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How To Choose The Right Fuel Injectors

When it comes to engine performance, the buck ultimately stops at fueling. After all, if you don’t have enough juice to go with that cool air and hot spark, you’re ultimately selling yourself short. Worse still, a fuel system that can’t keep up with the rest of your setup can ultimately lead to engine damage. And that’s why it’s important to make sure the fuel injectors you’re using are up to the task.

Obviously, if the injectors haven’t been maintained and they’re not operating efficiently, significant performance and engine durability issues can occur. If their health is unknown they should be flowed and cleaned and replaced as necessary. So you have the sheer functionality aspect of it to consider. But the biggest factor to consider outside of the injectors simply being worn out or contaminated is the duty cycle range you operate the injector in.

The data logs in certain EFI products will provide this information, (we prefer Holley EFI) which ultimately tells you whether or not the injector’s capacity is able to keep pace with the engine’s demands. Duty cycle is the percentage of time an injector is open during one injection period (which could be one or two engine revolutions). If it’s 10%, it indicates that your injector is open 10% of the time and closed for 90% of the time at that operating point. 100% duty cycle indicates that the injector is open all the time, it is “static” and has no more flow capability .

Ultimately, you really don’t want to see it over 85%. Anything above that is too high. And you risk the engine running lean, which could lead to very big problems, even with an engine that’s naturally aspirated.

So it’s clear that we need to keep the beast well-fed. But it’s also important to keep in mind that over-feeding can lead to other issues as well. But where’s the sweet spot? Are there other design factors to consider as well? Let’s break it down.


High Impedance Vs. Low Impedance

In the simplest terms, the difference between the two is the resistance of the coil in the injector. A low impedance injector usually has about 2-3 ohms of resistance across the injector coil. This can be measured with a basic volt or ohm meter.

Generally speaking, high-impedance injectors are more often found in street applications. They use a “saturated injector driver” in the ECU which is more cost effective. High Impedance injectors usually measure 10-14 ohms.

Low impedance injectors, on the other hand, use what’s called a peak-and-hold driver. “With a really large injector and higher fuel pressure, more current allows the injector to open faster and more consistently. That driver regulates anywhere from 4 to 10 amps of current to open the injector, then it reduces the current from that level down to 1 to 2.5 amps to keep it open. So the additional current is able to open the injector quicker, then it reduces it to make sure the current load isn’t too high. There’s really no downside to a peak-and-hold injector other than the cost – and the fact that none of the factory systems come with that type because of the cost.” And for all these reasons, low impedance injectors are most often seen in purpose-built race applications.

To determine which type of injectors your vehicle uses, you need to measure the impedance of the OEM injectors and choose injectors that match the impedance, otherwise you run the risk of damaging the vehicle’s ECU. Some aftermarket ECUs can drive both high and low impedance injectors, however, so in those cases it’s best to refer to the ECU’s documentation to verify what the system supports.


Body And Connector Styles

EV1, EV6, and EV14 are common injector body style designations that were internally established by Bosch. While these body styles are widely used throughout the automotive world, Bosch is not the exclusive manufacturer to the industry – Delphi, Denso, and others also supply injectors to specific brands.

And that’s important to note because more often than not, the body style will indicate the type of connector the injector uses, and in some cases that connector may be OEM-specific.

Still, odds are your system uses one of the two most common connector styles – the older Jetronic / Minitimer style connector, which uses a rectangular housing and two flat connector prongs, or the newer USCAR connector, which is square and uses narrower, rounded prongs.


Flow Rate And Sizing

In simplest terms, static flow rate is the maximum amount of fluid that an injector will flow at wide open operation. But as mentioned earlier, bigger doesn’t necessarily mean better in every situation. The thing to remember is that you don’t want an injector that’s too small, but you also don’t want an injector that’s too big. There’s detriments to both.

We know that injectors that are too small can cause a lean condition in the engine, but what happens if you venture too far in the opposite direction? The biggest issue comes from operation in the injector’s non-linear range due to oversizing.

During normal operation when you open and close an injector – half on and half off – it will essentially flow at 50% of its capacity. And as you open or close it more or less, it will flow more or less linearly. But when you get the pulse width down low, in its non-linear range, all of a sudden you’ll say, ‘I want 3% less fuel’ and the system and it will give you 30% less fuel.

And that can cause a range of performance and drivability issues. “You don’t want to run an injector in its non-linear range because it’ll be very difficult to tune,” he says. It will also adversely affect idle quality and throttle response. There are tables in Holley EFI and OEM ECU’s to factor this out, but the user needs to have the linearity data from the injector to enter which most don’t have.

If you’re trying to tune the injector end angle for peak power on a race motor, you don’t want a duty cycle of more than 40-60 percent. You don’t want to spray an injector during the overlap of the intake and exhaust valves opening, otherwise your fuel goes in the intake valve and right out the exhaust valve and into the exhaust. So in the software you would program in where you want the injection to start and end to avoid that. But if your duty cycle is too high, you have no choice in the matter, and you’re wasting fuel.

So where is that elusive sweet spot for a given application? The basic formula you can use to determine injector flow is the following:

Horsepower x BSFC [Brake Specific Fuel Consumption] divided by the number of injectors, divided by the max duty cycle you want to run

You need to know what the BSFC is of the engine you’re running. That’s how many pounds of fuel the engine uses per horsepower it makes – basically a measurement of the engine’s efficiency. And the type of fuel you use can have a big impact on that: If you’re using methanol, for instance, it uses two-to-four times more fuel than gasoline to create the same amount of energy, so you need an injector that’s two-to-four times bigger with a methanol system than you would with a similar one running on gasoline.O