Chapter 6: Open-loop operation on modified vehicles
As mentioned before, the open-loop operation of the ECU under acceleration conditions is 'guaranteed by design'. This is only possible if all the assumptions that went into the ECU programming are true. Of course, since the designers were programming for a factory car, they took the design parameters from a stock car and built them into their programming.
In order to guarantee that sufficient fuel reached the cylinders, the programmers had to consider fuel pressure, injector flow rate, injector dead time, spark time, spark duration, valve timing and a host of other engine operational parameters, down to such minutae as battery voltage. Using these values, they figured out how much fuel could be delivered in a given time, added a hefty safety margin and based their ECU program on that.
This is great, until the owner of the car begins to modify the engine to produce more power. By doing this, at least one (probably more) of the engine operational parameters is changed, and the ECU program is no longer as accurate as it was. Under closed-loop control, the changes will be detected by the ECU, which will automatically compensate for the altered conditions and maintain the correct air/fuel mixtures.
Open-loop control, however, is a different story. Without feedback control, there is no way for the ECU to compensate for (or even be aware of) the modifications and the changes they have wrought to engine operation. If enough modifications are done, it is possible that the ECU will simply not provide enough fuel to the engine, resulting in more generated heat. Enough extra heat can easily cause severe damage to an engine.
We have already seen that the ECU begins open-loop operation when the car is under acceleration. This is perhaps the worst time for racers, who often run at full accleration for prolonged periods of time. Since the ECU does not do it, it is up to the operator to monitor the engine condition and make sure that enough fuel is getting into the engine. The factory provided no gauges to achieve this, and it is necessary for the driver to have auxiliary instruments installed that let him or her monitor the important aspects of engine operation.
The most popular gauge is an air/fuel gauge. This handy little device is essentially a voltmeter with a display. When hooked into the oxygen sensor signal (the same one the ECU itself uses) the operator can watch the oxygen sensor signal to be certain that the engine has sufficient fuel. Other possible instruments include an exhaust gas temperature (EGT) gauge, an injector duty cycle monitor, and a pressure (boost) gauge to directly monitor the turbocharger.
When it comes to how rich DSMs must run during full acceleration, the problem isn't a lack of information but a surplus of information. That is, there is lots of information out there, but it's all different. There are many reasons for the discrepancies, including differences in location, weather conditions, individual automobiles, fuel quality, measuring instruments and techniques, and so forth. Because every setup is different, there is no 'magic' number that can definitively be stated as the 'perfect' number for DSMs.
However, there have been some general guidelines laid down over the years. Authorities in the field have stated that DSMs must run an absolute minimum of 850 mV from the oxygen sensor. Those who use EGT sensors have recommended a maximum operating temperature of less than 1700 degrees Farenheit (925 degrees Celcius). Different owners have different preferences, but the majority tend towards oxygen sensor readings of at least 900 mV, and EGTs below 1600 degrees F. Several racers have expressed a serious preference for even more conservative values. However, it is possible that some cars under some conditions would be able to safely exceed these values without harm.