In essence, a turbocharger is a simple device. Basically, a high-tech pinwheel that pushes air into an engine, it's powered by the combination of exhaust heat and pressure, harnessing power that would otherwise be lost out of the tailpipe. This makes turbocharging an ideal way to increase the power of an engine far beyond what natural aspiration will allow. Turbochargers make power more efficiently than superchargers do, because the engine isn't using power from the crankshaft to drive it. And over time, a turbo kit will likely cost less than nitrous (taking into account the cost of refilling the bottle), so turbochargers may be the best power-adder around.
As simple as turbochargers are, much mystery still surrounds them. Flip through a turbocharger manufacturer's catalog, and you will find dozens of different models in a dizzying array of sizes. On top of that are dozens of graphs and cringe-inducing mathematical formulae that may make you want to poke your eyes out. Which one is best for my car? Where do I install them? How much power can I make? Like so many of life's nagging questions, the answer to all of these is, "it depends."
The short answer is you can make as much power as you can afford to spend. Want 2,000 hp? A turbocharger will certainly get you there, though you need an engine and drivetrain that will support it, though its operating range will be quite narrow. Here's a simple rule of thumb: Turbochargers can basically double the power output of a well-made stock engine. And that's just the starting point. 1,000hp builds aren't outrageous, especially on a modern V8, and the engine combination needed to make that much power is remarkably mild. With a pair of turbochargers, it is not difficult to build a 1,000hp daily driver.
In this article, we touch upon some of the major points to consider when planning a turbo build of your own, offer advice from the pros, and document some effective, homebuilt combinations. First, though, let's define some terms.
Inducer, Exducer, and Trim
Notice how the blades of the compressor and turbine wheels are narrow at the top and wider at the base? The calculation of the difference in diameter between the top and bottom of the wheel is the trim. Generally, a larger trim number means that particular wheel will move more air than one with a smaller number.
The terms inducer and exducer refer to the parts of the turbine and compressor wheels that face the inlet and outlet sides of their respective housings. On the compressor wheel, the small diameter is the inducer because it draws air in from the center opening. On the turbine wheel, the bottom is the inducer because exhaust gas enters from the side and exits through the center opening.
Area Radius ratio (A/R) refers to how much the volume of the housing decreases from the air inlet to the compressor or turbine wheel. In colloquial terms, think of this number as a calculation of how much smaller the volute (the "snail shell") becomes as it twists to the center of the housing. The area-ratio figure has the most profound effect on the exhaust side. A smaller A/R increases the velocity of exhaust gas to the turbine wheel faster than a larger A/R housing will. This affects how rapidly the turbocharger will spool up and start to make boost in the intake.