Airflow Testing of Carburetors

Airflow testing of a carburetor consists of two measurements. First is the airflow itself, measured in cubic feet per minute (CFM). Second is the vacuum under the carb that pulls the air through the carb. This vacuum is more technically called the pressure drop, and is measured in inches of water with a device called a manometer. The scientific notation for this pressure drop is ΔP (delta P). The standard ΔP for 4 barrel carbs is 1.5 inches of mercury (1.5" Hg), which is equal to 20.4 inches of water (20.4" H2O). An airflow test simply consists of installing the carb onto the airflow bench with an appropriate adapter, holding the throttles wide open, raising the airflow until the ΔP equals 20.4" H2O, and reading the airflow amount. Yes, but it isn’t that simple.

Measuring the airflow is the easy part. Measuring the ΔP accurately is more challenging. The vacuum probe in the flowbench that measures the ΔP is supposed to measure just the vacuum. The poor choice of the type of probe used, and/or the poor placement of the probe in the flowbench, can allow the probe to be affected by air velocity and air turbulence, making the ΔP reading from the probe inaccurate. This inaccurate ΔP reading, in turn, makes the airflow reading inaccurate, and depending on the direction of air velocity and turbulence around the probe, the airflow reading can be dramatically higher or lower than the real airflow. The stock ΔP probe in my Superflow bench was seriously affected by air velocity and turbulence around the probe. So I replaced it with a different type of probe in a different location, and now the ΔP reading is much less affected by air velocity and turbulence.

Testing with the new ΔP probe and different carb to flowbench adapters still gave surprisingly different airflow results with the same carb. It became evident that the pressure recovery varied greatly from one adapter to another. When a carb flows air into a cavity (an adapter or a manifold plenum), the air recovers most of the pressure that it had when it was above the carb. The extent of this pressure recovery is determined by the airflow characteristics of the carb, the carb spacer (if used), and the intake manifold. Higher pressure recovery means that the air has transitioned smoothly from the carb to the manifold runners. If all of the changes in airflow direction can be achieved with a minimum of turbulence, then the total airflow will be higher, helping the engine make more power. The large variation in pressure recovery also brought up another question. How does the pressure recovery of any of these carb adapters on the flowbench compare to an actual racing intake manifold?             

To answer this question, I built a carburetor flow fixture that incorporates a single plane small block Chevrolet intake commonly used on 700 HP oval track engines. A new style ΔP probe is used to measure plenum vacuum, and another is used to measure the vacuum drawing air through the ports of the manifold. By having the actual pressure recovery of a real racing intake manifold, the flow testing of carbs and spacers is now much more realistic. As I had suspected, the pressure recovery of the manifold is better than some of the adapters I used before, and worse than others. My standard test for total carb airflow now consists of measuring airflow in relation to plenum vacuum with the carb bolted directly to the manifold (no spacer). Carb spacer testing and development takes into account the vacuum that the bench is pulling on the manifold ports. This way I can  measure the effects of carb spacers on the overall pressure recovery capability of the carb/spacer/manifold combination.  © Norm Schenck