Technical information on fuel mixture settings for all types of vehicle.
How do weather factors like altitude, temperature and humidity effect the fuel
mixture set-up? Here are some simple explanations that may help you fine tune
the system to get the best performance from your engine.
AIR DENSITY THEORY
If you find a fuel set-up that is perfect, then whenever the Air Density
is the same as it was for that performance, the same fuel set-up will produce
the same perfect result.
This theory works well providing that there are no other influencing factors,
but you will quickly find with experience that the other influencing factors
usually play only a small role in fuel set-up and can usually be determined
fairly easily.
When you have found two different fuel set-ups that work very well, Jet-Plus
will provide the ability to calculate the fuel set-ups for all other conditions.
AIR DENSITY
Air Density (Kgs/m3) can be calculated from the Temperature, Barometric Pressure,
and Humidity using the formula [Barometric Pressure(Pa)/Virtual Temperature(k)*287(Gas
constant)]. Virtual temperature is a fictious temperature dependant on the amount
of moisture in the air.
For more information on Air Density there are many websites available, most
of which refer to air density as Relative Air Density or Corrected Air Density.
When the term Relative Air Density is used there seem to be a number of things
that these figures are relative to, and produce different results specific to
the purpose designed. Corrected Air Density is generally used for weather forecasting
and related issues and uses barometric pressure corrected for altitude. For
our purpose we do not care what the air density is relative to, and we want
to know what the Air Density is right where we are racing, not at sea level,
so that is the reason Jet-Plus uses Actual Air Density in Kg/m3
for its calculations.
As the air density increases, your air/fuel ratio becomes leaner. As the air
density decreases, the air/fuel ratio becomes richer. If you are driving up
a high mountain, at the top, the motor has less power because you have less
air and the air/fuel ratio becomes richer.
TEMPERATURE
When the temperature goes up, the air density decreases, thus you have less
air available for combustion and without adjusting your fuel system, your mixture
becomes richer. The same works in reverse. As the temperature goes down, the
air density increases, giving you more air per cubic metre, and without adjusting
your fuel system, the air fuel ratio becomes leaner.
BAROMETRIC PRESSURE/ALTITUDE
As you increase altitude the barometric pressure reduces and the air becomes
less dense, meaning there is less air available, making it necessary to adjust
the fuel mixture to maintain the correct air fuel ratio.
HUMIDITY
When the humidity increases, it decreases the air density because the moisture
is taking up more of the air space and reducing the amount of air available.
Another possible influence on performance is as humidity increases the octane
requirements ease. The formula is something like... for every one gram of water
increase per one kilogram of dry air the octane decreases by 0.25 to 0.35.
SUMMARY:
The bottom line is to make sure to pay very close attention to your air/fuel
ratio on cold, dry days with low humidity and when at sea level to avoid engine
damage.