S5 Intake Manifold vs Throttle Body Bore

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Diameter of Primary Runners After Upper Plenum: 1.125"
Area of Primary Runner: 1.987"
Area of Both Primary Runner Combined: 3.975"

Diameter of Secondary Runners After Upper Plenum: 1.468"
Area of Secondary Runner: 1.692

The Cross-Sectional AREA of one TB bore is about 1.792 with the shaft. you can see that the combined area of the primary intake runners is bigger then the TB bore, even further more when you include the area of the plate shafts. You can see there is some room for improvement.

The Intake ports on upper plenum are 1.777" and get reduced down to the size above. The Upper Plenum internally is shaped like 4 velocity stacks.

To remove airflow restriction you can remove some material from the plates shafts to potentially gain more airflow. You can also have your TB ported before the plates for a velocitizing effect and potentially gain more, its not cheap to have it done though.

Please see [86-91 Throttle Body Shaft Modification]

See Also

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[FC Community Service Manual]