with Weather Computers
(c) 1999 Family Software
Racers use weather computers to predict ET, determine the proper air/fuel
ratio, and to analyze vehicle performance. When used properly, the computer
can help to achieve the utmost in consistency. Following are a few hints
and tips to help get the most out of a weather computer.
Air/Fuel ratio management
The air/fuel ratio for a gasoline-burning engine must be about 12.5
to 1 for the car to respond properly to changes in atmospheric conditions.
There is no benefit to running a lean mixture.
Find a day when the air density is average for the racing season in
your area, then richen the fuel mixture until the car slows down. You
shouldn't have to re-jet the carburetor until the air density changes
by 4 percent or more. By adjusting the fuel mixture for an average air
density of 96 percent, for example, the air can range for 92 to 100
percent without having to re-jet.
A quick way to check for the proper fuel mixture is to inspect the car's
header collectors. The color inside them should be very dark gray or
black. If the color is white or light gray, the mixture is way too lean.
As the air density changes over the racing season, you will notice a
change in color, lighter as the air density increases and darker as
After adjusting the fuel mixture properly, the car should run on the
predicted ET consistently. If it doesn't, variables are affecting the
combination. A computer will help determine these variable so that they
can be eliminated. Begin by correcting every ET to Standard Pressure
(STP, also known as sea level conditions) and recording each run in
a racer's log. In correcting the ET, we remove the effect that the current
atmospheric conditions have on the run. Therefore, the STP ET is the
elapsed time the car would have run under perfect weather conditions.
This creates a basis to compare on ET against another regardless of
the weather conditions. Once all variables have been removed, the STP
ET should be almost identical for every run.
The ET Predictor II computer has a unique feature for locating these
variables. STP Run Segment Analysis is for quarter-mile runs on which
60, 330, 660, 1000, and 1320 foot times are recorded. In the absence
of an onboard computer, it is the best method to get maximum consistency
and performance from your race car. After entering the interval times
from your time slip, the computer calculates the six individual track
segments of 0-60, 60-330, 330-660, 660-1000, and the 1000-1320 foot.
The computer then computes an STP ET for each of these segment times.
By simple comparison of the STP segment ET's, the user can analyze what
the car is doing at any point on the track and revealing, to one-thousandth
of a second, any variable that exists in combination or driving style.
It's a good idea when predicting dial-in to also predict the 60 foot
and the 1,000 foot times. As eliminations progress, comparing the predicted
60-foot time to the actual 60-foot time will show exactly what is happening
to the traction factor. Moreover, if a racer hits the brakes to avoid
breaking out, he or she will be able to tell exactly what the car would
have run by comparing the predicted 1,000-foot time to the actual 1,000-foot
By comparing the STP ET's from every run, the user can find out exactly
how consistent his or her car is. If the STP ET's vary by .020, then
the racer may want to dial .010 or more under the predicted ET to avoid
breaking out. This is especially true for the car being chased. This
driver needs to have the confidence to run it out the back door if necessary.
Methanol is becoming more popular in bracket racing. A rumor concerning
methanol is that changes in air will not affect performance. Not true.
Any naturally aspirated engine running methanol at a proper air/fuel
mixture will respond to changes in air density. The effect, however,
is just half of what it would be with gasoline. So, when using methanol
fuel, and a methanol calibrated weather computer, it becomes twice as
easy to run right on the predicted ET.
Always use the readings from your own weather gauges. What someone else
is doing will not help you. Keep your gauges outside, in the shade,
and shielded from the wind. Even with the doors wide open, a trailer
will absorb, and retain, radiant heat from the sun. The readings never
will be quite accurate. This is the same reason why you cannot use the
weather readings reported on your time slip. Depending on the time of
day and location of the sensors, temperature readings will vary by 10
degrees or more from the actual ambient temperature.
Don't ever let anyone tell you that you can read your gauges in the
staging lanes. The asphalt and surrounding race cars radiate more heat
than you can imagine. This is absolutely the worst thing that you can
do. Anyone telling you to do such a thing is probably trying to sell
you a bill of goods. The units that are being sold for that purpose,
simply do not work well. The only way to use one of these devices is
to keep it back at the trailer. The whole idea in ET prediction is to
measure the change in the ambient conditions from one time of the day
to another. Not to measure the air in the staging lanes. This in NOT
Use a hood scoop or fresh air system of some sort. Under-hood temperatures
are much higher than ambient air and will result in poor and inconsistent
performance if allowed to enter the carb inlet.
Finally, to take full advantage of your weather computer, when the track
announcer calls your class for the first round of eliminations, don't
be the first one into the staging lanes. Without a weather computer,
a racer tries to make a time trial as late in the day as possible, then
wants to be the first one to enter the staging lanes for the next run
in the hope that the air hasn't yet changed. Because everyone can't
be first in line, you'll be in good company. When it comes time for
you to run, you'll still know exactly what the ET is going to be. You
will run on your dial. Chances are, your competitor, will not.
For technical help or other questions you may have please write
3164 Surrey Lane
Aston, PA 19014