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,
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.
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.
Throttle Stop Timer Duration
Predictor II - Sharp EL-5500 II / Sharp PC-1270
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