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With todays turbos,transmissions w/high stall & lock-up converters,you can have a very streetable car,quick turbo spool up, that can also be very fast w/a single turbo.
Depends on what your goal is.
Bragging rights ,saying you have a twin turbo 6?
You want the best of both worlds,very mild manner street car using the small turbo & the larger turbo takes care of the mega HP for the top end?
For the most part,a well planned out single turbo system is all what most people only need & will satisfy you very well.
MBHD
