Plain Talk about Deception and Flow Numbers
February 13, 2012
by Harold Bettes
Airflow numbers without reference test pressure numbers are virtually meaningless. It might tell you more about the claimant than they wanted you to know.
We all want to pay attention to higher airflow numbers because we have learned that more power is often the result of improving airflow. However, airflow numbers expressed in CFM (cubic feet per minute) do not mean anything unless you also know the test pressure at which the airflow number was collected.
Many unscrupulous modifiers or manufacturers will flaunt a high flow number without referring to the test pressure that would quantify the flow in CFM. This deceptive practice is becoming more rampant in the marketplace as struggling businesses try to be competitive.
If you happen to see or hear that a cylinder head might flow 450 CFM, at first it sounds pretty awesome until you apply a little logical analysis. If the 450 CFM number was collected at 60 inches of water (‚ÄúH2O) what does that mean if everybody else is using 28‚ÄùH2O as the established test pressure standard?
Comparing various test pressures and airflow is very simple once you learn the simple rules that airflow follows. In the condition outlined above where 450 CFM was done at 60‚ÄùH2O the airflow at 28‚ÄùH2O would be only 307.4 CFM.
The arithmetic of comparison is easily done. The ‚àö(28/60) = .683 and then the .683 x 450 CFM = 307.4 CFM. This method of comparison is called the square root of the pressure ratio. Conveniently enough if you wanted to compare flow numbers gathered at 28‚ÄùH2O to 60‚ÄùH2O the multiplier would be the reciprocal of .683 or 1/.683 = 1.464 which would be multiplied times the airflow number taken at 28‚ÄùH2O.
So, if someone attempts to impress you with an airflow number always ask at what test pressure the number was collected or recorded. Then you can compare the data with your own calculations. That goes for carburetors, throttle bodies, cylinder heads and manifolds, or even flow benches.