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As others have noted, the purpose of a turbo is to mash as much air (and by extension, Oxygen to burn with the gas) into the cylinder as possible.
The result of this is increased fuel burned per combustion cycle, and thus more power per stroke.
HOWEVER. Over the years the art of turbo metering has grown more advanced. In the dark early days there was little to prevent TOO MUCH compression, which in and of itself is capable of flashing the fuel, causing very destructive detonation (also called "pinging," "knock," or "preignition."). The result of this can be broken, holed or burned pistons, rods, heads, valves or cylinder heads. So in the old days before more advanced sensor technology, turbos were generally quite limited in the power they were allowed to produce.
As years passed and electronics developed (especially at Saab, the real masters of turbocharging in its heyday), more and more of the turbo's potential was able to be harnessed. The Aero probably represents the point when this was maximized, though newer turbo technology does offer small improvements still.
The big thing to know about is the turbo's wastegate. This is literally a gate that opens to vent excess turbo pressure off before the engine goes kablooie, and it's controlled by a small amount of turbo pressure fed to a diaphram. This diaphram in turn is controlled by a valve controlled by engine electronics, which, when they sense impending doom, send a signal to the valve which diverts more pressure to the diaphram, in turn opening the wastegate valve and diverting turbo pressure away.
When you're leadfooting it in your Aero, the engine's electronics are constantly "feathering" the wastegate to keep you right on that line between maximum boost and overboost. A frequent bugaboo in any turbo car is the loss of turbo thrust, which will usually trace back to a malfunction somewhere in these works (somehow too much pressure is getting diverted away).
The valve that controls the wastegate is called the Boost Pressure Control (BPC) valve, and you'll hear a lot of discussion of this item when people are experiencing boost problems. Another frequent malady is the wastegate's control rod falling off, leaving you with no boost. This is a nice, cheap repair; others can be more difficult to track down.
There's also a "blowoff" or "hooter" valve which is a less frequent failure item, but still relatively common. Its purpose is to divert high pressure intake air away from the closed throttle butterfly when you let off the gas. Without it, every time you close the throttle, a shock wave would form in the intake tubing, traveling backwards toward the compressor side of the turbocharger, slowing or stalling it altogether. This in turn causes the exhaust turbine to stop spinning, resulting in a sudden buildup of exhaust backpressure, causing stumbling or stalling. This is usually due to crummy old vacuum lines or a bad hooter valve.
Most of these items are relatively easy to diagnose and not too expensive to put right. On rarer occasions the engine's brain is at fault; rarer still, turbo maladies are chronic and never seem to fully clear up.
All told, once you understand the anatomy of the turbo, it's not a hard system to maintain and/or repair.
posted by 69.177.23...
, Fri, 3 Mar 2006 11:27:28, Fri, 3 Mar 2006 11:55:59, Fri, 3 Mar 2006 14:26:34, Fri, 3 Mar 2006 18:20:44, Fri, 3 Mar 2006 05:10:54, Fri, 3 Mar 2006 11:45:35, Fri, 3 Mar 2006 17:59:40
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