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EFI Tuning Fundamentals: High and Low Impedence Injectors

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High and Low Impedence Injectors

05.31

00:00 - We've just covered the injector outputs in the ECU.
00:03 But we also need to be aware of the actual injector design, as this can affect the way the ECU controls the injector.
00:10 Injectors are broken down broadly into two types.
00:12 These are known as high impedance or low impedance.
00:15 The injector drive hardware in the ECU must be compatible with the type of injector you're using.
00:21 You can easily tell what sort of injectors you have by using a Multimeter or a digital volt meter to measure the resistance across the two electrical terminals of the injector.
00:31 When we're dealing with injectors, the resistance is often referred to as impedance.
00:36 This is just a different name for the same thing.
00:39 Low impedance injectors will have a resistance of approximately 0.5 through to around five ohms.
00:45 While high impedance injectors will usually have a resistance between about eight and 15 ohms, so it's really easy to tell them apart.
00:53 A low impedance injector is also known as a peak and hold injector, while a high impedance injector may be referred to as a saturated drive injector.
01:02 This refers to the way in which the ECU needs to control the injector.
01:06 The saturated drive style is the simpler of the two and doesn't require any real sophistication on the part of the ECU.
01:13 When the ECU wants to inject fuel, the injector drive switches to ground, completing the electrical circuit for the injector so it can then operate.
01:21 In this style of injector, the resistance of the injector controls the current in the circuit, and prevents damage to the injector drive in the ECU.
01:30 A peak and hold injector, on the other hand, requires a more sophisticated driver in the ECU that can control the current in the injector.
01:37 A peak and hold drive provides a high initial current to quickly open the injector, but once the it's open, less current is needed to keep it that way so the current is then reduced to a holding current.
01:49 Hence the term, peak and hold.
01:51 With these sorts of injector drives, a four to one ratio of peak current to holding current is common, and that means the injector might initially be provided with a four amp peak current, and then a one amp holding current.
02:02 The reason that peak and hold injectors exist, is that the high peak current provides more energy to drive the injector open quickly.
02:10 This can allow the injector to open and begin supplying fuel quicker, reducing the lag between the ECU signalling the injector to open, and fuel actually flowing.
02:20 This lag is known as the injector dead time or latency and we'll be covering this in detail in a future module.
02:27 Peak and hold injectors were designed to help combat the slower injector operation that was a result of manufacturers producing higher flowing injectors back in the 90s.
02:37 As the injector size increased, so did the size and the mass of the internal valve mechanism that needs to move in order for the injector to open and start passing fuel.
02:45 This heavier internal mechanism had more inertia and hence it took longer to open.
02:51 The higher initial current offered by a peak and hold injector driver went some way towards improving this situation.
02:57 These days, the modern crop of EV14 based injectors that have become prevalent in the performance after market are all high impedance.
03:06 This isn't to say that low impedance injectors are extinct, but you're much less likely to come across them.
03:12 Regardless of this, though, it's still important to understand the difference between high and low impedance injectors, and how to identify what you have.
03:20 As far as the ECU is concerned, some are compatible with either type of injector, while others are only compatible with high impedance injectors.
03:28 Check the instruction manual to be sure though because using the wrong injector type can do expensive damage to the ECU's injector drives and possibly to your engine too.
03:38 If you're performing an injector swap on a car that still runs the factory ECU, it's important to make sure that you match the impedance of the injectors being removed, so testing the stock injectors should be one of the first jobs you perform before pulling out your credit card.
03:53 Many factory cars produced in the '90s and 2000s came equipped with low impedance injectors, yet their ECUs were actually fitted with saturated injector drives.
04:03 From what we've just discussed, this sounds like a mistake, however to prevent damage to the injector drives in the ECU, an external ballast resistor pack was wired into the injector circuit to add impedance to the injector drive circuit and control the current.
04:18 This sort of a system makes the low impedance injector look like a high impedance injector as far as the ECU's concerned.
04:25 If you're upgrading injectors on this sort of car, then you have two options.
04:29 If you fit a new set of injectors that are also low impedance, then you've got nothing to worry about and everything will work as intended.
04:36 If, on the other hand, you're fitting a high impedance replacement injector, it is important to remove and bypass the ballast resistor pack, or the overall impedance in the injector circuit will be too high, affecting the injector operation.
04:49 The other scenario you may come across, is where you want to fit low impedance injectors to an ECU that has a saturated injector drive.
04:57 In this case, you need to increase the impedance provided in the injector circuit by way of adding a ballast resistor pack.
05:04 The easiest option for doing this is usually to use a ballast resistor pack from a factory vehicle, as they're designed for this exact task and are easy to mount and they're easy to connect.
05:14 So in summary, it's important to understand that there are two types of injector you're likely to come across.
05:20 And these can have important implications when it comes to the ECU.