Base Line Performance and Predicting ET
(c) 1999 Family Software
horsepower output of every engine is affected by the pressure, temperature, and
humidity of the inlet air. Therefore, in order to provide a common basis of comparison,
it is necessary to apply a correction factor to account for the difference between
standard air (STP) conditions and the actual conditions current at the time and
location of measurement. The Society of Automotive Engineers adopted a standard
method to be used for correcting observed power at full load. This method is used
as a basis for determining maximum horsepower levels with a dynamometer.
You cannot, however, base ET prediction on horsepower output
alone. Since the performance level of every car is different, the effect of the
air will vary accordingly from one car to another. Two cars with identical horsepower,
but having different power/weight ratios and other variables will respond in different
degrees to the same change in weather conditions. Therefore, an accurate method
of ET prediction may be based on an established performance level of the vehicle.
The Family Software ET Predictor Software and ET Predictor II Pocket Computers both use such a system,
consisting of a Base Line Horsepower Correction Factor (HCF) and a Base
Line ET. These two factors are determined by using a record of past performance.
This record can often be as simple as the data from the last run, if the car responds
perfectly to varying weather conditions, or an average from a group of runs if
it doesn't. This Base Line ET is then 'corrected' to what it should be, based
on the current weather conditions given and essentially becomes the ET prediction.
On gasoline burning engines, the air/fuel ratio, must be about 12.5 to
1 in order for the computer to accurately predict performance . A lean
running engine will not respond properly, perhaps making less horsepower when
the computer says otherwise. Generally, if the air gets better and the car slows
down, the engine is too lean. However, if the car picks up too much for the current
air, it is probably too rich. An engine running a slightly rich fuel mixture is
perfect for ET prediction based on air changes.
How close the actual
ET is to the predicted ET depends on any other variables that relate to the consistency
of the car's performance. To help in determining these factors, the ET Predictor
also predicts your 60 foot time and if you run at a track using the Compulink
timing system, it can also predict 330-foot, 660-foot, and 1000-foot times. It
can be used in this manner to analyze the other variables in your combination,
just as if you had an on-board computer such as our DataMasterä Sportsman
You are going to need a racer's log or notebook and gauges
to read air pressure, temperature, and humidity. We like to use an 'uncorrected'
barometer or digital altimeter and a digital thermo-hygrometer. Start out by making
several runs and immediately AFTER each run record all of the information from
each time card along with the matching weather conditions for that run.
Since we are going to be establishing a base line, it is vital to keep other variables
to a minimum. To be competitive in bracket racing and in the Super classes, you
usually have to run within thousandths of a second of your dial-in. All racers
watch rear tire pressure very carefully, but what about front tire pressure? As
the ambient air temperature changes, so does the air pressure in all your tires.
Granted, the amount of change will depend on the volume of air in a given tire.
But, even a slight variation in rolling resistance can affect your ET, so ALL
tire pressures should be the same for every pass. This is not the time to be jacking
tire pressures around to alter your reaction time! Likewise, burn-out time, starting
line RPM, staging depth, shift points, and other factors should all be kept as
constant as possible. Always fill your fuel tank to the same level for every run
down the track. If you don't think 5 or 6 lbs. of fuel will make a difference,
think about it the next time you breakout by .002 in the final round. It's a longer
ride home when you lose.
The amount of time between runs can also be very important. Many
cars will run 1 or 2 hundredths slow on the first run of the day. One cause may
be due to power train and chassis lubricants that have not yet reached proper
operating temperatures. Running the engine and drive train with the car on jacks
is part of the solution, however, this does nothing for the heavy grease used
in some front wheel bearings. Surface rust on exposed brake rotors and drums may
have some effect, especially if your car sits outside during the week. If your
car behaves in this fashion, later on by examining your racer's log, you may be
able to ascertain just how long the car has to sit before this factor comes back
into play. But, first you must know its happening before you can do anything about
it. Anyway, don't be surprised if your first run of the day gets thrown out of
our computation. Try not to make the runs too close together. Automatic transmission
fluid will heat up and lose viscosity resulting in a drop in line pressure and
decreased converter efficiency. These are just some of the variables to watch
out for. When you start to analyze the data you are collecting, more ideas will
surface than you could imagine. The first step is collecting the data and computing
a Horsepower Correction Factor (HCF) for every pass you make.
The HCF is the correction factor defined by the current weather conditions
for the exact time when that particular run was made. For example, on the first
time run an example car ran a 9.352 ET with a 60' time of 1.348 under weather
conditions of barometer: 29.82, temperature: 73, and relative humidity: 56. After
entering these conditions into your computer, it will report the current Horsepower
Correction Factor as 1.0374, Vapor Pressure as .458, and the Air Density as 95.736.
This is the 'air' for your first pass. If you were going to use this one run as
your base line performance, your BASE LINE ET would be 9.352, your BASE LINE 60
FT would be 1.348, and your BASE HP FACTOR would be 1.0374. You would use the
same procedure for every pass and write the HCF down in you log. Don't forget
to write down the air density. Variations in the air density of more than 4 per
cent on gas burning engines can require a jet change to make the car run right
on the prediction.
If you are using an uncorrected barometer, always
enter 0 for the elevation. When using an altimeter to obtain the air pressure,
always enter 29.92 for the barometer and then enter the altimeter reading for
the elevation. You can use your computer to predict ET from any one run or a group
of runs. You can't predict too accurately off a bad run and picking the best run
is not always that easy. But, by correcting your ET to standard pressure conditions
(STP), you can very quickly see what's going on. This creates a basis to directly
compare one ET against another regardless of the weather conditions associated
with the run. An example car ran a 9.298 on run # 3 and then a 9.396 on run #
4. At first glance, you would assume that this is a very inconsistent car and
the 9.396 was a very bad run. However, at STP, the 9.298 would equal 9.203, while
the 9.396 equals 9.211. In reality, the 9.396 pass is only .008 worse than the
9.298 after correction to standard air. The car is simply responding to changes
in the weather in exactly the way it should be and is indeed very consistent.
After we have determined the best run of the group, we can further analyze the
other runs through Family Software's exclusive Run Segment Analysisä . The
computer calculates individual segment times for the 0-60', 60-330', 330-660',
660-1000', and 1000-1320' foot distances, then corrects these segment times to
STP. Using Run Segment Analysisä , you can see exactly where on the racetrack
the last run differs from the base run. You can easily determine track conditions,
the effect of head and tail wind, and other variables regardless of the weather
conditions. Doing this can tell you what variations in performance occurred at
six (6) different points on the track. Similar to having an on-board computer,
the analysis of this data will help you in perfecting the consistency and performance
of both car and driver.
Once you have established a base line performance
level for your own car, you will be able to accurately predict ET's by entering
into the ET Predictor program, these factors and the weather conditions present
at the time you will be making the run. In the same manner, you can predict 60',
330', 660', and 1000' times and chart them as well.
3164 Surrey Lane
Aston, PA 19014