Flow Bench Fabrication
As a longtime reader, I remember Smokey writing about using a GMC 6-71 supercharger to run a flow bench. I would like to build a flow bench, and I have a 6-71. Before I start this project, I need to know the following:
1. How fast the supercharger needs to spin.
2. The displacement of a 6-71 supercharger or another way to find out the cfm capability of the supercharger.
3. What depression (inches of water) I should expect.
4. The disadvantages of using a supercharger versus a lot of small vacuum pumps.
Thanks for your help.
Via e-mail
Before addressing these four questions, it may be helpful to share some initial comments about air-bench building.
You should determine the purpose for which the bench will be used. For example, do you plan to flow an entire cylinder head at one time, complete induction systems or just individual inlet paths, flow capacities of mufflers, or entire engines? The amount of pump you need is largely dictated by the amount of air to be measured. Early SuperFlow benches provided pressure drops on the order of 5 inches of water. One scholarly old airflow instructor I once had asserted, "You can't tune a summer breeze!" Since you aren't precisely duplicating unsteady state engine airflow on a steady-state bench, measurement sensitivity includes the ability to move sufficient air volume.
Now the 6-71 blower you are contemplating should be sufficient for most any airflow work. Depending upon how precise you want data to be, the use of an airflow meter to measure the blower's output will provide you good preconstruction information. Without this, a blower speed of about 4,000 rpm will be more than adequate. There are some opinions that suggest flow data need not be cfm-specific since correlation to a running engine is, at best, just a correlation.
Depending on how you configure the airflow meter (or orifice plate size), a working depression of 22 to 28 inches of water is common among many experienced bench operators. This differential will provide adequate flow measurement sensitivity to test piece changes (ports, heads, manifolds runners, and so forth). I suggest you apply correction factors that include ambient air temperature. You will probably find that variations in inlet air temperature have more impact on data than barometric pressure. Additionally, depending on how you may or may not try to regulate air temperature in the bench's environment, you will probably find that supercharger to be a significant contributor to inlet air temperature. All the more reason to correct for this influence.
While you didn't indicate how a "lot of small vacuum pumps" can be configured, chances are good you'll be better off using the supercharger--for a variety of reasons.
Jim McFarland
AutoCom Contemporary Auto-motive Communications & Research