You're Grounded Young Man
From March '04 issue of TPR magazine, by Klaus Almendinger
Recently a spate of new products have
come to market that improve 'grounding' in cars. Sometimes the
claims of what these products do are fairly broad. The usual claims
of horsepower improvements and lowering fuel consumption are pretty
standard for all automotive aftermarket products, including seat-covers,
floor-mats and cup-holders.
Let's look at the issue these products try to adress. Every electrical system
in the car (excl. alternator) uses electrical power. Electrical power is defined
as voltage (the 'pressure' in the line) times current (the flow volume of electrons
in electrons/second). Electricity flows from the plus side of the battery through
the device back to the negative side of the battery. The negative side of the
battery is also connected to the chassis or frame. This is called 'ground'. In
a lot of cars the negative side of a device is connected just to the nearest
hunk of metal in contact with the frame. This can cause problems under some circumstances.
Ever been under a nice hot shower while somebody else in the house
flushes a toilet? The nice warm water stream suddenly changes to scalding hot?
The reason for that is easily explained and the same principle applies to grounding
issues. When water flows through a pipe there is a certain pressure loss along
the pipe. The pipe creates a resistance to the water flow. The smaller the pipe,
the larger the resistance. This resistance lowers the pressure at the end of
the pipe (shower head) from the original pressure where the water main enters
the house. If another outlet on the same water pipe also uses water, the flow
in the pipe to both increases and more pressure loss results. So now your nice
balance of hot and cold water is upset because less pressure is available on
the cold water side of the shower valve.
In electrical systems it works the same
way. Anything that conducts electricity has a certain resistance to the electrical
flow. The electrical flow is measured in Amperes (A) or Amps. 1/1000th of an
Ampere is a milliAmp (mA). In electrical formulas the symbol for current is I.
The electrical pressure (equivalent to water pressure in a plumbing system) is
measured in Volts (V). Resistance is measured in Ohms (formula symbol R) The
bigger the resistance of a supply wire, the more voltage builds up along that
wire (V = R*I). But because there is only a fixed supply voltage available from
the battery, the resulting voltage available for the electrical device is less.
Bigger wires have lower resistance and therefore lower losses.
If multiple devices
share the same electrical return path (for example the frame), the currents of
all the devices add up on the way to the battery and there is less voltage available
for each, depending on the total current between the ground point and the battery.
Returning to our plumbing example above: If we would run a separate pipe from
the water main to each water faucet in the house, there would be no scalding
in the shower because the pressure losses from any faucet would only affect that
faucet. Of course the other solution would be to run 3" pipes all through the
house to minimize pressure losses. This of course would be much more expensive
than running only pipes as necessary. It also only minimizes the shared loss
effect, but does not eliminate it, like separate pipes would. The grounding systems
on the market are the equivalent to that 3" pipe.
In wiring the electrical system
in a car the best way is to run separate ground wires from each system to a common
ground point. The common ground point should NOT be just a bolt where lugs from
each device are stacked in top of each other. The contact patches between the
lugs create their own resistance and ground problems. The very best way is to
bolt a short piece of copper bar near the battery to the frame. Connect the copper
bar to the battery with a heavy duty ground strap. Drill and tap the bar for
each ground return. The heaviest current user in a car is the starter. It can
take up to 800 Amps of current. So it should have its own ground strap directly
to the battery. So should the alternator. Its wires carry the second most current
after the starter.
Electronic fuel injection and ignition systems have their
own caveats though. Ignition systems create very high current pulses for a very
short time. This is especially true for Capacitive Discharge Systems. Those need
their own ground wire to the common. EFI systems rely on different sensors in
the car. Throttle position sensor, Intake and coolant air temp sensors, Manifold
pressure sensors and so on. These sensors typically have 2 or 3 wires. When they
have 2 wires, one is typically the signal and the other ground. 3 wire sensors
need a 5V supply, signal and ground. DO NOT connect the ground of these sensors
to the common ground as described above. The EFI computer, as any electrical
device can only see its own ground and references all measurements to that.
The EFI computer also switches the injectors on and off. Injectors use relatively
high currents, and these currents have to flow back to the battery through the
EFI computer's ground wire. This causes a voltage drop on that ground wire. Were
the sensors grounded to the common ground as described above, the ECU would see
only the sensor voltage minus the voltage drop of its ground. Instead ground
the sensors directly at the EFI computer to its ground. Sensors only take a
few mA of current anyway, so the additional drop on the EFI ground caused by
them is irrelevant.
Another issue arises with sensors that create a very small
signal, like thermocouples for EGT and cyl.-head temperature measurements. In
an engine compartment a lot of current pulses from ignition and injection systems
flow around. Any electrical current also creates a magnetic field. The two wires
from the sensor (signal and ground) create a loop, which acts as antenna to pick
up these magnetic fields. To avoid that, either use shielded wire or simply twist
the wires together. Each twist creates a smaller loop, which picks up less of
that noise, but also adjacent loops pick up a signal that's inverted from the
loop before. This way the induced noise voltages in each loop cancel each other.
Audio systems in cars also need to be connected to this 'star' ground. The human
ear is the most sensitive organ we have. The difference between the loudest noise
(pain threshhold) and the quietest noise we can hear is over 1 million to one.
So any electrical noise from inadequate grounding can be amplified by the audio
system to hearing level.
Now to the claims of some of the grounding systems regarding
10 times better impedance. We talked about resistance of the wires before. Resistance
applies to steady-state current (DC). Steel has about 10 times the specific resistance
of copper. So the material must have 10 times the square area of copper to achieve
the same resistance. This is not a problem when using the frame as ground.
The material with the lowest resistance known is silver. Followed by copper (a few
% higher) High frequency currents (and pulses contain high frequencies) make
the wire behave differently. High frequency resistance is called impedance and
depends on the frequency of the current. Very high frequency currents tend to
flow not evenly in the whole cross-section of the wire, but only on the surface.
Therefore multi-stranded wire creates more surface area for the high frequency
currents to flow, hence lower impedance. But this effect is only important for
VERY high frequencies in the upper radio frequency range. If the electrical system
in your car produces high currents with frequencies that high, your car would
shut down TV and radio reception for miles around. Frequencies that high are
just not normally encountered in a car. The advantage of multi-stranded wire
in a car is flexibility and vibration resistance.
As to claims of better performance
and lower fuel consumption: The EFI systems injects fuel according to the amount
of air drawn into the engine, measured with it's sensors. More performance can
only be had if more air/fuel enters the engine. No electrical system can increase
the air-flow, so no more fuel flow either. The only effect a better grounding
system can have is if the sensor grounding was so bad before that the EFI computer
misread the sensors due to ground offsets. This can be inexpensively remedied
by following the grounding guidelines above. Some of the better EFI computers
utilize 'differential' inputs. They measure both, the signal and the ground of
the sensor itself and calculate the difference. This way they become independent
of any grounding issues.
As to ignition system grounds, one of the performance
parameters in an engine is ignition timing. Timing is derived from sensors on
the flywheel or crank. These sensors create reference pulses. A pulse can either
be there or not. If not, the car stops. No additional grounding changes that.
As to the ignition system itself: as long as there is enough spark energy to
light the fire, the engine runs. More ignition energy does NOT increase power,
just spark-plug wear. Corresondingly, if the ignition system is adequately grounded no amount
of additional grounding will do any good.