Fullscreen HTML5Med Res (??MB)HTML4

CAR PROJECT 2: A Fuel Cut Defeater For Cars With Variable Boost Control Note: prototype board shown. Are you intending to build the Variable Boost Control described earlier? If so, you will need to built this simple Fuel Cut Defeater (FCD) to eliminate the standard factory fuel cut which typically occurs at boost levels about 16-17psi. The unit is not adjustable, so no external controls are required. Design by DENIS COBLEY T HE FUEL CUT DEFEATER is another simple “interceptor” design. It modifies the signal from the engine’s MAP (Manifold Absolute Pressure) sensor and stops it from exceeding a particular level before feeding to the ECU. Therefore, the ECU does not sense the over-boost condition brought about by the Variable Boost Control and hence does not cut the fuel supply via the injectors. The circuit is built on a small PC board and there are just four external connections: ie, +12V, 0V (chassis), MAP sensor and the output to the ECU. The circuit (Fig.1) is very simple and is based on a TL072 dual FET-input op amp package. IC1a is connected as a unity gain buffer (with its output connected directly to the inverting input) so that there is negligible loading of the MAP sensor signal. Input protection for IC1a is provided by the series 10kW resistor and by diodes D1 & D2 which clamp any large signal transients. Level clamp IC1b and diode D3 act as a level clamp once the output signal from IC1a exceeds the threshold at pin 5 of IC1b, as set by the three resistors and 5.6V zener diode ZD1. In fact, IC1b works as an inverting comparator, with the voltage at pin 5 set to around +3.9V. For output signals from IC1a of less than +3.9V, the output of IC1b will be close to +12V and diode D3 will be reverse-biased. Hence, IC1b has no effect on the output signal from IC1a and it passes unmodified to the ECU, via the 1kW and 680W resistors. However, once the signal at pin 6 of IC1b exceeds +3.9V, IC1b’s output goes low (0V) and diode D3 will be forward biased and it will conduct to shunt the output signal to the ECU, clamping Variable Boost Control – continued from page 37 ECU to the boost control solenoid and cut it. This should be done around 50cm or more from the ECU to make it difficult to detect. The end going to the ECU goes to the green wire on the Variable Boost Control, while the end going to the boost solenoid goes to the blue wire on the Variable Boost Control. Note that these connections should be made with male and female bullet or quick connect crimp 38  Silicon Chip connectors to ensure the integrity of the connections. Using these also means that you can quickly restore the standard boost connection, if you need to. The Boost control pot VR1 needs to be installed on your car’s instrument panel. Setting the boost level As already noted, you MUST have a boost gauge fitted at all times to monitor the boost levels. Drive the car up a long hill in third gear and set VR1 to provide the required boost level. Warning: exceeding factory boost levels can reduce the life of the engine and transmission. Finally, note that you may need the companion Fuel Cut Defeater, described in this issue, to remove the factory fuel cut which is typically set to activate at boost levels above SC about 16 or 17psi. siliconchip.com.au Fig.1: the circuit is based on a TL072 dual op amp (IC1a & IC1b). IC1a operates as a unity gain buffer, while IC1b works as an inverting comparator. it to a level of about 1.4V. Thus the ECU does not see MAP signals above a certain threshold and so it cannot cut fuel as it normally would in an over-boost situation. This last factor means that it is extremely important to ensure that you always monitor the turbo boost on your car if you are using this Fuel Cut Defeater in conjunction with the Variable Boost Control. Assembly & fitting Assembling the PC board (see Fig.2) is straightforward but make sure that you install op amp IC1 and the diodes the right way around. When you have finished the assembly, check all your work and then connect a 12V supply to the PC board. Make sure that you connect the supply with the correct polarity otherwise you are likely to damage diodes D1 & D2 and the op amp. Check that the voltage at pin 8 of IC1 is +12V (or close to it), with respect to pin 4. The voltage at pin 5 should be close to 3.9V. Note that this setting is designed to suit a Subaru WRX and may possibly need adjustment to suit the MAP sensors in other vehicles. When installing the PC board in your car, the +12V wire can come from any point which is switched by the ignition switch. The Earth wire can be run to any convenient point on the car’s chassis. You will then need to identify the wire from the MAP sensor to the ECU solenoid. The one to use has about 1.4V on it at idle. Cut this wire at around 50cm or more from the ECU to make it difficult to detect. The end going to the MAP sensor ECU goes to the green wire on the Fuel Cut Defeater (FCD). The end going to the ECU goes to the blue wire on the FCD. Note that these connections should be made with male and female bullet or quick connect crimp connectors to Fig.2: follow this parts layout diagram to build the Fuel Cut Defeater. Note that diodes D1 and D2 face in opposite directions. siliconchip.com.au ensure the integrity of the connections. Using these also means that you can quickly restore the standard boost SC connection, if you need to. Parts List 1 PC board, code 05102071, 50 x 33mm 1 TL072 dual FET-input op amp (IC1) 3 1N4148 small signal diode (D1,D2,D3) 1 5.6V 400mW zener diode 1 100mF 16V PC electrolytic capacitor 1 100nF MKT polyester or monolithic (code 104 or 100n) Resistors (0.25W, 1%) 1 10kW 1 3.9kW 3 1kW 2 680W Fig.2: this is the full-size etching pattern for the PC board. Check the board for etching defects before installing the parts. February 2007  39