Items that are conduits for air flow, like manifolds and automobile cylinder heads, often need to have the air flow efficiency checked for quality control purposes, or can often be improved in some way. Since air flow improvements through internal combustion racing engines often lead to higher power and efficiency, a flow bench can help determine the best method for improving that air flow.
Internal combustion cylinder head intake ports are usually the best place to improve air flow for the off road racer because of the constraints of casting such an item at a reasonable cost. High volume castings often lead to compromises on the ideal port over the port that actually leaves the factory. By reworking the factory cast ports by hand using a hand grinder with rotary stones and carbide bits, the factory cast ports can often flow air much more efficiently. This is often referred to as "Porting" or "Porting a head".
While porting a cylinder head can often lead to improved air flow efficiency for racing engines, this is not always the case. Working areas of a port that do not need improvement can lead to less flow and lower velocities in a port. That is why top racing engine builders employ a flow bench in not only determining the final result of their work, but as a valuable tool during the porting process to determine the areas of a port that need improvement, and what areas should be left untouched. A racing engine builder may flow test a single port 200 times or more while determining the optimal shape and contours for that particular head casting and valve

This info is from Frequently asked questions at the Flow Performance website: http://www.flowperformance.com/faq.html#bookmark3

Pictures are from K&W Equipment shop with Flow Performance Data Acquisition.


Cabinet with Fixture for Holding Heads and Computer Data Acquisition

Screen Shot of Flow Performance Data Acquisition

Head in Place to be tested with Dial Gauges for Flows vs. Valve Opening

Another Way of Accessing Flows