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Turning Up the Boost
There are many misconceptions about "turning up the boost." Boost pressure is only one part of a complicated balance of getting more power from an engine. Boost is developed by the pressurization of air that is NOT going into the engine. If all of that air was going into the cylinders, there would be no boost pressure. It's like trying to cut off water flowing from a garden hose: the more you restrict the water, the higher the pressure. If you don't restrict the water, the pressure doesn't rise. The boost pressure in the plenum is caused by the restriction of air entering the cylinders and the turbo attempting to force more in.
We are trying to get a greater volume of air into the cylinders. That air, along with a greater amount of properly proportioned fuel, will result in more power. Our four valve per cylinder engines are nearly 98% efficient at filling the cylinders. The only way we can get more volume of air into the cylinders is by pressurizing the air. But just because you put twice the "pressure" of air into a cylinder, doesn't mean that you have twice the volume. Pressure ratios aren't linear.
Back to the garden hose. If you had a hose with a good nozzle on it and 30 psi of water pressure, you might be able to fill a bucket in about 30 seconds. If you were able to increase the water pressure to 60 psi, the bucket would fill in about 22 seconds, not half the time. That same hose will squirt farther across the lawn, but doesn't actually get much more water to the plants.
This brings into play the pressure ratios and volume. We often think of pressure as "boost pressure." Remember that we are sitting at atmospheric pressure, typically around 14.7 psi of "boost" (29.92 in. Hg or 760 mm Hg), so any pressure that we want to develop really needs to be referenced from absolute pressure. We are at 14.7 psi, so if we want to run 14.7 psi of "boost," it is really 29.4 psi, or a 2:1 pressure ratio. Again, as mentioned, twice the pressure does not mean twice the volume of air.Sizing a Turbo
When sizing a turbo, you must look at the displacement of the engine, the operating RPM, and the desired pressure ratio. With these factors we can find a compressor wheel that will make the required volume. Manufacturers produce compressor maps to aid in this selection.
Often a smaller compressor wheel is used to get quick spool up. (Smaller wheels have less mass, so they're easier to turn.) What results is a turbo that may make 15 psi of boost at 3500 RPM, but as the engine speed increases, the boost may actually drop. The boost drops because the smaller wheel can't produce the volume of air needed. This is especially true of the S4 Esprit turbo.
A side effect of creating boost is heat (hence the need for a chargecooler). A small compressor wheel spinning at its upper limits may be able to create more boost, but a significant part of it is from heat not from actually compressing more air. It is simply "beating" the air harder. A serious downside is the increased speed that the small compressor wheel must spin. With a wheel at its upper limit, when the throttle plates close, there is a major spike in the thrust loading of the compressor wheel due to the sudden back pressure generated.
It is actually possible to use a slightly larger compressor wheel, spinning slower, running a lower pressure ratio with a higher volume, and due to greater efficiency and cooler intake air, get more power from an engine.
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