not sure I understand this, but I've noticed some Audi's driving around town. usually the higher model Audi A8's (Upper 60K+) are badged "supercharged" but also most definitely have a turbo....explain?
Compound turbocharging (or compound super/turbo charging) is a great way to get the benefits of a small turbocharger / supercharger, as well as the benefits of a big turbocharger / supercharger.
Think of it this way - if a supercharger makes 8 psi of boost by itself, it's boosting the air feeding it at a ratio of 1.5:1. If you add a turbocharger that also has a boost ratio of 1.5:1, then you've now boosted the intake charge at a ratio of 2.25:1.
The benefit is you can run a fairly small second booster that spools up fast and would usually run out of airflow for the motor. Because when that small supercharger starts to run out of steam, the big turbo will begin to boost the air entering the small supercharger. The supercharger will continue to compound the air it's fed, therefore 'keeping up' with the big turbo.
Matching the big and small turbo is the tough part.
links:
http://www.atsdiesel.com/ats2/templates/template_06.asp?p=2029422272&c=36
Wikipedia quote:
Compound Turbos[edit source | editbeta]
Compound Turbo Charging (One turbo compressor wheel feeding the other) Compound turbocharging is not the same as twin turbo charging. A compound turbo utilizes different sized turbos feeding off one another to do the job of one turbo. Air enters the low-pressure turbo (the larger of the two) and is fed into the high-pressure turbo (the smaller of the two), then directed into the engine or intercooler. This is why this type of turbo charger arrangement is referred to as Compound Turbo System. Compound Turbo Chargers work in series. Multiple turbo chargers are also commonly referred to as twin turbo chargers; twin turbos are just that "twins". The two identical sized turbo chargers are the same in nature or identical is size and work in parallel. Typically each turbo is used to do one half of the work and is not in a compound configuration. The air that is compressed into the engine is not fed from one turbo compressor to the other; the air that goes to the engine is simply split between the two of them.
The compound turbo charger set up offers all of the attributes of a small and large turbo charger without the negative effect of either one of them. There is a small turbo charger and a large turbo charger plumbed in series to one another. The small turbo charger is referred to as the high-pressure turbo charger and the large turbo charger is referred to as the low-pressure turbo charger or the atmosphere turbo charger. In a "Compound" turbo charger set up the small turbo is responsible for generating quick turbo response and rapid air flow right off idle. As the engine is accelerated from idle the small turbo begins to produce boost immediately while the large turbo slowly starts to produce a positive pressure to the small turbo. The large turbo when matched properly should be supplying boost pressure to feed the small turbo shortly after the small turbo is making boost in to the engine or intercooler. If the large turbo charger is not matched properly it will cause the small turbo charger to stall and the low-end performance will suffer.
Example: 2008-2010 Ford Super Duty (6.4L Power Stroke diesel engine)