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Intake Manifolds sometimes include measurements by amount of flow per unit of time, and involve a lot of complicated air flow effects. How are these flow values measured and/or calculated?
edit: Apologies that this wasn't more clear, I'm not interested in how the engine determines the flow; this is rather straightforward and answered in other questions. What I want to know is: If a "performance" manifold is advertised as "X cubic feet per minute" of airflow, how was that value determined?
With modern engines, it is measured with a MAF meter, a device that meaures air flow by using a hot platina wire or film that cools down when air flows past it. The temperature is an indication of the air flow. Other cars approximate the airflow with the engine speed and the manifold pressure.
Carbs have many ways to do it. Constant depression carbs have a piston that uses the vacuum in the manifold to raise the piston accordingly. Other carbs don't use the manifold pressure of flow at all. They just have an acceleration pump that squirts in extra fuel upon throttle increase.
Most of the times, the air flow really is only used to determine how much fuel to inject, or how much to raise the piston in the case of SU carbs.
In labs testing engines, we used manometers (specially calibrated with constants), temperature etc and the gas equations to get mass or volume flow rates...
