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  • Sequenta-What? - Under Pressure

Sequenta-What? - Under Pressure

Serial-Sequential Turbocharging

Jason Sands
Jan 21, 2014
Photographers: Jason Sands
If you’re a diesel performance enthusiast, you’re probably already familiar with what a single turbo is, what compounds are, and even how variable-geometry turbos work. But the times, they are a changing. The ’11-to-current 6.7L Ford has a unique turbo that employs two compressor wheels and one exhaust wheel, which doesn’t really fit into any of the above categories cleanly. It still has a variable exhaust side for quick boosting, and when it comes under boost it arguably hits harder than any of the other trucks on the market today. The idea was to package the flexible and quick-spooling nature of parallel twin turbochargers (one turbo for each engine bank) into an easy-to-package and cost-effective combination.
These out-of-the-box turbocharger ideas are getting more and more popular. Recently, when the Cummins ISV5.0 was announced, our prying eyes deduced that it will most likely have “two-stage serial-sequential turbocharging,” in the Nissan Titan.
Huh? What’s That?
It’s actually not as bad as it sounds. The “two-stage” part of the system refers to the fact that there will be compounding involved, as in, a larger turbo will blow into a smaller one, and the turbos’ pressure ratios will be multiplied. The “serial-sequential” part refers to the fact that there are two different-sized turbochargers, and one of them is bypassed at a certain rpm.
"Most people will dismiss odd compressor arrangements…"
How it Works
In a standard-sequential system, one small turbocharger is used from idle to about halfway through the engine’s powerband. At a certain rpm point, exhaust energy is diverted to a larger turbo to spool it up and to wastegate the small turbo. Then, at high rpm and full power, the smaller turbo is bypassed completely, and the larger turbo takes over.
In a serial-sequential system, both turbos start out in a compound relationship, with neither turbocharger being bypassed. With this arrangement, high boost levels can be created at part-throttle situations, which results in improved emissions, fuel economy, and towing power. Since the smaller turbo is so tiny, however, it becomes a restriction at full-throttle, high-rpm operation, so it is bypassed completely, and the engine runs solely on the larger turbocharger. Since it only needs to make 300-ish horsepower from 5.0L, a single turbo can create more than enough boost pressure for optimal performance.
Best Setup Ever?
Certainly, the 6.4L Power Stroke and 3.0L in the BMW 335d have some pretty killer compressor arrangements, but if our information is correct (which Nissan and Cummins are keeping quiet about, by the way), the setup on the new 5.0L could take the cake. We should also note that “best” is all relative; in an industrial-type application, we’re sure the single VGT turbo on the commercial version of the 5.0L Cummins works just fine.
A Brave New World
OEMs are progressing past simple single-turbo setups and into new and exciting systems that could change the industry. We’ve already heard rumors that one of the Big Three might be using a super-turbocharged arrangement, which could provide even better response than any turbo setup.
It’s unclear whether the aftermarket will follow and start marketing new and unique turbocharger setups, or whether the number of modifications and controls such as custom housings, billet wheels, and wastegates are already doing the job for enthusiasts.
While many people will dismiss odd compressor setups with a shrug of their shoulders, you never know, a super-turbo, series-sequential, or two-stage serial- sequential setup could be just what the doctor ordered for your Power Stroke, Cummins, or Duramax.

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