OK, if you want to play the semantics game let's get really techincal. I should have said "no additional constriction". All bends restrict airflow in the sense that the change in direction slows down the air stream and interrupts the laminar flow. Thanks for pointing out the obvious. The point you're having a hard time accepting is that you were wrong about the bends being a significant constriction due to the diameter reduction from non-mandrel bends. But that isn't the case - like I said, they aren't mandrel bends, but they are about as close as you're going to get without a mandrel bending machine.

Sorry dude, but you are wrong. A single 2.5" exhaust isn't even enough for a 400 HP gasoline engine (Mr. Vissard's formulas prove it - 2.2cfm x 400HP=880cfm and 880cfm/115cfm per square inch = 7.65 square inches - slightly more than the area of a 3" exhaust). Even if it were enough for a gasser it would still definitely NOT be adequate on a diesel. I'd be extremely surprised if you could even get a 3L diesel's output up to 400 HP if you're choking the exhaust down to 2.5".

Even the 6.9 Fords (170 HP) came with an exhaust system comprised of two 2.25" down pipes that Y in to a single 2.5" - and it SUCKED!I know they are over twice the displacement of the 3L, but they are also less than half the 400HP 3L you refer to. Simply upsizing the downpipes to 2.5" and everything after the Y to 3" made a HUGE difference on my first F250 diesel, and it wasn't even a turbo! Just changing the exhaust I picked up around 10-15 HP.

My current F250 with the aftermarket Banks turbo system has two 2.5" downpipes to a 3" Y up-pipe, and a 3" pipe from the turbo back, and that's way too restrictive IMO. Even though the non-wastegated turbo only makes around 8 PSI at 3000 RPMs with your foot on the floor and pulling a load uphill. I wish there were room between the bellhousing and firewall to put a 4" pipe from the turbo back and I'd do that upgrade in a hearbeat.

Unfortunately you have to "roll" the tranny tunnel seam on the firewall just to get the 3" in there, so a 4" pipe just ain't in the cards...

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown