The fuel control system for Unfair is complicated by the need for the car to have multiple personalities. On one hand it needs to be able to cruise nicely, but when pushed hard, the fuel demands increase dramatically. During cruise the fuel pump may only need to deliver 4-5 gallons/hour @ 42 psi, but when running an eight-second pass, it will need 135 gallons/hour at 70 psi. By using a fuel control system that utilizes pulse width modulation (PWM), Unfair can have the best of best worlds.
Pulse width modulation is a very fast-acting electronic switching mechanism. The pulse is a square wave function where the switch is either on or off. In the on position, full battery voltage is applied to the pump. The switching function time is called the duty cycle. If the switch is turned on for 50 ms, then off for 25 ms, and repeated, the duty cycle is 66 percent [50 ms / (50 ms + 25 ms)]. By switching the fuel pump on and off very quickly, the pump only sees battery voltage when the PWM switch is on. Unlike a rheostat that would reduce voltage and increase amperage/heat, PWM simply reduces the speed of the fuel pump without the heat.
GM uses PWM technology in mass production. The new Camaros, CTS-V Cadillacs, and many others use similar PWM technologies. Why would GM choose to use a more complex fuel delivery system? The answer has two parts. The first is reliability. By slowing down the pump, wear is decreased and the chances of a warranty return are minimized, plus the pump noise is significantly reduced. The other is that the car can also have a dual personality like Unfair. Turn down the fuel pressure and flow during cruise and idle, then increase them to give the fuel injectors greater fuel delivery capability.
However, OEMs integrate the engine PCM and the fuel system control module (FSCM) to make it work. The FSCM is not programmable, so making the needed changes for Unfair would not be possible. That’s where VaporWorx comes in. The VaporWorx kit is a stand-alone system that requires no PCM commands to operate and can be tuned to suit the engine needs. The fuel pressure sensor output can also be monitored using the Stack instruments (Super Chevy, July ’11) to display actual fuel pressure and set alarm limits. This will also allow data logging so that fuel vs. MAP pressures can be compared and engine tuning changed as needed. The average output voltage of the PWM controller can also be monitored. Though not the true voltage, most sensors cannot react fast enough to the PWM signal and hence display an average voltage. This will give a good indication of how close the battery voltage matches the output of the PWM controller.
Walbro Engine Management
6242 Garfield Street
II Much Fabrication
228 E. Sunset