Throttle Stop Prediction

(c) 1999 to 2013 Family Software

Proper throttle stop management is essential to vehicle consistency and predictability. A timer controlled throttle stop has several critical adjustments. These are the throttle position, or stop adjustment, the timer delay setting, and the timer duration setting. The throttle position adjustment limits how far the throttle plates are closed on activation. The timer delay setting is a variable, input by the user, in seconds, tenths, and hundredths, that delays the point at which the device will activate. The timer duration setting, also input by the user, determines the length of time the stop is to remain activated. This exact combination of throttle position adjustment and timer delay setting determines how dramatic of an effect the timer duration setting will have on elapsed time. This relationship, in fact, defines the ratio that exists between timer duration and change in elapsed time. A ratio of 2 to 1 would be established if one (1) second of timer duration was equal to a 5 tenth change in ET. Using a smaller duration setting is one way to achieve better consistency, because, the longer the stop is on, the less linear the ratio can become. This is true unless you have a perfectly linear throttle stop ratio. On a low gear throttle stop program, engine RPM will increase greatly the longer the stop is on. The car is accelerating while on the stop. This makes your throttle stop ratio (and Throttle Stop Factor) a variable! To compensate, the throttle stop is activated sooner, when vehicle speed is lower. Ideally, the system should perform a brief, hesitation in performance. Racers talk about the "cruise RPM", however the cruise MPH is the actual determining factor. With a torque converter, vehicle MPH is not 100 percent dependent on engine RPM and neither is your throttle stop ratio. Driveshaft RPM (MPH) is. In most cases, no matter how "flat" your "cruise RPM" curve is, the car is still accelerating,

Throttle Position Adjustment
The most important step in throttle stop setup is to determine a permanent throttle position adjustment. This will depend greatly on the type of throttle stop and how responsive the induction system is. The perfect combination would have no variables associated with the activation or deactivation of the throttle stop. Possibly, direct port fuel injection with a throttle linkage type actuator would be the best choice. Here, the throttle could be closed almost completely, then reopened, without incurring any hesitation due to poor throttle response.

On carbureted engines, it may be advantageous to limit the closing of the throttle plates to the operating range of the main enrichment system. This is mandatory, when using a throttle-body style unit mounted under the carburetor, because the accelerator pumps do not function on deactivation of the throttle stop. Otherwise, an unpredictable bog or surge will certainly occur. With the actuator located in the linkage, the accelerator pumps will function. However, if the throttle plates are closed to where the main system stops flowing, then the accelerator pumps are being overly relied on for good response. Provided that the volume of fuel delivery is adequate, larger total jet area, higher float levels, and increased initial ignition timing all help to improve throttle response. Ultimately, the throttle plates should be closed as far as possible while still maintaining maximum consistency of throttle stop performance. This can only be achieved through direct experimentation and analysis. A highly accurate data recorder, like our DataMaster (tm), is mandatory for throttle stop racing. Start by adjusting the throttle plates to a position just above the point where the venturi boosters begin to flow strongly.

Timer Delay Setting
Without an on-board computer, time slips must be used to analyze each run. In this case, and with some low gear setups, it's sometimes best to set the timer to activate the stop after the car has tripped the 60-foot clocks. Otherwise, the traction factor becomes an unknown. An inconsistency could be due to traction or throttle response. Since the timer delay begins counting before the elapsed time clocks do, it is necessary to add the vehicle's reaction time to the 60-foot time to compute the proper timer delay setting. The roll out is the elapsed time from when the trans-brake is released to when the front wheels clear the stage beam and start the timing system. In an example car, the average 60-foot time is 1.250 seconds, and the roll out is .250. The 60-foot time plus the roll out equals a total of 1.500 seconds. So, the timer delay must be set to at least 1.500 seconds or the stop will activate before the 60-foot mark. For a safety margin add one tenth to this computation. For this example then, 1.60 would be entered as the timer delay for each test run.

