posted February 16, 2003 09:15 AM
An auxiliary venturi dispenses fuel into the air flowing through a main venturi based solely upon the velocity of that air. A carburetor has no way to sense the mass of the air flowing through it. Since the process of combustion proceeds on the basis of mass- so many pounds of fuel mixed with so many pounds of air- you have to alter the amount of fuel delivered by the main system to keep the same air/fuel ratio in the face of changing weather conditions.
Though temperature, humidity (more correctly, vapor pressure) and barometric pressure all come into play, if you keep track of the temperature you’ll be in pretty good shape. Vapor and barometric pressures play only a minor role in determining fueling requirements (though they play a major role in ignition advance requirements), especially if you race at the same track most of the time.
You can do a very good job of keeping up with fueling requirements with a good thermometer alone. Hang the thermometer in some place out of direct sunlight and where the air circulates freely. If you have an enclosed trailer, hang it near the front of the trailer, well away from the ceiling and walls (or any surface that can absorb and re-radiate heat). If you don’t have an enclosed trailer, hang it somewhere else that fulfills the above requirements (don’t hang it under a trailer or vehicle near the ground, readings will be way off). Try to hang it in the same place every time for the sake of consistency.
All you have to do then is find a good jetting baseline and make a note of the temperature at the time. When the temperature drops the air is denser, meaning that you have more oxygen available for combustion and will need to richen the main system to compensate. Of course, the reverse is also true. In practical terms a temperature change of 10º to 12º F. requires a change of one Holley jet size to maintain the baseline air fuel ratio. If your baseline temperature is 75º F. and the temperature drops to 65º F. or so, you’d need to increase one jet size; if the temperature rises to 85º F. or so, you’d need to decrease one jet size. A change of 20º to 25º F. would require a two jet change, etc.
The effects of vapor pressure and barometric pressure get a little more involved- if you’d like to discuss this further feel free to call me at 704.482.7588 any day up to 8:00 PM EST.
Earl Parker II www.parkercarburetion.com
[This message has been edited by Earl Parker II (edited February 16, 2003).]