With the release of the 6.7L Dodge Cummins came numerous advancements in technology. One of the key mechanics of a Cummins engine was also totally redesigned, the turbo. With the 6.7L Cummins, Holset unveiled their Variable Geometry Turbo (VGT) with integrated exhaust brake.
The Holset VGT uses a turbine stage where the swallowing capacity is automatically varied while the engine is running. This permits turbine power to be set, providing sufficient energy to drive the compressor at the desired boost level wherever the engine is operating. This is achieved by varying the area of a "nozzle", which is a set of guide vanes that control the flow of exhaust gas through the turbine.
Conventional designs pivot the vanes to achieve different nozzle areas, such as on the Duramax trucks. The Holset VGT is different in that the vanes do not pivot side to side, but slide front to back, which ensures a high level of durability and reliability (less moving parts).
The sliding nozzle ring changes the opening through which the exhaust gases flow onto the turbine wheel. This change in the geometry of the turbo increases the boost as the nozzle is closed down. Reducing the opening increases exhaust manifold pressure and increases the turbo's speed. As the nozzle ring opens up, the exhaust pressure reduces and the boost decreases.
However, as with all things, there are pitfalls of new technology, especially when other things are altered. One issue that has become clear on these new trucks is that with even the few moving parts within the Holset VGT, they are susceptible to becoming "gummed up" due to excessive soot levels, causing the nozzle to become stuck in a single position. Also, as with all things "stock," they are only capable of so much, and in this case, airflow. Industrial Injection has become one of the first companies to begin to offer not only Hybrid type turbos that retain the variable nozzle, as well as complete single stage replacement turbos, and even TWIN TURBO applications.
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