Don't Get Blown! (Part 2)

Turbine "Trim" describes the relationship between the size of the inducer (outer, driven edge of the turbine wheel) and the exducer (smaller side of the fan through which exhaust gases exit the turbine housing). Engineers vary the ratio of the two to balance the turbine's ability to spool quickly versus the high-speed shaft horsepower the turbine can ultimately deliver to drive the compressor and make boost. (Source: Maximum Boost, by Corky Bell)

Four cylinder engines--which only produce exhaust pulses every 180 degrees of crankshaft rotation--need every trick in the book to improve turbine performance at low engine RPM. 205-horse 2.3L turbo Mustangs used an extra-low first gear for better torque multiplication compared to the 210-horse 5.0L V8. MR2 Turbos used a twin-entry turbine housing to boost turbine performance. VATN turbos have been effective on Chrysler 2.2L fours.

The parts of a turbocharger. It is critical to understand that the compressor and turbine are two completely separate devices linked by a common shaft. (Source: Maximum Boost, by Corky Bell)

Garrett electrically-assisted turbocharger, designed to eliminate lag in spool-up to boost.

Same turbo, different boost. Nothing changed in these Project MR6 chassis dyno pulls but the maximum level of boost permitted by a computer-controlled wastegate.

1991 V6 MR2 "Project MR6" produced a selection of horsepower runs: naturally aspirated, supercharged via TRD blower kit, turbo-only, turbo-only, higher boost, and then compounded with both blower and turbo. Figures are rear-wheel horsepower, typically down by at least 15 percent due to drivetrain and tires losses.

Schematic of a turbocharged engine. Exhaust drives the turbine, which drives the compressor to force more air in the engine so it can burn more fuel. This creates more exhaust to drive the turbine, which provides more power to turn the compressor to make more boost. To some degree, the more power a turbocharged engine makes, the more it can make. (Source: Maximum Boost, by Corky Bell)

Garrett Variable Area Turbine Nozzle (VATN) turbo reduces turbo lag with dynamic variations to the area through which exhaust gases enter the turbine housing. VATN turbos accelerate exhaust gases through small nozzle size when exhaust energy is low to quickly spool the turbo, but increase nozzle size as exhaust energy increases with boost and engine RPM--thereby reducing backpressure as an adverse side effect as engine output increases.

Solids-modeling of a Garret compressor wheel/rotating assembling. Computer-Aided Design and computer-controlled five-axis milling machines enable the wheel castings to be finish-milled with little human intervention.

Toy Poodle and Great Dane Garrett turbos. The goal is not too big, not too little, but "just right."

Turbo system schematic. It is critical to understand that the compressor and turbine are close together, but completely separate devices that do not pump between each other except through the engine.

Garrett Variable Nozzle Turbo (VATN). Note the row of vanes surrounding the turbine wheel, which can be rotated to greatly change the speed of exhaust gases contacting the turbine wheel. The VATN turbo is designed to eliminate the tradeoffs in turbine housing nozzle (A/R) of conventional turbochargers.

Turbo schematic, showing oil moving through the center section to lubricate twin bearings (which can be journal or ball bearings). The same basic "frame" turbo design can support a variety of compressor and turbine wheels, and a selection of turbine housing with various A/R turbine nozzles sizes.

Garrett aluminum compressor wheel de-installed from the turbine shaft and wheel, which are friction-welded together. (Source: Garrett)

Garrett GT12 and GT60 rotating assemblies, illustrating the scalability of the centrifugal compressor and gas turbine: The GT60 (larger), capable of 1450-2000 horsepower, has up to 40 times the pumping capacity of the GT12 (smaller), which is good for 50-125 horsepower.

Cutaway of Garrett TR30 racing turbocharger. Note the V-band clamp that hold the compressor and turbine housing to the cartridge, making for quick, easy, repeated removal under racing conditions.

Garrett twin-entry turbine turbocharger. This design can be effective at improving spooling performance by increasing exhaust energy at low RPM on engines with fewer cylinders by isolating the exhaust pulses from adjacent combustion events until they actually contact the turbine wheel.