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. 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. 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. 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. 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. 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. 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. 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. 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 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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