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Racing is a competitive sport that requires much more than superior mental and physical ability on the track. Having a race car, or for that matter, an engine, that is developed to its ultimate potential for that class of racing can make an OK driver look great and a good driver untouchable. This is why teams in the top forms of racing have engine departments that are constantly developing and testing engine components and engines for more power and durability.

One piece of equipment that is required in these engine shops is a flowbench because it can help an engine department document whether changes in the combustion chamber, intake, and exhaust tract improve the ability of the engine to get more of the air/fuel mixture in the cylinder and therefore make more power. Because flow-benches are used by race engine shops across the country this article shows how a flowbench works, how most engine development people test with a flowbench, and how to interpret the flow numbers to help you become a more educated customer and a more successful racer.


A flowbench measures the resistance of airflow through a passage by either sucking or blowing air at a specific pressure through that passage. On intake tract passages like carburettors, intake manifold ports, or cylinder head ports, the air is drawn through the passages into the flowbench, while on cylinder head exhaust ports or headers, air is blown out of the flow-bench. The pressure of the air flowing through a part is measured in "inches of water" on a flowbench. This is measured on a manometer, often called the test pressure meter and resembling a large thermometer. It is filled with a coloured fluid and placed vertically with the lower end open to the atmosphere and the upper end attached to the cavity at the base of the test fixture.

With the flowbench turned off, the test pressure meter should read zero, but when the blower motor is turned on, the fluid will rise up the meter, which has gradations spaced an inch apart-thus "inches of water." The number of inches the fluid rises up the meter correlates to the lack of pressure, commonly referred to as vacuum, in the space below the test fixture. A control on the flowbench, called the flow control knob, opens and closes a valve that will vary the pressure differential or "inches of water" across the port being tested. An inclined manometer, called the flow meter, is used to determine the percentage of flow the passage being tested allows. By changing the size of the opening, called the orifice plate, between the passage being tested and the blower, the maximum quantity of cubic feet per minute (CFM) of air that could be flowed through that part can be altered. The flow meter has one of its ends plumbed to the cavity area just below the test fixture, and the other end plumbed to the cavity below the orifice plate but before the flow control plate, which is attached to the flow control knob. The pressure differential is represented on the flow meter in percent of flow.

If you were testing a bad-flowing passage you would get lower percentage numbers, while a good-flowing passage would have higher percentage flow numbers. Multiplying the percentage flow times the maximum flow for the orifice size at which you are testing (which is determined for that orifice and provided with the flowbench), you can determine the actual amount in CFM of air that passage flows. With the head bolted to the Flowbench and the 'valve installed with a light spring, a valve actuator is needed to open the valve a specific amount for each test point. This is just a threaded bolt with a dial indicator that rests on the valve so that when the valve is opened by turning the bolt, the dial indicator reads the amount the valve opens.

Flow-testing is usually done at 0.100, 0.200, 0.300, 0.400, 0.500, 0.600 and possibly 0.700-inch valve openings. Some people test at closer intervals but usually these intervals suffice. Before the test on the intake port is run, the net valve area must be determined. The valve is in the head because that is the way the engine will be run. The air/fuel mixture must flow around the valve so the testing with air must be done with the valve or else the flow numbers are completely useless.

To calculate this, use this equation: Net valve area (in2) = 0.785 [(diameter of valve ) 2 - diameter of stem ) 2 This is the actual opening at the end of the port and by referring to a chart you will know what CFM setting the flowbench should be set at to perform the testing. That port and valve area combination should be tested at the same CFM every time so that the data can he related to each other. To begin a test, open the valve to the predetermined lift, turn on the blower, open the intake flow valve to set the test pressure, and read the percent flow. Record this. To determine the CFM flowed, multiply the percent flow times the max flow setting the flowbench was set at. Repeat this on all the lifts for that combination.


Put simply, flow numbers provide a representation of what the passage would do in a running engine. When comparing flow figures for a carburetor, intake manifold, cylinder head port, or whatever, don't just look at the CFM of air the part flowed and compare that to someone else's testing without knowing what test pressure (inches of water) the test was conducted at.

Often, purposely or not, CFM numbers will be compared, and the higher CFM number was tested at a higher test pressure (which will provide higher CFM numbers). To correct test pressures, multiply this conversion factor times the flow figures you wish to convert to a different test pressure. As long as you are comparing apples to apples, the data will help you make an educated decision, if you compare apples to oranges (data from different test pressures), you are going to make a poor decision that probably won't get you going faster.

While flow-testing can be very helpful in determining what modifications will allow more air and fuel to enter the combustion chamber and the spent gasses to escape in the same amount of time, the engine should be tested on the dyno and/or the racetrack to determine whether the changes really work. The flowbench is testing airflow at ambient conditions not the air/fuel mixture the engine is fed at. Temperatures vary everywhere from freezing to 1200 degF so other engine variables should not change the actual flow.

Flow numbers are used daily to provide insight into what is occurring inside an engine. While there has been a lot of testing and scientific papers written on airflow, most engine builders or head porters rely heavily on experience with testing stock and modified components to determine what improves performance and what doesn't. That might sound foolish, but in a very competitive atmosphere where any gain is critical, information that can make you power is very valuable. As a customer, you should be able to get a straight answer to a question regarding flow figures and why a component is improved because of flow testing.

Hopefully, this basic overview of flow-bench testing and the data gathered from that testing will help you go faster at the races.


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