Article by Will Handzel in Circle Track
magazine
GET IN THE
KNOW ABOUT FLOWBENCHES.
How a Flowbench works
Flow Numbers
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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. |
HOW A FLOWBENCH WORKS
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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. |
Top
WHAT THE FLOW
NUMBERS MEAN IN THE REAL WORLD
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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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