Originally Posted by stressdoc
Twin scroll is another way of getting boost at low rpm/pressure -- a small impeller with little inertia, and then a big one for serious boost (like the turbo in the diesel).
Sorry but this is not true. Twin-scroll just means that exhaust gases (from the six cylinders in this case) are split in two separate streams of flow (one flow from each 3-cylinder bank) which enter the turbine separately and spin the same impeller which has two sets of fins, one for each flow. The advantage is the higher pressure in each flow and the alternating exhaust pulses in each flow path (after a cilinder fires the exhaused gas from that cylinder creates and exhaust "pulse") which create the high turbine RPM faster than a single flow.