Turbochargers come in all sizes and types to meet all needs. Today's turbos may include design elements engineered to reduce turbo lag, increase longevity and boost performance through material science, ball-bearing center sections and even variable geometry. Turbos are as advanced and refined in their designs as any engine to which they are bolted.
The sort of turbochargers traditionally used on automobiles and trucks all follow the same basic design parameters. The exhaust-side compressor housing (scroll) is made of some sort of cast iron or steel for longevity under extreme temperatures, and the "cool-side" compressor housing is made of aluminum. These scrolls use a single large inner channel, against which air is compressed by the turbine blades.
A standard turbo uses investment cast turbines on the intake and exhaust side that are tuned to be most efficient through a set RPM range. The center shaft joining these turbine blades is suspended by a solid bearing similar to those found on rod or main bearings. The bearing is cooled by engine oil.
Almost all modern performance superchargers (like the Garrett GT Series) use some sort of ball-bearing center section in place of the original solid bearing. These ball-bearing center sections allow the turbine blades to spin more freely, which decreases low-RPM turbo lag and increases the turbo's high-RPM potential. Ball-bearing center sections also produce less friction, which helps the turbo to run cooler and last longer.
Some turbochargers (like those used on older Mitsubishi Evolutions) have a center divider running up the middle of their exhaust-side compressor housing. This effectively divides the scroll into halves, which forces exhaust gases to move through the channels faster. These high-velocity exhaust gases push harder on the exhaust impeller at low RPM, which greatly reduces turbo lag.
One drawback of traditional turbo design is that the exhaust-side directional housing blades are cast into one solid form and represent an inherent compromise. On one hand, blades that angle straight out from the center will offer fantastic high-RPM efficiency but will take a long time to spool up. More sharply angled blades get moving very quickly, but inhibit flow and will not spin very fast.
Modern "variable vane" turbochargers use a lever system similar to those found in old mechanical distributors to alter the vane angle electronically. These turbos were first used on the Dodge Shadow-based Shelby CSX and are currently in use on 3.6L Porsche 911 Turbos.
