MOVING OVER THE Earth’s surface are roughly 50 enormous air masses of varying temperature
and pressure that may each cover thousands of square miles. These regions are
constantly bumping into one another at
invisible borders we call “fronts” to create
the daily weather changes we experience.
In addition, the gravitational pull of the
moon creates atmospheric “tides” that are
especially noticeable at the equator. Think
of these great air masses as forming a separate atmospheric terrain moving ever eastward over the spinning planet’s geography.
Meteorologists call them high or low
pressure areas based on their weight compared to the mean sea level air pressure.
Air pressure can be expressed in a variety
of international units. In America, it’s traditionally given in inches of mercury and
29.92 in.-Hg. is considered the standard
atmosphere. But a more popular measure
is lbs./ sq.in. Visualize a square inch on the
ground at sea level, supporting a square-inch column of air stretching to the top of
the atmosphere. The standard weight of
the air above that inch would be 14. 7 lbs.
Pressure varies with altitude as a result
of gravity, and just as water pressure gets
extreme at great depths, atmospheric
pressure at high altitudes is radically less.
NASA has averaged conditions year-round
and notes that air pressure will typically be
one-half of sea level at 18,000', one-third at
27,480' and one-tenth at 52,926'. Our bodies are very sensitive to atmospheric pressure, causing our ears to “pop” with climbs
of just a few hundred feet as we ride.
Air is mainly nitrogen (78.08%) but
the oxygen component ( 20.95% of the
total) is what matters most to humans and
our internal combustion engines. As the
amount of oxygen drops, so does available
energy, whether it’s found in our muscles
or burned with fuel to turn the rear wheel
of a motorcycle. As a rule of thumb, a
1000' gain in altitude decreases air density—thus oxygen and hp—by 4.5%.
Temperature is also a very important
factor in air density. As temperature rises,
the air molecules become excited and
bounce off one another more vigorously,
increasing the space between them and
reducing the amount of oxygen in a given
volume of air. Low temperatures increase
oxygen density. Of course, low pressure
and low temperature are typically found
together in nature, which complicates
the simple mathematical relationship of
elevation to air pressure. When flying in
airliners, for instance, outside air tem-
perature at an airliner’s cruising altitude
of 35,000 ft. is consistently very close to
65° F below zero.
As an example of how temperature
affects horsepower, consider that if temperature increased from 70° F to 90° F
during a day’s ride, your engine’s power
would drop 2.9% if all other factors
remained constant. And the air density
difference between 105° and 35° could
create a hp difference of 10%.
Humidity is the third most important
factor in air density calculations. Because
water vapor rising from the oceans and
land sources displaces air molecules,
it reduces the available oxygen. Relative humidity near the earth’s surface is
usually above 30% and the saturation or
dew point is very sensitive to temperature.
Consider that 100% relative humidity at
86° F means the air will contain as much
as 3% water vapor, reducing available
oxygen and hp by that amount. With
100% relative humidity at 68° F, the air
contains slightly less than 2% water vapor.
We’ve probably all experienced how
our engines seemed to run better in certain weather conditions, and this wasn’t
an illusion created by an especially memorable ride, perhaps on a balmy evening,
returning home from a special date. Older
carbureted engines were very susceptible
to the weather (two strokes even more so).
In stock condition, their main jets tended
to create a slightly rich fuel/air mixture to
avoid overheating, so that in conditions
of low temperature, higher than average
barometric pressure and low humidity, air/
fuel mixtures would be optimized and give
a noticeable boost to power output.
Modern fuel-injected engines con-
stantly monitor the oxygen content of the
exhaust to adjust air/fuel ratios in order to
meet emissions targets. Prevailing ambi-
ent conditions are pinpointed by a variety
of sensors that naturally include pressure
and temperature, allowing the ECU to
automatically adjust for altitude at the
same time. With their fuel delivery no lon-
ger confused by less than expected oxygen
content, their driveability remains good.
But they aren’t unaffected by weather, and
power still drops with reduced air density.
Engine manufacturers and consum-
ers both need consistency to compare
the output of engines tested in differing
weather conditions, so the various inter-
national standards organizations have
created correction factors. It’s said the
original SAE (Society of American Engi-
neers) standards were based on average
weather conditions in Detroit, as it was
home to the biggest automobile factories.
The old SAE J607 standard was based
on ideal weather conditions of 60° F,
zero humidity and sea level air pressure
of 29.92 in.-Hg. that were clearly meant
to inflate power ratings. But the current
SAE J1349 standard (in effect since June
1990) gives 77° F, zero humidity and a
pressure of 29.234 in.-Hg., which is both
more realistic plus it reduces power-ad-
justed insurance rates.
MCN’s rear wheel dynamometer power
and torque numbers are likewise corrected
for temperature, pressure and humidity to
the SAE J1349 standard, so that our dyno
test results can be fairly compared regard-
less of the ambient weather on the day of
the test. If, for instance, the test was done
on a day when the temperature was 114° F,
a 1. 22 correction factor would increase the
corrected hp to bring it into line with what
that same engine should have created at 77°
F. Or, if very cool coastal air was the pre-
vailing condition, the tested numbers might
be multiplied by .97 to decrease them.
Our quarter-mile test results are also
corrected. Because our Area 51 is nearly
a mile above sea level—near and at the
same altitude as a now defunct NHRA
sanctioned drag strip, we use the strip’s
NHRA correction factors to achieve sea
level performance as nearly as possible.
Correction factors are never perfect,
but what real world performance measurement ever is?