Useful Calculations for Comparing Flow Numbers

The necessity of flow-number comparison is something that anyone involved in flow testing must endure. Even if the number comparison is done on the same components and flowbench, it is important to know how to compare the numbers so the time and effort is worthwhile. The comparison process is necessary to evaluate published numbers vs. your own developed flow numbers. In flow testing, you’ll learn that you must ask (or qualify) at what test pressure the flow numbers were recorded.

The SuperFlow instruction manuals provide a chart for comparing flow numbers from one test pressure vs. another test pressure. If your copy isn’t handy or has been misplaced, we outline the information you need to know below.

The chart in the SuperFlow instruction manual for flowbench operations is based on the square root of the pressure ratio method. If you have flow numbers at a known test pressure and want to compare those numbers at a different test pressure, it’s easy to do.

As an example, if you have flow numbers at 10”H20 test pressure and would like to know what the flow should be at 25”H20 test pressure, the formula is: TBD on how to present it. You would multiple the flow numbers taken at 10”H20 test pressure by 1.58 to see what the flow should be at 25”H20.

Predicting Horsepower based on Airflow Numbers

The performance coefficient SuperFlow develop dis based on very accurate empirical data, and even 30+ years after its introduction into the marketplace, it is still a good indicator of power capability of an engine based on its airflow. Today’s engines are more efficient because of airflow improvements that were generated by thousands of people searching for more power.

The prediction of horsepower based on airflow numbers can be applied if the test pressure is known. The results are a good estimate of the engine’s capacity to make power if everything in the system is optimized to take advantage of the airflow available. An accurate estimate of the power capacity of the engine is dependent upon having accurate flow numbers for the complete airflow system, including the cylinder head, manifold, carburetor or fuel injection system.

The power coefficient varies with the test pressure. The following can be used for a quick evaluation of airflow numbers at different test pressures.

The equation: HP/cyl=Cpower x Test Flow, where Cpower = Coefficient of power, Test Flow = cfm flow at the same test pressure that the Cpower is applied.

Cpower for:

  • 10”H20 = .43
  • 15”H20= .35
  • 25”H20= .27
  • 28”H20= .26

These numbers assume the engine is using gasoline for fuel.

Example: If you have system airflow numbers recorded at 25”H20 and the flow as 200 cfm, the calculation would be HP/cyl = .27 x 200 = 54HP/cyl. If you were working on an eight-cylinder engine, then 8 x 54 = 432HP capacity. This assumes that each port flows the same number. More accurate results can be applied if the same calculation is made for each port of the airflow is now the same.

Predicting Peak Power rpm based on Airflow Numbers

Predicting the rpm at which peak power will occur, based on airflow, is an additional useful way to evaluate airflow numbers and an easy way to see the effects of changing engine displacement.

The equation: RPM at peak power = [(Crpm) / (displacement / cyl)) x cfm]

Where Crpm = Coefficient for peak power rpm calculation, displacement / cyl = displacement per cylinder in cubic inches, cfm = cubic feet per minute from flowbench data taken at a given test pressure.

Crpm for:

  • 10”H20= 2,000
  • 15”H20= 1,633
  • 25”H20= 1,265
  • 28”H20= 1,196

These numbers assume the engine is using gasoline for fuel.

Example: Using the same numbers that were applied in the previous example for power/cylinder above (200 cfm at 25”H20 and the engine is an eight-cylinder engine with a displacement of 355 cubic inches), 355/8 = 44.375 cubic inches per cylinder. Applying the equating and solving for rpm at peak power, where RPMpp = (1265/44.375) x 200 = 5,701 RPM.

Just for fun, what would happen to this number if the engine was 455 cubic inches? Now the engine displacement divided by the number of cylinders yields an entirely different number. So, 455/8 = 56.875. Applying the equation for rpm at peak power, RPMpp = (1265/56.875) x 200 = 4,448 RPM.

Many applications involve a specific need to know the airflow of engine components and how the engine uses air. Applying simple equations can compare the airflow of components. Many relationships can be enhanced if the airflow is known including valve timing (camshaft selection), inertia tuning factors of intake, power per cylinder capacity and the rpm at which peak power will occur.

You can learn some very interesting things by studying airflow through engine components and the engine itself. Because the engine is a self-driven air pump, many of the characteristics of the engine are set by its capacity to flow air.

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