This machined section on the...
This machined section on the exhaust outlet adapts the Aurora's turbine housing to work with the factory exhaust downpipe.
As such, the Aurora 2000 model offers some substantial improvements over the stock turbo. The ATS Web site's data charts indicate the Aurora's airflow is 877 cfm versus 555 cfm for the Holset-roughly a third more. The output of the factory turbo is 295 hp, while the Aurora can produce 466 hp (depending on other modifications to the injectors, intake, and exhaust-all of which had already been done on this particular truck).
Spool time is also reduced with the Aurora, dropping from 6.1 to 3.7 seconds. Interestingly enough, even though the airflow and potential power-output numbers are much higher, the increase in max boost is not that much greater. The stock turbo can make around 35 psi, and the Aurora is good for 40 psi. That's because the Aurora works smarter-not harder-by increasing the compressor's output. We won't get overly technical about the internal design of the compressor wheel; it features several improvements in the blade shape and angle along with the diameter and housing.
The diameter of the compressor...
The diameter of the compressor discharge outlet opening on the Aurora (left) is bigger than the stock turbo as well-211/42 versus 2 inches.
"Small changes in size can make huge changes in the compressor," Cannon says. "(The Aurora 2000) also produces less heat for a denser air charge and more airflow."
Another significant difference in the Aurora line of turbos is what it doesn't have: an internal wastegate. Typically, this feature is used to relieve backpressure and also allows for a smaller turbine housing. How did ATS dispense with this item?
"The turbine wheel and housing are matched to the volumetrics of the engine," Cannon says. "They absorb all the energy of the gas for more efficiency." With less restriction in the turbine housing, no wastegate is needed, so there's more flow and less drive pressure. This configuration, however, requires meticulous matching of the turbine and compressor and a different type of internal design.
 |  Diameters also make a big...  Diameters also make a big difference. The Holset compressor inlet opening measures about 2 inches, while the Aurora is 3.5 inches. |  While the overall size of...  While the overall size of these turbos is roughly the same, the Aurora's turbine housing is clearly bigger and can flow more exhaust than the factory housing. The factory turbo requires a wastegate to bleed off exhaust-drive pressure that would otherwise overspin the turbo. |