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Raceday Variables

Racers are constantly aware of the variables in racing. Throughout the day, many weather variables and mechanical items are changing. From week to week, even more things change. These variables must be checked, adjusted, and noted constantly. During eliminations, there are factors changing every round. You must take these into consideration before selecting your dial-in and your lane. Here is a short list:

• A headwind can slow the car by two- tenths of a second.
• Heat and humidity may require a jet change.
• Ignition timing can vary over time due to worn parts.
• Valve lash also changes due to worn parts and requires occasional readjustment.
• A leaky pan seal may require additional transmission fluid if it goes unchecked, creating a trans slip or shift problem.
• Unheated engine, transmission, or slicks can slow the car by one-tenth of a second.
• Spark plugs eventually get fouled or widen their gap and need to be replaced or cleaned.

Rubber

Characteristics of the rear tires such as tread loss, rubber hardness, side- wall deterioration, and air pressure changes (due to temperature of the tire) affect the slicks ability to maintain traction. If the slicks show a grain pattern, it's time to reverse them. Many people compare tire pressures to see what works for other cars. This should not be done. Race cars using 4-link suspensions with a high and short instant center will hit the tire hard and therefore need more tire pressure than suspensions that use a long instant center. Sidewall construction can also affect the tire pressure requirement.

Effects of Heat

You must keep the fuel and fuel lines as cool as possible in the engine compartment. Gasoline boils at 150 degrees F., which is easily obtained in the summertime. Isolate the lines with insulation. Between runs, lift or remove the hood to allow the rising hot air to escape instead of providing heat to your fuel line and carburetor. If the carb float bowl reaches 190 degrees, the fuel will vaporize. Hot fuel can cause erratic problems throughout the staging lanes and on the track.

What causes high underhood temperatures? Usually, high-compression engines run at higher rpm, lean-jetted engines, and engines run over- advanced (ignition timing) create higher temperature problems. In addition, headers increase underhood temperatures due to extra surface area of the exhaust system. I tried header insulating wrap to reduce heat in the engine compartment. The wrap may have done its job, but the results were undetectable in my car.

A well-worn cam can also cause overheating. If the exhaust lobe does not efficiently vent the hot gases, the gases get trapped in the cylinder. This causes an increase in residual head temperature that is hard to identify and leads to burned valves, pistons, and rings. How can we detect a worn lobe? Unfortunately, a compression check doesn't indicate the condition of the exhaust lobes at cranking speed. It tests the sealing abilities of the valves and rings. However, a dial indicator placed on the valve retainer while rotating the engine by hand can show us the valve lift. Worn lobes show up as reduced valve lifts.

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.

Barometer

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.

Temperature

A lower temperature (air inlet) will make more horsepower.

Humidity

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.

Wind

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.

Altitude

A higher altitude (elevation of track) will decrease an engine's horsepower. This is important if you race at various geographic locations.

Sunshine

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.