the stock computer into releasing more fuel. Since the FPR is purely mechanical, the control unit knows only that opening the injectors for "X" amount of time produces "X" amount of fuel at the factory pressure - by increasing the amount of pressure the regulator supplies, more fuel is released with every pulse of the injectors.
Most 900 Turbos came from the factory with a 2.5bar fuel pressure regular - some later high performance models came with a 2.8bar unit. If you are even considering anything above the stock horsepower rating, you will want to upgrade to one of these 2.8bar units which should be good for 170-190 horsepower. Beyond that, you'll need something more. Non-turbo Saabs featured lower-flowing injectors coupled with a 3.0bar regulator - as such, acquiring a 3.0bar regulator is as easy as scouring the local junkyard or talking to the dealer. Additionally, Bosch does produce a 3.3bar unit which is usable on Saabs - originally designed for Porsche (I believe), it's a drop-in replacement for the stock unit. Do be aware of a few things: First, while increasing the fuel pressure will provide more fuel, it is not a 1:1 ratio. The ultimate rate of flow will be determined by the injector, and at some point increasing fuel pressure will simply be inadequate. Second, fuel injectors are invariably designed to work at a specific pressure - most are optimized for 3.0bar FPRs, and can become unreliable if operated at higher pressures. Third, since this methodology involves fooling the computer, problems can arise. If the computer cannot determine precisely how much fuel is being metered, rich running can result. At high engine speed, under high boost, a rich mixture is a good thing if not downright desirable, but at low engine speed a rich mixture can result in poor idle and poor performance, not to mention poor gas mileage.
With those issues in mind, a very good solution is a rising rate fuel pressure regulator. Whereas normal FPRs are fixed rate, a rising rate unit provides a non-linear increase in fuel pressure. For example, while a stock fuel pressure regulator will provide 2.5bar of pressure over manifold pressure, a rising rate unit could provide 3bar of fuel pressure for every bar of pressure in the intake manifold - at 1bar of boost a fixed FPR would provide 3.5bar of pressure, that rising rate FPR would provide 4bar of pressure; at 10bar of boost, a fixed unit would provide 12.5bar of pressure, that rising rate unit would provide 40bar. Of course, that's a ridiculous example, but you get the idea. The obvious advantage of a rising rate fuel pressure regulator is that it can provide low pressure at idle, and high pressure at full boost, overcoming the limitation of the fixed unit. If you're planning on running high boost, you should absolutely invest in a rising rate fuel pressure regulator.
Next, consider how much power you're looking to make and size your fuel delivery system appropriately. The stock injectors are sized to provide a maximum of about 200hp with a 2.8bar fuel pressure regulator. While you can eek more power out of them by increasing fuel pressure, you should consider an upgrade. Saabs use very common high-impedance injectors - they can be had nearly anywhere. SPG9 and Swedish Dynamics both various sizes of upgraded injectors, and I strongly recommend giving them a call for help with your application. If you feel up to the challenge of sizing your own injectors, Summit Racing stocks a full line of injectors that will work in any Saab. For reference, stock 900T injectors are rated at 21lb/hr at 2.5bar of pressure - that's roughly equivalent to a 24lb/hr injector at 3.0bar of pressure.
Once you've dealt with the common fuel system upgrades (injectors and FPR), there are some other upgrades and improvements you may wish to look at:
First, consider the fuel pressure rail. On a 900, it's round, but on a 9000 it's square. The reason? More recently that you'd imagine, it was determined that square fuel rails are actually superior to round ones, and suffer less pressure drop across them. No one currently makes square rails for 900s, but with a minimum of work a 9000 unit can be adapted - cylinder spacing and injectors dimensions are identical between the two cars, so it's a matter of designing a simple mounting system. I'll post pictures as soon as possible of an adapted rail.
Ignition System: The ignition system fitted to all 900s and early 9000s is roughly the same - basic electronic distributor-type ignition. A sensor in the distributor provides engine speed information to the ignition module. The ignition module determines proper timing and fires the ignition coil. High voltage energy is sent to the distributor, and (not surprisingly) distributed to each of the car's four cylinders. On early cars, the ignition is based on a simple magnetic pickup; on later cars (1982+), ignition is based on a Hall-Effect trigger which is more reliable and more accurate. The type of ignition on your car is mostly irrelevant, though earlier cars can be easily updated to Hall-Effect ignition if desired.
As with the fuel injection system, there are two approaches to tuning the ignition system - work with it or jettison it. While basic upgrades can be done which will improve the car's performance in general, ultimate performance will only be had by replacing the ignition system with something more flexible. The reason? A fundamental limitation in the Turbo's ignition system: While RPM-based timing is determined by a fairly primitive, but reasonable, device, the system has no way to make timing adjustments based on boost. Regardless of how much boost the car is actually running, the system makes one change - at 5psi timing is retarded 5 degrees. This setting is wholly inadequate for running big boost (not enough retard), and is actually hurtful to low-speed performance as it is a one-time, all-at-once change and not gradual.
That said, there are four tunable aspects to the stock system. The obvious one is the coil - the part that turns your car's 12v electrical system into a high-energy system capable of "powering" a spark plug. The stock coil is rated at about 50,000v