Article By Colin
Lloyd in Dragster Australia magazine.
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"In
designing a race engine, or for that matter a road
engine, out of all the components that go towards
making that engine a success, the cylinder head is
the major player". It is the one piece that can make
or break your next project motor, so it's not
surprising to find that a need to test cylinder
heads and the modifications done to them is
essential for engines to go faster.
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In
the 90s no
self respecting head or engine shop would be seen
without a flow bench.
Despite this, I know of a few cylinder head
manufacturers who interestingly don't have one! A
flow-bench simply measures dry airflow through the
ports of the head in a rather static fashion. This
is in contrast to a running engine which has
differing pressures across the head, opening and
closing valves, temperature gradients and, of
course, an air fuel mix traveling through the ports.
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Flow-benches
haven't changed much since their commercial
introduction in the early 80s, The main development
has been that today some interface directly with a
computer, mostly to increase testing speed and
repeatability. But, despite this, the principal of
operation remains the same with this being a
pressure differential across the head that promotes
air to flow through any open conduit, intake port,
exhaust or, for that matter, plug hole.
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The
air flow is measured on most commercial machines by
comparing it to that of a known orifice plate. It is
then read as a percentage of that plate flow at a
known test pressure. If you take a look inside a
commercial flow-bench, the first thing you notice is
how simple it seems ... which it is, but, behind the
simple mechanics is some very complex math's -
Bernoulli theorem as 'well as a heavy use of
calculus.
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But
enough of that stuff. Like most simple devices,
things rarely go wrong or in error unless some of
the rules of scientific testing are not followed.
Was the test checked for internal or other leaks and
temperature compensation it needed? Was the intake
tested with a radius entrance or not? Maybe a
manifold was fitted. Was the exhaust straight off
the head or did it have a stub pipe attached or even
a complete exhaust system? All these things need to
be documented as they will affect the final result.
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As
we already know, flow-benches measure air flow in
cubic feet per minute (cfm) through a piece of
conduit exhaust pipe, manifold, muffler, inlet and
exhaust ports. The pressure differential across the
test piece must be high enough to provide a
reasonable velocity of air through the conduit to
simulate the performance of the test piece based on
its application. For
cylinder heads and their related components, 100 to
500 ft/min in steady state testing provides enough
velocity to simulate performance in a running
engine. This pressure differential is measured in
inches of water (for imperial types like me) and the
range is usually between 10 in. to 30 in. of water
test pressure. This will provide the velocities we
spoke about.
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THE INFORMATION
NEEDED
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At
first thought, the measurement of cubic feet per minute as
provided by the flow-bench gives us the information needed, ie,
the number of cubic feet of air passed in each minute. CFM can
roughly be converted to horsepower potential with simple math's:
120 cfm and 10 in. of water test pressure converts to 412 Hp for
an eight cylinder engine. Likewise, 190 cfm and 25 in. of water
test pressure is 412 HP. This is from the same port tested at a
higher test pressure hence more flow. The calculated horsepower
number is based on a certain level of volumetric efficiency (VE)
achieved by the engine. Many components of the engine design
affect the VE. For example, you've just bolted freshly ported
heads onto a 350 Chev motor. The porter told you these heads
flow 570 HP, but he didn't tell you that you had better get rid
of your two barrel manifold and the 350 Holley bolted to it,
therefore the engine doesn't come any where near 570 HP. This is
possibly an extreme case, but it's easy to get the drift. The
bottom line is that CFM is what the flow-bench tells you, but it
doesn't tell how this was achieved or how to realize the
potential in a complete motor.
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Another
important aspect of port design is port velocity. A poorly
designed port flowing 250 cfm, for instance, will undoubtedly be
larger in cross section than a well designed one flowing the
same 250 cfm. Both heads might produce the same power as
measured on an engine dynamometer, but the smaller port will
have a wider power band, more torque at the peak rpm. probably
have smaller, lighter valves and perform better, hence it
will be faster on the race track. At Headsense I've been mindful
of good port velocity for many years, often sacrificing those
last few cfm to get the desired air speed at particular
positions of the port. Our software allows me to determine
average port velocity (at our test pressure) at any point along
the port conduit. The need for high air speed is to help fill
the cylinder at all engine rpm, given that with long duration
camshafts, cylinder filling before top dead centre and
after bottom dead centre is important to achieving maximum power
so high air speed and lots of it gives us energy in the inlet
runner. Einstein's formula for energy, E = MC2, is as applicable
here as it is in other areas of physics. M is mass or the flow (cfm).
C is velocity (ft/mm).
Energy, therefore, increases to the square of velocity.
So you can see that velocity plays a vital part in this
equation. The need for a balance of flow and velocity is very
important. To have one without the other won't give you a motor
with a wide torque band and its track performance won't be good.
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BALANCE
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The one
thing in common with all race winning engines is undoubtedly a
good balance between flow and velocity. As is often the case, a
race engine falls short of the potential head HP, but it can
also exceed it. The primary reason for both cases is volumetric
efficiency, also as RPM increases friction and pumping losses
come into play but by and large it's the level of VE attained
that is the major influence. An example would be the 400 cubic
engine of Brisbane Group Three race? Michael Varney which was
built and tested by Headsense. This small block Chey is a 13:1
comp, single four barrel, avgas-type motor with AFR heads. This
motor produces 650 BHP and 560 ft/Ib of torque under 7000 rpm.
The VE at the torque peak is 109 percent and an amazing 106
percent at the HP peak with one carburetor. The heads flow 623
BHP and the motor has a very wide operating range. Track
performance is also very good.
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So
far we've talked about inlet port testing and the engine
requirement on the inlet side. The exhaust, whilst not having as
much influence as the inlet system, still has an important role
to play. The flow-bench measures exhaust flows the same way as
the inlet except in the opposite direction. Instead of a vacuum
under the head we develop positive pressure and force air past
the valve and out the port. The airflow is compared in the same
way as the inlet test and we end up with cfm of exhaust flow.
Exhaust ports do not make power, they only limit what already
exists. By power I mean the ability of the motor to make
torque at high rpm. That's what power is, torque over time.
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A
poor flowing exhaust port will have trouble expelling the
cylinder of waste combustion over the decreasing time as the rpm
increases, so if the waste can't exit as the rpm rises, the
torque output drops. This occurs even if the inlet has enough
flow and speed to support more output. "If it don't get out it
won't go in." Opening the exhaust valve earlier is one way to
combat this problem, giving the exhaust even more duration
(time) to complete its job is pretty well a universal method for
power production. There is a catch - too early exhaust valve
opening will start to lose torque at low rpm. This is important
to low compression engines and auto transmission race cars. The
importance of high flow and velocity in the low lift portion of
the exhaust cycle (to about 70 percent of the total exhaust
lift) can't be emphasized strongly enough. This phase is called
"blow down" and is where the piston is descending toward BDC and
the cylinder volume is expanding. At the same time we are trying
to expel the cylinder of waste combustion. High cylinder
pressure is the only reason this takes place. The more we can
get out before the piston turns around and has to pump it out,
the better off we are. Hence the need for very good low lift
exhaust flow.
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The
bottom line is this: A flow-bench is a measuring tool. It's how
the engine builder responds and interprets the results that is
the big difference between the engine that wins the race and the
others behind.
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