August 25, 2000


Going with the Flow
Flow bench numbers 101

You have read the hot rod magazines, heard your friends swear flow numbers out of their mouths, but now it is time to get the real deal on what flow data information represents.  With the help of a friend, Tom Deskins, a few friends and a class on fluid dynamics from my college days,  I was able to piece together an explanation the rest of us could understand.

When a part is tested on a flow bench there are two numbers that are going to be reported:  the rate at which the air is flowing through the part and the pressure drop across the part. The flow rate at which air flows through the part is given in cubic-feet per minute (cfm).  The pressure drop at the time where the flow rate is recorded is given in inches of water.  Put the two together and you have the flow bench information.

Flow rates are understood by most people, but a short explanation is in order to make sure everyone is on the same page.  A flow rate is a combination of two things, volume and time.  Flow rates tell you how much of a substance is moved over a set time period.  Since we are talking about air flow through car parts the units of measurement will be done in the same units that are commonly used in these applications,  cubic feet per minute.

The pressure drop is the second number that will be reported.  This is simply the amount that the pressure decreases over the distance of the part.  Another way to think of this is the difficulty the air had flowing through the part.  The harder the air flows through the part, the larger the pressure drop.  Most flow benches have a gauge that reads the pressure drop in inches of water.  

Take a look at the following fabricated flow bench information to use as an example.  This part flowed 500 cfm at 12 inches of H2O.  Another way to read the information is your part flowed 500 cfm with a 12 inches pressure drop.  The 500 cfm is simply the amount of air moved over one minute in time while the 12 inches in H2O represents the pressure drop across the part. 

In most cases, when you get a part tested on a flow bench you have to determine under what conditions you want the part to be tested.  This usually results in the person with the test part to choose a pressure drop at which they want the test to be performed.  For example:  I want to have a throttle body flowed on a flow bench.  I would ask that the throttle body be flowed at a certain pressure drop.  Some people choose 28 inches of H2O and others choose a different pressure drop such as 12 inches of H2O.  The person running the bench would increase the flow rate until the pressure drop across the throttle body was 12" of H2O.  The operator would then record the flow rate.  The result would look something like this; the throttle body flowed 500 cfm at 12 inches of H2O.  If the part were flowed with a greater pressure drop, i.e. 16 inches of H2O, the flow would increase.  The opposite would be true if the pressure drop was decreased.

That covers the basics for understanding what the data from a flow bench represents.  Now, you should have a good idea of what the numbers represent and how the numbers are obtained.


Author:   Eric Barger
Editors:  Tom Deskins & Kelly Barger

- Eric Barger

Works Cited

1.  Deskins, Tom.  Interview.

Web Author: Eric Barger
Copyright 1999 - 2002  Eric Barger.  All rights reserved.
Revised: June 07, 2007.