Originally Posted by rcounts

Actually wouldn't that be 1.5 L per rev since each cylinder only fires/exhausts every other rev?

However, at 3000 RPMs that's still 4500 LPM, or roughly 159 dubic feet per minute. That's still a fair amount of volume to have to push through a pipe with a cross section of just under 5 square inches (pi x r squared = 3.14159 x 1.25 x 1.25 = 4.91). Whereas with a 3" pipe you're pushing that same 159 cubic feet per minute through a pipe with a cross section of just over 7 square inches (3.14159 x 1.5 x 1.5 - 7.07). While 2.16 square inches difference may not sound like much, it is roughly a 44% increase (2.16 / 4.91 = .4399).

Diesels run best with NO back-pressure at all (as MTUPower said - a vacuum on the exhaust would be better still) so 44% increase in flow area = a 44% decrease in back pressure. Gotta' be some benefit to that - though you'd need to run the 3" all the way to the outlet of the turbo to get the most benefit out of it...