As modern automobiles get smaller and more economical, their engines also have gotten smaller and more economical. The problem is that a small economical engine in a car feels like a car with a small economical engine.
One way, however, to get big-engine feel without big-engine thirst for fuel is called turbocharging. And turbocharging is now being used by more and more car makers who want their cars to perform as cars should.
One of the first car makers to offer turbocharging on cars designed for normal street use was the Swedish Saab company, which as early as 1977 intro- duced the Saab Turbo, and which is still a leader in the turbo field.
Being a pioneer in the field, Saab now offers the first Isecond generation turbo.ll
More Fuel + More Air = More Power
Simply speaking, turbocharging means that the carls exhaust is used to drive a turbine which forces more air into the combustion chambers, and with the air, more fuel. The more fuel and air, the more power.
That seems simple and easy, but it isn't always that simple and easy. One problem is that as long as the exhaust spins the turbine, the engine intake system gets charged with more and more pressure -- called boost. If left to its own devices, the turbo would eventually destroy the engine. So a safety valve, called a wastegate, allows the exhaust gases to bypass the turbine when the boost reaches a specified level. That boost level is what determines the car's performance; the higher the boost, the higher the speed.
Another problem with turbocharging is what we have all heard from time to time from almost all car engines: knock. Knock, or "pre-ignition," means that the fuel/air mixture in the cylinder ignites at the wrong time, something that can easily destroy a car engine.
"Knock" Can Be A Problem
Because of the boost pressure in turbo engines, knock is a more serious problem in these engines than in those that are normally aspirated, something that has forced turbo engine makers to put in additional safeguards. These include lowering the compression ratio considerably, which unfortunately causes the problem of "turbo lag" -- because of the low compression, it takes a while for the turbocharger to build up the boost to respond to the gas pedal.
Knock, or pre-ignition, is a function of the gasoline used in a car. The octane ratings you find on most gas station pumps indicate the quality of the fuel and are actually a measure of the fuel's resistance to knock.
But the fuel quality can vary much more than is indicated on the pumps. This has forced turbo car makers to tune their engines to the lowest possible grade fuel likely to be encountered, causing some turbos not to feel like turbos at all.
APC Solves Problem
The Saab APC system -- Automatic Performance Control -- has solved this problem...and solved it in a way that is the envy of the auto industry.
Not only does APC protect the engine against the ravages of knock, but it has also improved the fuel economy of the Saab Turbo by up to 10 percent and increased performance considerably.
Auto enthusiast magazines who have tested the Saab Turbo with APC have recorded 0-60 mph figures between 9.3 and 10 seconds, while the Environmental Protection Agency reports fuel economy figures for the Saab Turbo with APC at 21 mpg city and 34 mpg highway for the manual five-speed transmission version; and 21 city and 29 highway for the automatic transmission version.
APC automatically adjusts the performance of the engine by regulating turbo boost to the quality of the fuel being used. A knock detector (somewhat like a microphone) is mounted on the engine block and continuously monitors the engine for knock. The detector sends signals to an electronic control unit, which gives the necessary instructions to a solenoid valve, next to the waste- gate valve, controlling the turbo boost pressure. The control unit is also in contact with a pressure transducer on the intake manifold and can balance turbo boost so that it is always correct, regardless of the condition of the fuel or the engine load. The control unit and the solenoid valve are unique Saab components and basically represent the intelligence of the Saab APC system.
Fuel Is Used 100%
If low octane fuel is used in the APC engine, the engine tends to knock earlier. The knock detector senses this and informs the control unit, which instantly instructs the solenoid valve to open the wastegate. This reduces boost pressure and knocking stops. The whole sequence takes only a fraction of a second, and the driver is normally unaware of what is happening, although knocking can sometimes be heard briefly, especially under high engine load.
APC ensures that the fuel is always fully utilized. This improves engine efficiency, which together with the rather high compression ratio of 8.5:1, reduces fuel consumption. The high compression also offers better throttle response and thus avoids turbo lag at low speed driving and acceleration.
Since the APC system lets the engine use all of the energy in the fuel, regular or low octane gasoline is sufficient for everyday driving. With higher grade fuel (the engine is tuned to use fuels with pump ratings of between 87 and 92), the fuel economy is maintained, and the performance is correspondingly improved.
The APC system is standard equipment on all 1983 Saab Turbos sold through Saab's 315 dealers nationally.