$25 Additional Injector Drive


This note contains many modifications which may void your vehicle's warranty, or even be illegal in some areas. It is your responsibility to check the validity, safety and legality of anything here.

If you want an all-singing, all-dancing controller this is not for you. If you want an effective, low cost solution to getting a little extra fuel into the engine when it needs it, then read on.


Additional injectors under the control of the factory computer are one solution to dealing with increased boost, and possibly a fuel-cut defender. The problem is, how do you drive the additional injectors?

The simplest answer would be to wire an extra injector in parallel with the original. This will work if your computer has the current ability to handle it, but many don't and a blown computer is very expensive. A better way is to use the drive signal to control, but not drive the additional injector. Sounds simple huh? Actually it is...

The advantage of this method is that if we add just one injector, the ECU, via the readings from the Exhaust Oxygen Sensor (Sometimes called a Lambda, or EGO Sensor) will normally be able to keep the mixtures in check.

Remember that this circuit will not drive "Peak/Hold" injectors. I have decided to create a new version which will. More detals soon.

The Circuit

The circuit is hardly original. It's a textbook Darlington switch, which requires just 700 microAmps from the computer to turn on. Two transistors, a diode and a resistor are all that you really need, other than a suitable fuel injector of course. Here's the parts list:

Label Item
Q1 BC548 Transistor
Q2 TIP31C Power Transistor
Q3 BC548 Transistor
D1 10A 500PIV Diode
R1 12K 0.5W Resistor
R2 10K 0.5W Resistor
R3 12K 0.5W Resistor
SW1 Switch

The schematic for the driver is available in two forms:
Download the Protel Schematic file by clicking here
Or as an Adobe PDF file here
If you use Express PCB, you can use this file to send away for the board to be made up. This artwork will create two identical boards.


It would be best to make a printed circuit board, but it really is not essential. Perf-board will do nicely if you are careful.

The TIP31C transistor will be carrying several amps, so it needs a heatsink. If you mount the circuit in an aluminium box you can use the box as the heatsink, but be careful to insulate the TIP31C with a proper mounting kit. It's case (the collector) is at 12V and if it shorts to earth things will go very badly indeed.

Given the current that a fuel injector can draw, a heavy-duty relay is required. Headlight relays are ideal for this task. This way, only light-duty cable needs to be run to the switch, which is best mounted in the cabin.

The mounting and plumbing of the injector is entirely up to you, but don't wire it in just yet.

Picking an Injector

This is tricky. Since an additional injector is triggered from a single original injector, the engine will receive 1/(cylinders) extra fuel than the computer expected if a same size injector was used. This is probably going to be way too much fuel, so a smaller (ie lower flow rate) injector is going to be required, or use a pressure restrictor in the line to the additional injector.  Experimentation is going to be required, and remember that if the computer operates in closed loop mode (almost all factory ones do) it will attempt to trim back the overly rich mixture, which will help somewhat.

Installation and testing

Power for the driver is supplied from a switched 12V supply. Make sure that power goes away when the ignition is OFF.

The open end of R1 is connected to the switched side of the target injector.  With the engine running, one side of the injector will be at a constant 12V, while the other will oscilate, because it is being switched to ground every time the injector "fires". This is side you need to connect to.

To test the unit, don't connect the new injector just yet. Rather, use a 12V lamp in it's place, and hook everything else up. With the switch off, the lamp should be OFF.

Start the engine. The lamp should still be OFF.

Now, turn the switch on. At idle, the lamp should be on dimly or even flickering. As revs increase, the lamp brightness will also increase, indicating that duty cycle of the original injectors is being followed.

If all went well, switch off, shut down and you can now connect the new injector.

In practice, you can use the switch input via a microswitch on the throttle, so it only operates at full throttle, or perhaps a boost pressure switch to trigger when the unit reaches a preset boost level.

Going Further

So what if you need even more fuel? You can run multiple copies of the circuit, connected to other injectors.

A more clever approach would be to monitor the injector rate and/or duty cycle to determine the best time to switch in extra injection. This is something I'm working on at present, so watch this space!

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Copyright, 1997 - 2011 Christopher F. Moran.
No part of this work may be reproduced without prior authorisation.
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While every possible care is taken to verify the material published on these pages, the authors accept no responsibility for any content or any effects arising from using the content.  You should check with local authorities regarding the legality of any modifications listed here.

Always drive your car sensibly and according to the relevant laws in force where you are.
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