Effects of Weather
The weather or atmospheric conditions affect a race car so much that it is important for a racer to monitor these conditions in order to explain why his Pontiac runs faster on some days and slower on others. I race at several tracks on the East Coast and I have seen my car change by .40 second throughout the year. In most cases, the weather was responsible for the lackluster horsepower output; other times it was the sun’s slippery effects on the track. In a gasoline engine, the amount of fuel (jetting) that can be run through the engine depends on the quality (density) of the air it ingests.
The weather cannot be controlled, but through careful observation and measurements, a racer can adjust his dial-in to compensate for changes in the weather. If a racer is experimenting with engine tuning like jetting or shift points, or trying out a new camshaft, it is important for him to look at the difference that the weather makes from week to week. I suggest that you carry your own set of instruments instead of relying on other sources due to the possible differences in calibration. Here are the six variables that you must monitor to recognize the weather-induced changes in your racecar.
A higher barometric pressure will let the engine make more horsepower. Use richer jets to take advantage of the denser air and the car will run quicker.
A lower temperature (air inlet) will make more horsepower.
An increase in humidity (water vapor) will decrease an engine’s horsepower. Leaner jetting will optimize the air/fuel ratio, as noticed by the exhaust gas temperature (EGT); however, little power is regained.
A headwind will slow a full-bodied race car while barely bothering a dragster. Note direction and speed. A typical 10- to 20-mph headwind will slow a car 1-3 mph.
A higher altitude (elevation of track) will decrease an engine’s horsepower. This is important if you race at various geographic locations.
A hot sun will elevate the track temperature and decrease traction at the starting line. More important, clouds and nightfall will suddenly increase a car’s traction abilities. A sudden change in the 60-foot time due to tire spin will be amplified throughout the run. For example, if my car slips by .04 second in the 60-foot segment, an additional .02 second is lost in the remaining part of the run.
In addition to these factors, always question the accuracy of the instruments that you use to measure this data. For example, this year I found that my high-precision barometer changed readings (.22-inch Hg difference) when only the temperature changed (70 to 90 degrees F.). Now I must add a temperature factor to the barometer reading before computing the density altitude figure.
In drag racing, the winner is usually the driver who makes the least of mistakes. By recognizing and compensating for the variables, you will be focused at minimizing your mistakes.