The 911 Turbo has always been synonymous with performance. Now the car is more capable than ever thanks to a new twin turbo system featuring Variable Turbine Geometry (VTG).
On a conventional turbocharger, the exhaust flow drives a turbine that is connected to a compressor in the air intake tract. By ‘squeezing’ the incoming air, the amount of oxygen in a given volume is increased. Since compression also causes an increase in temperature, the air must be passed through an ‘intercooler’ unit. With more oxygen present in each cylinder charge, more fuel can be burned yielding greater energy. Since higher exhaust pressures generate corresponding loads on the intake side, the intake pressure must be carefully controlled in order to protect the engine. On the new 911 Turbo, the boost pressure is limited using wastegate valves that bypass excess pressure around the twin exhaust turbines.
Another important factor is the size of the turbo unit. Since a smaller turbine has a lower mass, it generally responds more quickly to increasing pressure, spinning up easily to its optimum speed. The key disadvantage of using a smaller turbo is that the back-pressure generated at higher engine speeds causes a significant reduction in performance. Resistance is caused by the smaller cross-sectional area through which the exhaust is required to flow. Larger turbo units, which create lower back-pressure at higher rpm, take considerably longer to spin up under power due to the large cross-sectional area and relative inertia of the heavier turbine. Generally, this type of turbo will only be effective in the medium rpm range. This phenomenon, known as ‘turbo lag’, means there is virtually no turbocharging effect at lower engine speeds. To overcome this problem, the twin water-cooled turbochargers on the 911 Turbo feature Variable Turbine Geometry (VTG). With this technology, the gas-flow from the engine is channelled onto the turbines via electronically adjustable guide vanes. By changing the vane angle, the system can replicate the geometry in all types of turbo, large or small.
With Variable Turbine Geometry (VTG), it is possible to achieve higher turbine speeds, and thus higher boost pressure, at lower engine rpm. Cylinder charging is significantly improved, with a corresponding increase in both power and torque. Maximum torque is reached at lower rpm and is retained across a wider rev range. A full 460 lb.-ft. is available from as low as 1,950 rpm up to 5,000 rpm. Every throttle input is met with exceptional response and phenomenal acceleration. When the boost pressure reaches its maximum value, the guide vanes are opened further. By varying the vane angle, it is possible to achieve the required boost pressure over the entire engine speed range. As a result, there is no need for excess-pressure valves as found on conventional turbocharged engines.
Matching the superlative performance of the car is the efficiency with which it is generated. In spite of the increase in power and torque, the 911 Turbo offers a further reduction in fuel consumption.
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