In order to test for consistency of the desired throttle position adjustment, enter 2.00 for the timer duration and make several runs in the same lane. Without a weather computer, all runs must be made under exactly the same conditions. However, a good weather computer can be used to correct all the 60-foot times and 1320-foot times to Standard Pressure (STP). This process removes the effect that weather conditions have on the run. Handily, this allows direct comparison of any run without regard to atmospheric conditions. Next, select all the runs where the STP 60-foot times are almost identical. Compare the STP 1320-foot time of each of these selected runs. Any variance between runs accurately reveals the consistency of the current throttle position adjustment. Continue this process, opening the throttle plates slightly, until the best results are achieved. The throttle position adjustment should never be changed after this point. Our ET Predictor II weather computers and RaceLog Pro software, have a proprietary Run Segment Analysis (TM) program that will show exactly how much a throttle stop, or other variable, is effecting performance at each point on the track.

Timer Duration Setting
Once the throttle position adjustment and timer delay setting are established, a ratio exists between the amount of timer duration and reduction in performance. When the value of this ratio is known and throttle stop performance is consistent, we can accurately predict the timer duration setting required for any elapsed time.

In order to obtain the existing ratio, it is necessary to compare a minimum of two throttle stopped runs, one fast and one slow run. You need to have at least a one (1) second difference between timer duration settings in order to see the effect of the throttle stop. Less than that and the small variations in traction and/or wind will effect the computation greatly. You'll always get a different result. You want to find your ratio using the middle of your throttle stop curve. So, if your stop duration on average will be somewhere about 2.500 seconds, make your first run with 2.000 seconds in the timer duration, then make your second run with 3.000 seconds in the duration. If the STP 60-foot times are good, these two runs can be used for comparison, otherwise the process should be repeated. Subtract the STP 1320 ET of the first run from the STP 1320 ET of the second run and divide the result by the difference in timer duration. For example, the first run netted an STP ET of 8.750 at 2.000 duration. The second run netted an STP ET of 9.250 with 3.000 in the timer duration. The result of 9.250 minus 8.750 is .500. So, one (1) second difference in timer duration equals .500 seconds of reduction in performance. The ratio then is 2 to 1 (1 / .500 = 2) and the throttle stop factor is .500 (1 / 2 =.500).. If throttle stop operation is consistent and the throttle stop ratio is linear, this factor should stay the same as long as the same Cruise RPM is maintained. If the system cannot be made to perform consistently, or the throttle stop ratio is not perfectly linear, it is possible to use an average throttle stop factor, computed from several pairs of test runs. Our software has a built-in program to easily calculate the throttle stop factor from two or more runs.

Predicting Throttle Stop Timer Duration
Predicting the throttle stop timer duration is easy, once the throttle stop factor is known. Using a throttle stop factor of .500, previous stop duration of 2.50, and a predicted elapsed time of 8.75, the new timer duration would be 2.50 seconds, in order to slow this car to an 8.90 index.

ET Predictor II - Sharp EL-5500 II / Sharp PC-1270
To predict throttle stop timer duration using the ET Predictor II computer, the user presses Program Start Key #7. The program begins by displaying the current value for the throttle stop factor, in this example 'TS FACT: 0.1125'. The user then presses Enter to input a new value for TS FACT. The program prompts with 'TS FACT?'. The question mark shows that the program is awaiting new input. To use the currently stored value, as in this case, just the Enter key is pressed. The next value to be displayed is the predicted ET, and appears as 'PRED ET: 8.855'. The predicted ET is updated every time the ET Prediction program is run. Again, a new value can be entered here, or the current value can be accepted. The same procedure is followed for entering the index (INDEX) and base (or previous) timer duration setting (BASE TD). The base timer duration is needed to predict the new timer duration. It is the duration setting used on the same run that is being used to predict the ET from. Finally, the predicted timer duration is output as 'PRED TD: 2.40'. The user may elect to print out the results. The program will prompt, 'HARD COPY?(Y/N)'. The 'YES' button is pressed for a hard copy, or the 'NO' button, to skip this option.

For technical help or other questions you may have please write or call:

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

Back to Articles