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 (“H2O) what does that mean if
everybody else is using 28”H2O 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”H2O the airflow at 28”H2O would be only 307.4 CFM.
The
arithmetic of comparison is easily done. The √(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”H2O to 60”H2O the multiplier would be the reciprocal of .683 or 1/.683 =
1.464 which would be multiplied times the airflow number taken at 28”H2O.
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.
