Establishing Base Line Performance and Predicting ET

(c) 1999 Family Software

The 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 Computer.

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.


Family Software
3164 Surrey Lane
Aston, PA 19014
(610) 497-5561

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