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It lowers losses to have larger tubes up to the throttle body. The pressure losses due to friction in a tube are directly related to the speed of the flow running through the tube.
Set the throttle body at something, 76mm, then run a 76mm tube 3 feet up to the throttle body and pull a vaccum. Measure the speed of the air at the inlet of the tube. Now run exactly the same experiment but start with a 100mm tube that gradually drops to 76mm at the throttle body. The speed of the air at the inlet is lower. If you look at the flow rate through each of these for a given vaccum level, the decreasing tube will have a higher flow rate. It's analogous to running a set amount of electrical current through a fat wire or a thin wire at a specific voltage. The thin wire heats up more due to resistance.
I think I showed the attached pictures before of flow entering a "T". When the center stream of the flow leaves the entering section and hits the back wall of the T, it stops or stagnates. The flow along the sides of the entering tube is pressed out by the stagnated flow from the center of the tube and makes it's way out towards the outlet of the T. In other words, the flow makes it's own wedge. Adding a wedge, without many, many experiments or fluid modeling, is more likely to hurt than to help with the airflow.
The more interesting thing about the picture is the big "dead" zones as the flow near the walls leaves the entering tube. This is known as separation. The tube in the T section may as well be half as large because the air isn't doing anything in the corners. If the entering tube where rounded as it merges with the T, the air could more easily follow the contours of the transition and use more of the volume of the tube. There is a potential for a HUGE benefit to airflow.
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