SuperFlow recommends that one picks a standard test pressure (in inches of water). This makes all of one's results directly comparable. This is similar to applying the same voltage across a resistor to measure the current going through it.

SuperFlow also recommends that one takes readings throughout a range of valve lifts. Not just at the maximum lift of the valve. The most important part of the flow frequently occurs just as the valve is closing. The last bit of air on the intake makes the difference whether the engine obtains 90% volumetric efficiency or 100% or 110%.

To begin a typical test, one sets a cylinder head on top of a cylinder adapter, with a dial indicator on the valve. A threaded bolt-type device is used to push the valve open. Lighter valve springs are used to make the test-valve-actuation easier. The lighter spring must be stiff enough to keep the valve from being sucked open during the test.

SuperFlow recommends that engineers always use a radiused inlet guide when testing a cylinder head by itself. A radiused inlet guide has a rounded shape to guide the air into the test piece. A sharp edge at the port entrance will cause the air to flow in toward the center of the port rather than following the port walls. The edge acts like it is choking the airflow. The flow difference can be as much as 30% due to the sharp edge effect.

The radiused inlet guide directs the air straight into the port with a small loss, just as it would be if an intake manifold was connected. When an operator tests a head without a radiused inlet guide or an intake manifold, the results will be very different. So it is critical to use a radiused inlet guide. The radius should be about half the width of the port. For a 2" wide port, use a 1" radius radiused inlet guide as the minimum dimension. The same radiused guide should of course be used for all comparative tests.

Air is moved through the cylinder head by an air pump or blower. This is similar to a vacuum cleaner motor. Most flowbenches use more than one blower. The blower sucks the air in and blows it out at a higher pressure on the other side. These blowers will draw, depending on the model of flowbench, between 70" and 110" of water vacuum. Inches-of-water is a pressure measurement that is based on the height of a column of water that the pressure will support. For general reference, one atmosphere is 403" (10.3 m) of water. One psi (6.9 kPa) is 27" (68.6 cm) of water, approximately. By comparison, the more commonly used SF-600 flowbench only uses a pressure of 25" (63.5 cm), which is slightly less than one psi.

As air is drawn in through the port, the flowbench must measure the pressure drop. The test pressure meter measures this pressure drop. It does it by simply comparing the test pressure just below the cylinder to the atmospheric pressure. As the flowbench creates suction below the cylinder, it pulls on the fluid column and the amount of pull determines the height, and that indicates the test pressure. It works just like a pressure gauge. The column of water requires very little calibration, and it remains very stable over many years. Some flowbenches have been in operation for more than 20 years without service. This column of water is called a manometer.

A second manometer is used to measure the actual flow. The flow in the flowbench is measured by comparing the pressure loss across an orifice. This is a sharp edged orifice. It’s just a hole in a thin plate. The pressure drop across the orifice, as the air flows through it, is measured by measuring the pressure above the orifice and below it. The pressure difference is measured on a manometer again. This manometer has a slanted tube to read more than 0 to 6" (0 to 15 cm) or 0 to 13" (0 to 33 cm) of water on the bench. By inclining the fluid column, the scale is extended. So if the actual height is 13" of water, by slanting the tube on its side, the scale is effectively 25" long. The slanted manometer is easier to read accurately at low pressures.

It is important to maintain the same test pressure as the valve is opened. Open the valve from 10% of its diameter to 20% of the diameter, and the flow might go from 100 to 200 cfm. The test pressure will drop if one does not adjust anything else. Before yone was measuring the test pressure at 10", now it might be 8". You'll need to adjust the flow level back to the test pressure. This is what the Flow Control Knob does. It has a valve very much like the valve in the engine, with a thread on the end. The Flow Control opens and closes an orifice. Adjust the test pressure back to the standard test pressure for every valve lift. Then read the flow meter.

The flow meter displays percentage flow of the orifice maximum capacity. If an orifice has a maximum capacity of 100 cfm and the reading is 90%, the flow is 90 cfm. That principle is used in all the flowbenches.

The flowbench must be accurate through a wide range of flows. At very low flows, down at the low end of the flow meter, one cannot read it very accurately. It’s a small change compared to the total flow, so one loses sensitivity in flow measurement. To offset this, we uses multiple orifices. Sometimes using as many as ten ranges.

This provides good sensitivity at very low flows and high flows. On a SF-600 flowbench, there are 6 on the SF-1020 10 ranges from 25 cfm to 600 cfm. When running a test, select a smaller range for low valve lifts and a higher range for high valve lifts, making sure the maximum orifice range selected is slightly greater than the flow of the valve for best sensitivity. The FlowCom will do this electronically, and it has a much wider range. Consequently, one can use a single range with FlowCom for all the measurements.

Time is saved as the flowbench is able to flow air in both directions, because it saves moving the head around on the flowbench. To reverse flow, on some models, a system reverses the flow direction. There is a knob on the front of the bench, or levers, that change the flow to the intake or exhaust direction. One lever flips the connections to the manometers so that it sucks on the other end on each manometer.

The other lever changes the control plate in the flowbench to reverse the flow direction. Then the air blows backwards through the cylinder head. There is another control valve for either intake or exhaust test pressure control. To reverse the flow, close the intake knob and open the exhaust knob to let the exhaust flow occur and flip the manometer and flow direction levers. Then the air flows the other direction.

For an air flow motor of this type, the efficiency is only 50% to 60%. The rest of the energy becomes heat transferred to the air. As a result, the air gets warmer in the exhaust direction as it goes through the bench. The temperature does not affect the flow reading because the flowbench measures the ratio of the pressure difference across the orifice, to the pressure across the valve.

Both the orifice and the head see air at the same temperature, density, and humidity. A flowbench of this design provides results, which are independent of the atmospheric conditions. In fact, if one puts a cylinder head on the flowbench, and run it at a barometric pressure of 24" of mercury, and run it later at 29" mercury, with the same cylinder head, one will get exactly the same flow numbers. We don’t have to correct for temperature or correct for pressure.