The carburetor is bolted to a large vacuum chamber. The vacuum in the chamber is controlled, and airflow is measured through the carb with the butterflies wide open.
For testing two barrel carbs, the vacuum in the chamber is set of 3". For testing four barrel carbs, the vacuum is set to 1.5".
If you took the same carb, and tested it at both 1.5" of vacuum and 3" of vacuum, it would flow more air at 3" of vacuum. More vacuum equals more airflow; this should be fairly intuitive. So, this is why you need to convert the numbers, so that you normalize the airflow between the two different test methods.
There is no such difference in any kind of testing on the dyno. CFM depends on the pressure drop across the carb, and will be greater at 3" of vacuum than it is at 1.5" of vacuum. So, CFM is not CFM, because it is dependent on other variables. As a measurement, however, torque is torque; it can't vary with conditions the way CFM can. However, please note that the torque output of the engine CAN vary with conditions, such as temperature and humidity. That is why a dyno has a built in weather station, to calculate a correction factor so that test results taken in different weather conditions can be fairly compared.
Jay Brown
1968 Shelby GT 500 Convertible, 492" 667 HP FE
1969 R code Mach 1, 490" supercharged FE, 9.35 @ 151.20, 2007 Drag Week Runner Up, Power Adder Big Block
2005 Ford GT, 2006 Drag Week Winner, 12.0 Daily Driver
1969 Ford Galaxie XL, 460 (Ho Hum....)
1964 Ford Galaxie 500, 510" SOHC
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