The manual gearbox delivers 24 mpg on the highway test, and the Tiptronic S version 23 mpg. · Despite its segment leading performa

The manual gearbox delivers 24 mpg on the highway test, and the Tiptronic S version 23 mpg. Despite its segment leading performance potential, the Turbo Cabriolet does not carry a gas guzzler penalty. The Turbo Cabriolet is also certified as a Low Emissions Vehicle, category two (LEV-II) thanks to its engine's efficiency. Besides the twin turbochargers, the 3.6-liter boasts Porsche VarioCam Plus variable intake timing and lift system under the purview of its Motronic® engine controls. Power on tap, instantly Key to the turbocharged engine's potent efficiency is the use of variable turbine geometry (VTG). Essentially, VTG uses constantly adjustable turbine blades to vary the turbine's cross-section precisely to the exhaust flow regardless of engine load and/or speed. At low rpm, the blades close to form small air gaps. This causes the exhaust gases flowing through the small cross- section to accelerate, spinning the turbine quickly—as though it were a small, low rotating mass turbocharger. At higher rpm, with its inherently greater exhaust-gas flow, the turbine blades open, increasing their cross-sections and simultaneously lowering turbine speed, maintaining a constant intake charge pressure. The full range of movement of the turbine blades takes merely 100 milliseconds and is controlled by the engine's Motronic® electronic control module (ECM). Action of the variable turbo vanes is so precise that there's no need for a turbocharger bypass valve to limit maximum boost. The intake charge from each of the engine's two turbochargers is cooled by a dedicated intercooler, each located ahead of a rear wheel, as has been a Porsche Turbo hallmark. The result of this sophisticated engineering and complex manufacturing is a turbocharged engine that responds to the driver's demand for more power immediately. There is no time lag between stepping on the throttle and the expected rush forward of boost. In fact, the VTG delivers its rewarding surge of power as quickly as an engine with a mechanically driven supercharger, but without the penalty of the supercharger's parasitic power drain on the engine. More for those who want it To ensure the expected high level of engine response and long life, maximum intake pressure is set to approximately 14.5 psi, or 1 bar. The optional overboost feature briefly allows approximately ten-second bursts of 17.4 psi or 1.2 bar at the touch of a console-mounted button. On-the-road tests show that with Overboost, 50 to 75 mph (80 to 120 kph) passing time—without downshifting from second highest gear—is just 3.6 seconds, vs. 3.9 seconds without the use of overboost. Porsche engineers measure boost levels at sea level, under full load and at an ambient temperature of 68°F (20°C). Keeping it cool Though the VTG concept has been applied to diesel engines for some time, it has not been commonly used with gasoline engines due to the higher heat of gasoline exhaust. Diesel exhaust temperatures are much cooler than gasoline engine exhaust, which places far less thermal stress on the turbocharger's components.