tuning with an unleaded racing gasoline,
one needs to pay a little more attention
to some details than with leaded gasoline.
Spark plug reading becomes more critical
because it is harder to get color on the
plugs with unleaded. Getting the air / fuel
ratio correct is tougher because most unleaded
gasolines contain oxygen which adds to the
oxygen in the air. Looking at the exhaust
pipes will not tell much except that there
is no gray color from the lead. How do we
accomplish this task that was previously
done with the naked eyeball?
of this is really tough because we get accustomed
to one way of doing things and our world
begins to turn upside down as fuel parameters
and environmental regulations change. To
start the process you need a good light
to �read� the spark plugs. It has to be
a light with a magnifying glass associated
with it. There are several available on
the market. They resemble the instrument
that your doctor sticks in your ear to have
a look at whatever he is looking at down
there. Maybe for us involved in racing,
he is simply checking to see if the pathway
to the other ear is clear so that he can
see daylight out the other side.
is always best to start with a new set of
plugs. When reading the plugs, always look
at the entire set, and make jet changes
where they are needed. The carburetor does
not have to have the same size jets in all
four corners. In fact, it is very unusual
in a racing engine to have the same size
jets in all four corners of the carburetor.
This is due to differences in manifolding,
inertia forces, etc.
spark plugs needs to be done with proper
light / magnifying glass mentioned previously
so you can see all the way down (invert
the plug to read it) into the plug where
the insulator (white part of the plug) comes
through the steel case. What you are looking
for here is a slight coloring on the insulator
just above the steel case. This is called
the �fuel ring� and to have the correct
air fuel ratio, the �ring� should be visible
all the way around the insulator. If it
is spotty or very light, the engine either
has not run long enough to develop a fuel
ring, or it is too lean. A visible ring
that just offsets the white color of the
ceramic is usually a good sign.
contrast, leaded racing gasoline will color
a plug much quicker than unleaded and a
correct �Fuel Ring� will usually be a little
darker due to the lead in the gasoline.
To correctly read a plug with leaded gasoline,
it is necessary to have the same good light
/ magnifying glass identified above. They
are not cheap, so don�t try to steal one
from your doctors examining room.
the fuel ring as an indication of the correct
air-fuel ratio, another indication is light
shiny spots at or near the tip of the insulator
on Autolite plugs only.
These are difficult to see and show up on
Autolite Plugs only. It has to do with the
material used in the insulator. Not all
insulators are the same.
the spark plug insulator shows any sign
of small dark spots, especially at or very
near the tip, detonation is taking place
in that particular cylinder. Those dark
spots are sometimes referred to as �speckles�
and are very small pieces of aluminum oxide
that have been cooked off of the top of
the piston due to the abnormal temperature
/ pressure that occurs during detonation.
One cannot afford much of this before failure.
If this condition is observed, a determination
must be made as to whether the mixture is
too lean, or if there is too much spark
timing. Keep reading, we�re not there yet.
at the side electrode of the plug will help
to determine the correct spark timing. Ideal
timing is indicated when the side electrode
shows heat all the way to the case. If there
is not an indication of heat all the way
to the side electrode / case, then spark
timing should be increased. If the heat
line stops 1/16 th of an inch short of the
side wire/case connection, the engine needs
an additional � degree of timing.
spark timing appears to be correct, but
there are signs of detonation, the air fuel
mixture is too lean. The carburetion must
be made richer by going to larger jet(s).
is unlikely to occur in all cylinders at
the same time unless the spark timing and
/ or the air fuel ratio is way off. Normally
detonation will show up in one or two cylinders.
Richen only the corner of the carburetor(s)
that feeds the problem cylinders. Be sure
to verify that the detonation problem has
Changes and Density Altitude (DA)
use �Density Altitude� (DA) as a measurement
when tuning an engine. If, for instance
the DA is 1200 feet, this means that the
air at that racetrack has a density equal
to what air would be at 1200 feet above
sea level if standard conditions exist at
sea level. As the DA increases there is
less oxygen available, and the mixture needs
to be leaner. For every 750 feet that DA
increases, the jet size needs to be reduced
by one size. In contrast, for every 750
feet that DA goes down, jet size needs to
be increased by one size.
the �old� days, all that we had to be concerned
with is getting the correct amount of oxygen
from the air mixed with the correct amount
of gasoline being used. In tuning with unleaded
gasoline, we also need to take into account
the amount of oxygen in the gasoline (if
there is any). This part can become critical.
assume that a racer has his engine �dialed
in� on Rockett Brand 111 Leaded Racing Gasoline
and his sanctioning body is switching to
Rockett Brand 100 Unleaded Racing Gasoline.
The 111 has a specific gravity (SG) of 0.726,
and the 100 has a SG of 0.744. Normally
when changing from a gasoline with a lower
SG to one with a higher SG, one must lean
the mixture by going to smaller jets in
the carburetor. In this case it would be
about 2 jet sizes in a Holley carburetor.
That is not the correct move in this example
because the unleaded contains 2.7% oxygen
which in this case will negate the jet change.
The engine may like richer jets than it
did with the 111, but the jets used with
the 111 are a good place to begin. Just
try to remember what an old guy (he was
about 30) told me when I was 17,�When
in doubt, go richer.� This can
reduce the need to build (or buy) a new
above example is why it is critical to know
how much oxygen is contained in the unleaded
gasoline used. This is also the same reason
that racing with street gasoline is not
a good idea since one never knows how much
oxygen is contained in the street gasoline.
Know your racing gasoline, know your racing
gasoline supplier, know if it contains oxygen,
and if so, how much? Don�t depend on information
from your buddies. Call your supplier (does
this sound bad or what) and if he doesn�t
know, change suppliers.
an unleaded gasoline will not color a pipe
to the gray that we saw with leaded gasoline,
so don�t look for it. Exhaust pipes used
with unleaded gasoline will be slightly
black in contrast to the gray that was observed
with leaded gasoline.
of this takes experience. The more you do
it, the easier it will become. Take your
time, pay attention to details, and everything
will come together.
Your Nearest Distributor, Call: 1-800-345-0076