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 it decreases.
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 time.
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
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 ambient air!
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 or
3164 Surrey Lane
Aston, PA 19014