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Technical Curiosities:
The Variable-Geometry Turbo

December 10, 2011 by Matt

VATN VGT VTG VNT Turbo Shelby Porsche Aerodyne Turbocharger Variable Geometry Area Nozzle Turbine

The variable-geometry turbocharger (VGT) has many names: Variable-area nozzle turbo (VATN), variable-nozzle turbine (VNT) or variable turbine geometry (VTG), to name a few. The name it’s given depends primarily on the company offering the technology—Holset, Aerodyne, Garrett or Porsche, respectively—but its principle of operation is the same.

Rather than employ a wastegate to vent excess exhaust pressure in order to regulate turbo output, and thus boost level, the VGT uses a ring of movable vanes which encircle the exhaust turbine. A computer- or analog-controlled servo alters the angle of the vanes in response to engine, turbo and driver demands, regulating boost level and turbo response far more quickly and seamlessly than an ordinary wastegate. Boost threshold, turbo lag and many other standard turbo disadvantages are greatly reduced or eliminated altogether, and the device enables the turbo to fulfill the promise of making a smaller, more efficient engine truly feel and act like a larger, more powerful one when called upon, without the low-end lethargy or surges that accompany conventionally-regulated turbo engines.

VATN VGT VTG Turbo Shelby Porsche Aerodyne Turbocharger Variable Geometry Area Nozzle Turbine Drawing Schematic Diagram

The VGT’s downsides include, as you might imagine, added complexity, and until recently a lack of durability from the delicate vanes at higher boost levels, when the temperature and pressure of the exhaust gas pouring into the turbine is quite intense indeed. Still, in spite of its complexity over a standard trap door wastegate, there’s still no more straightforward and adoptable alternative method of controlling boost; the VGT doesn’t require engine developers to redesign the entire powerplant to accommodate it—just a few piping changes, sensors and lines of code in the computer.

Among the first production cars to use a VGT was the limited-edition ’89 Shelby CSX-VNT, a breathed-upon Dodge Shadow whose standard 2.2l turbocharged Chrysler K-engine was mildly reworked and fitted with a Garrett VGT. 0-60 time was average for the day, in the mid 6-second range, but the engine’s full 205 ft-lbs of torque were on tap from an impossibly low 2100 rpm all the way through redline, thanks to the turbo’s unique method of regulating boost. Concerns over reliability meant only 500 CSX-VNTs rolled off the production line, and enthusiasts would have to wait another 18 years before another VGT-equipped, gasoline-engine car would appear in the States: The ’07 Porsche 997 Turbo, whose 3.6l, 473-hp flat-six was fitted with a pair of Borg-Warner VGTs. The advantages of the VGT—lack of a wastegate simplifying plumbing in the rear-engined car, and the chassis-settling boon of ultra-linear engine response—made the design a natural and long overdue fit for the 911. Of course, while we wait for more automakers to adopt the technology, aftermarket companies such as Aerocharger and Holset have been faithful to offer kits and turbo upgrades to tuner shops and ambitious DIYers.

Editor’s note: This post is part of an ongoing series spotlighting obscure automotive engineering solutions. Read the other installments here:

Filed under: Porsche, Shelby, Technical, Technical Curiosities

4 Comments

  1. gary edwards says:

    The first variable-geometry turbocharger was the honda legend in japan,designed and manufactured in 1988,(though officially released early 1989)and not a limited-edition ’89 Shelby CSX-VN

    • Matt says:

      Thanks for the heads-up! My sources say the rollout dates for both were pretty similar, but I’ve amended the article to allow for some ambiguity.

  2. Juhani says:

    Turbocharged diesel-engined cars had variable geometry turbos become pretty much the default choice in the late 90’s to early 00’s, before Porsche (re-)introduced them to petrol-powered cars in the 997 Turbo. At least in Europe, where diesel-powered passenger cars are much more popular than in Japan or North America.
    Apparently, diesel engines are less destructive environments to the finicky variable vanes than petrol engines, don’t remember why exactly, but the operating temperature is a main reason.

  3. Mercedes-Benz used this technology, too, years ago (OK, maybe the 1990s). Not sure if it was on a gas or diesel engine, but they definitely did so, maybe with a KKK turbo.

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