Originally posted by Mr 2
Aftermarket parallel twins are actually less efficient than a single with the same potential outputs. The reason being is that there is more disturbance in 2 exhaust housings than one. The disturbance which causes velocity losses is the inside surface area of the housing. Air hitting the housing is slowed and since twins have twice this area there is more exhaust velocity lost.
When you go parallel twins, you have to remember you've only got three exhaust ports per turbo as opposed to the six ports for the big singles. Obviously, six ports create more exhaust energy than three, but the bigger heavier big singles offset this somewhat. With twins, two smaller turbos are doing the job of one large turbo and, apples to apples, the twins should spool quicker and, together, generate as much top-end power as the big singles.