The neck makes a major improvement to the flow because the air from the turbo is not laminar, it exits the turbo in a "corkscrew" flow like a tornado. The sharp crossectional change at the step up coupling slows the circular velocity quite a bit, making the turn into the IC much more efficient.
On the boost gauge the improvement typically results in almost TWO pounds more boost at the manifold. Which brings up another cheap improvement some of the newbies may not be aware of.
The wastegate port is on the outlet of the turbo, if you move it to the upper plenum on the manifold, the boost rises by a few more lbs, that's the difference in restrictions in the IC, t-body, and plumbing between the turbo outlet and manifold. If you leave the wastegate port connected to the turbo while you make mods to improve the air transitions in ALL of the interconnecting plumbing, (sharp edges on both ends of the up pipe, IC outlet, t- body inlet, slimming the TB shaft, rounding the leading and trailing edges of the throttle blade, rounding/blending the upper plenum transition from the TB to the inside of the plenum, etc.) you can see the results of your work in increased boost! THEN plumb the wastegate hose to the upper plenum (middle of the passengers side, drill and tap a 1/8" pipe thread for a hosebarb) and enjoy the "free" boost gain! Doing all of these mods usually gains another few lbs.of indicated boost. But with the wastegate hose connected to the plenum, you wouldn't see the difference, but the "butt dyno" will show it in transient response and a bit of HP improvement due to cooler air from less turbulence.
One HP in 10 places, is still a 10 HP gain....
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