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CAR PROJECT 1: Simple Variable Boost Control For Cars With Turbochargers Note: early prototype board shown. Quite a few cars have turbochargers these days and these can provide even more performance if the turbo boost pressure is increased. However, you don’t want permanently increased turbo boost as it increases wear on the engine, so you need a variable boost control, as presented here. Design by DENIS COBLEY Fig.1: the circuit is based on just two transistors and a couple of diodes. It intercepts the standard boost signal from the car’s engine management computer (ECU) and stretches it so that the signal to the boost control solenoid has a longer duty cycle T HIS VARIABLE Boost Control is a simple circuit to modify the factory boost levels. It was designed in the first instance to suit a Subaru WRX but it could be adapted to many cars and FWDs fitted with turbochargers. It employs a small PC board and requires only four wire connections to the car’s electrical system. Before we go any further we should 36  Silicon Chip warn that if you want to fit this project, you must have a turbo boost gauge permanently fitted. Operating variable boost above factory set levels can blow your engine if you don’t know what the boost level is. How it works The Variable Boost Control is a simple interceptor – it “intercepts” the standard boost signal to the Boost Control Solenoid from the car’s engine management computer (ECU) and stretches it so that the solenoid signal has a longer duty cycle. Most factory boost control systems use a variable width pulse signal to control the boost solenoid. This operates at about 14Hz to suit a Subaru WRX (1998 model). While the circuit of Fig.1 looks simple, it works quite well and has been fitted to several cars. The ECU boost pulse waveform is fed to the input which has a 180W 1W pull-up resistor. This is sensed by the ECU as the boost solenoid’s coil. From there, the signal is fed via a 4.7kW resistor to the base of transistor Q1 which inverts the signal at its collector. The inverted signal charges a 2.2mF capacitor via a 1kW resistor and diode D1 only to be discharged via 10kW trimpot VR1 each time the signal at the collector of Q1 is pulled down to 0V. The net result is a roughly sawtooth waveform with a slightly rounded leading edge and gently sloping trailing edge, the steepness of slope being dependent on the setting of VR1. This fairly rudimentary pulse signal siliconchip.com.au is fed to the BD681 Darlington transistor (Q2) via a 22kW resistor which drives the boost control solenoid. Fast recovery diode D2 is connected from the collector of Q2 to the +12V line. It is included to damp the spike voltages generated each time Q2 is turned off. Incidentally, Q2 inverts the signal back to the same polarity as the input, so that it drives the boost control solenoid correctly. A BD681 Darlington transistor is specified for Q2 since it has a high collector voltage, high gain and an adequate collector current rating (4A) to carry the currents of typical boost control solenoids. Note that in some cases Q2 may need a small flag heatsink. Note also that this circuit has no protection against reversed supply voltage so if you connect the supply leads the wrong way, both diode D2 and the Darlington transistor are likely to be instantly destroyed. The current path will be via the Darlington’s internal reverse diode and D2. You have been warned! The circuit operation is clearly demonstrated in the scope waveforms of Fig.2. The top waveform (yellow trace) is the input signal, a series of negative-going pulses. The middle waveform (purple trace) is present at the cathode of D1 while the bottom trace (cyan) is the output waveform at the collector of Q2. Note that the negative going pulses of the output waveform are substantially longer than those of the input waveform. Our test waveforms were made while the circuit was driving a resistive load rather than a boost solenoid but the operation was otherwise the same as would occur in a car. Building it The Variable Boost Control is assembled onto a small PC board measuring Fig.2: the top waveform (yellow trace) is the input signal (a series of negativegoing pulses). The middle waveform (purple trace) is the signal at the cathode of D1 while the bottom trace (cyan) is the output waveform at the collector of Q2. 38 x 30mm. The wiring diagram is shown in Fig.3. Note that the BD681 should be laid flat down on the PC board so that the whole assembly can be sheathed in a piece of heatshrink sleeving. Apart from two wires being needed to connect the 10kW potentiometer (VR1), you will need four wires to connect the Variable Boost Control to your car. These are as follows: • +12V IGN – Red • Earth (chassis) – Black • Boost wire from ECU – Green • Output to Boost Solenoid – Blue The +12V IGN wire can be tapped 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. Then you will need to identify the wire from the continued next page Fig.3: here’s how to install the parts on the PC board. Take care with the polarity of the transistors and diodes. siliconchip.com.au Parts List 1 PC board, code 05102072, 38 x 30mm 1 BC547 NPN transistor (Q1) 1 BD681 NPN Darlington power transistor (Q2) 1 1N4148 small signal diode (D1) 1 FR307 3A fast recovery diode (D2) 1 2.2mF 16V PC electrolytic capacitor 1 10kW potentiometer (VR1) Resistors (0.25W, 1%) 1 22kW 1 4.7kW 1 1kW 1 180W 0.5W Fig.4: this is the full-size etching pattern for the PC board. February 2007  37 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