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Flow Bench Facts

All you need to know about flow benches...

By Michael Knowling

Click on pics to view larger images

This article was first published in 2002.

Extracting more power from an engine, whether it's a Daihatsu 3-cylinder or a big block Chev, revolves around the ability to breath air. From the air filter, through the throttle, the head(s) and out the exhaust we're searching for the least possible airflow restriction. Not surprisingly, therefore, an instrument that measures airflow - a flow bench - is tremendously important.

What is a Flow Bench?

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A flow bench is essentially a glorified vacuum cleaner that flows air through a large diameter orifice. On most benches, the direction of flow can be altered so air is either blown or sucked through the test object - these settings are referred to as exhaust and intake modes respectively.

The machine automatically adjusts airflow volume to maintain a constant level of flow restriction through the test object. This restriction is expressed in inches of water and is known as the test pressure.

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Before any testing takes place, the flow bench must be calibrated so the maximum airflow volume is known at six different motor settings. This flow volume is expressed in cfm (cubic feet per minute) and is known as the flow scale. Once calibrated, airflow volume through the test object is shown on a manometer scale as a percentage of the flow scale peak figure. For example, a cylinder head that flows 50 percent of a 200 cfm flow scale is capable of 100 cfm.

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Here are the flow test results of a standard Toyota 1UZ-FE cylinder head. Across the top of the table you can identify different amounts of valve lift (expressed in thousandths of an inch), the Flow Scale heading refers to the maximum possible airflow in that particular motor setting, below that is the measured flow (as a percentage) and - finally - there's the calculated cfm figure. Note that the top group of numbers refer to the head's inlet port flow, while the second set below refer to exhaust port flow.

How Is It Operated?

The flow bench pictured here is a Superflow SF-600E owned by Frank Intini of Adelaide's F&M Cylinder Heads (note that Superflow is one of the most recognised flow bench manufacturers in the world).

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Before anything else, the first step is to grab an adapter that allows the flow bench orifice to make an airtight attachment to the test object. Most experienced flow bench operators have a variety of cylinder head adapters sitting ready to go, but you'll need to fabricate your own if you want to test something a little out of the ordinary. For example, to test an intercooler with 3-inch fittings on a Superflow SF600-E you'll need to drill a 3-inch hole through a flat sheet of metal and weld a section of 3-inch pipe around the circumference. Four 1/4-inch holes spaced at 5-inches square then need to be drilled through the flat sheet - this allows the adapter to be securely bolted to the flow bench. Seen in this photo is an adapter for testing carburettor barrels.

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Once the test object is mounted on the bench, the operator selects the appropriate flow direction and choses a flow range. As mentioned, there are six different flow ranges on the Superflow SF-600E; the six flow ranges on Frank's flow bench span from 35.4 to 610 cfm in intake mode, and 42 to 647 cfm on exhaust mode. The appropriate flow range necessary for a particular test can be guessed by the operator - they should know a ballpark figure for how many cfm a given test object should flow at a certain test pressure.

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For most purposes a test pressure of 28-inches of water is used, but this can be varied accordingly. A motorcycle cylinder head, for example, can be measured with a test pressure of 10-inches of water. Note, however, the electric motor can overheat trying to maintain small volume of air; the Superflow SF600E incorporates a small air temperature gauge to let the operator see when output temperature is getting out of hand (the gauge only works when the bench is running on exhaust mode).

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To obtain accurate cfm readings, it's often necessary to radius any sharp edges the bench may be drawing air through. It's common practice to stick a bead of plasticine around the edge of cylinder head ports to ensure there is no turbulence at the mouth; although it appears radiusing the edge would falsely improve airflow, we're assured it merely removes any "misrepresentation."

What Can Be Tested?

A flow bench can be used to test almost any engine component that conducts air.

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The traditional application for a flow bench is to quantify gains of ported cylinder heads; often a 'before' and 'after' test reveals where the biggest gains have been made. Note that the flow bench is ideal for evaluating cylinder heads because it can set to suck air through the inlet port and switched to blow air through the exhaust port, thereby replicating the flow directions inside the engine.

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Furthermore, the flow through cylinder ports can be measured at different valve lifts. Of course there's no point testing flow only when the valves are fully open - you need to measure flow through various increments of lift. As Frank says, the flow through the port at middling valve lift is particularly important because that's where the valve spends most of its time while the engine is running. Adjusting the valve opening on the flow bench is achieved using custom-made valve spring tool; your flow bench operator should have a collection of these suitable for your head.

Yet another benefit of using a flow bench to test heads is the ability to calculate the maximum potential power. Use the formula:

Potential Horsepower = 0.256 x cfm at 28-inches of water x number of cylinders

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Other than cylinder heads a flow bench can be used to look at airflow through mufflers, cat converters, inlet manifolds, intercoolers, air boxes, blow-off valves and more. You can even flow a full-length exhaust system if there's enough open space in the test room. Note, however, whatever you test must be clean - otherwise loose matter can cause damage to the machine.

How Much Is It?

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Frank from F&M says the typical charge to flow test a head is AUD$55 per cylinder - regardless whether it's 2, 3, 4 or 5-valve per cylinder. Costing for other types of testing are on a P.O.A. basis and, bear in mind, you might need to fabricate your own adapter. Whatever the exact charge, though, a flow bench is a great way to compare and evaluate an assortment of go-fast gear.


F&M Cylinder Heads
+61 8 8294 2515

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