Holley carburetors are calibrated for sea level operation and an inlet air temperature of 70deg Fahrenheit. Once you know the correct stock jetting for your particular Holley carburetor, you can determine whether you live or race at an altitude above sea level. For every 2000-foot increase in altitude, you can reduce the jet size by one size. If you had a carburetor which has a stock jet size of 80 and you live or race at 2000 feet above sea level, then you would use a #79 Holley jet in the carburetor. Similarly, a change in the carburetor's inlet air temperature may require a change in the jet size from the stock calibration. Many racers go a step further by combining all of the weather variables, temperature, barometric pressure, dew point and humidity with the altitude of the track they are racing at to determine the "density altitude". This is a "corrected" altitude above sea level. From there they can determine whether a jet change is necessary to maintain performance or whether to change their "dial in" (if they are bracket drag racers).

Drag racers should try to optimize jetting by looking for the jet size that gives the best MPH, rather than best elapsed time (ET).

AIR DENSITY CALCULATION

Air Density Formula: 32.2 x P/(T x R)

 P = barometric pressure in absolute lbs/ft2

(P) = normal barometer reading (which is in inches of mercury) x .4912

This gives you PSIA.

Then you must convert this to PSFA by multiplying by 144.

The shortcut would be to just multiply by 70.7328.

T = degrees R This is much easier. Just add 460 to the current temp in degrees F.

R = the gas constant for air = 1718.

The 32.2 is a conversion from Slugs/Ft3 to Lbs/Ft3

The complete equation results in: 32.2 x P/((t+460) x 1718) = Lbs./ ft2

This is as simple as it gets. If you throw humidity factors into the equation, it starts to get pretty hairy. The source I had for the equation stated that the effects of humidity are far out weighed by the effects of either pressure or temperature. If you need more details on that, USA today has some information on their web site