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Pick the right fuel injector 



This information is written to give an idea of ​​how fuel injectors work (not diesel or piezo) and facilitate the selection of injectors and clear up common misunderstandings. There are always deviations in practice, but the focus here is to keep it very simple, so there are only a few points to cover.


Fuel injector types

Fuel supply
Top fed injector that is fed with fuel through the top of the injector and mounted on a fuel rail (fuel rail) that distributes fuel to the various injectors. This variant is most often found on cars from the European and American markets.

Side fed injector that is fed with fuel from the side and mounted in a fuel rail that distributes the fuel to the various injectors. These are most commonly found in the Asian market.

The top fed injectors, regardless of brand, often use some kind of Bosch standard measurements, although it may differ slightly in size.



Compact body (33.6 mm between O-ring grooves)
Standard body (48.65 mm between O-ring grooves)
Long body (60.65 mm between O-ring grooves)

The injector tip then protrudes 11 mm from the upper O-ring groove in the standard version. There is also a longer injector tip that protrudes 24mm from the upper O-ring groove.

In addition to the above, there are a plethora of sizes adapted to specific car models, but then they are more difficult to adapt to the aftermarket, even if an injector from a certain engine can fit others as well.

The mounting holes or O-ring diameter of the injectors are often referred to as 14 mm. The collar on the O-ring groove is 13.2 mm and the largest dimension on a fitted suitable O-ring will then be 15.2 mm. These dimensions fit in 14.5 mm holes.

Long tip fuel injectors (xT)

Some injectors have an extension from the lower o-ring that measures 24mm. This can be inserted into the intake or removed using a lower adapter to get the standard 11mm protrusion.

Fuel injector size

Duty cycle of the injector

The guideline value for the maximum degree of utilization of injectors in the industry is 80% (0.8) but can vary greatly depending on the application and selected components.

Why you do not want to use the injector to the max depends on many different factors but the possibility of adaptation when driving due to external damage such as clogged filters, worn fuel pumps, quality of fuel, etc.

Fuel filters such as high pressure filters and pre filters as well as fuel pressure regulators are three of the most common causes of lost fuel pressure. Something to keep an extra eye on.

Tension, pressure and temperature are some other factors that affect and make you want the last 20% margin.

BSFC - brake specific fuel consumption

A measure of how much fuel per hour is needed for a given engine to produce a horsepower.

General values ​​for petrol engine you can use if you have no previous history of the engine to run on (At E85 add 0.15):
Engine N/A: 0.5
Compressor engine: 0.6
Turbo engine: 0.65

Units of measurement

It is common for different manufacturers and countries to use different values on injector measurements. Be careful with this or it will go wrong.

CM3 / min
G / min
CC / min
LBS / hr (LBS / hr * 10.5 = CC / min)
LB / hr


What should be mentioned first and foremost is that there are as many ways to calculate this as there are people. Also that you can take a lot more information into account to get as accurate a value as possible such as the engine efficiency, fuel density and energy content, etc., etc. But the important thing is that we do not choose an injector that is too small, then we work with the injectors properties and design to find a one that is suitable for the engine and application.

We now use general values ​​to determine the injector size:

(Engine power x BSFC) / (Number of injectors x Degree of utilization)

Example of a newly built 4cyl supercharged petrol engine which is estimated to have 400hp.

(400 x 0.65) / (4x 0.8) = 81.25 LBS / hr (81x25 * 10.5 = 853cc)

Here we have come to the conclusion that we should have at least 853cc injectors for the above mentioned engine and then you choose the nearest injector type that fits and works with the fuel pressure, injector image, spray angle, resistance, fuel compatibility and working temperature described on this page.


Fuel injector flow matching

When manufacturing injectors, small variations occur which mean that the flow on a size of injector can vary by several percent. What differs is due to precision in manufacturing and of course the extra manufacturing cost that would be if you were to make these exactly directly from the factory. Factors that affect the flow are coil winding, injector disc, valve seal to name a few examples.

The car manufacturers use injectors with the variations the factory must stay within and have very large margins of error instead.

The aftermarket often uses small margins of error due to high power outputs with smaller safety margins and limitations of testing possibilities in a virtual environment as well as practical tests in different conditions.

Therefore, it is good to flow match the injectors that are not made from the factory. In short, this means that, for example, 100 injectors are paird to flow within the new framework, which in the aftermarket is usually 1%, which is very good if you compare with other sources of error that exist that have much greater variation.

A ECU system can today compensate fuel on each cylinder together with separate lambda sensors and EGT sensors on each cylinder, but despite this it is best to have a set of injectors that flow equally on each cylinder.


Fuel injector resistance

Injector resistance
Resistance is a current limiting ability of an electrical circuit (eg a coil in a injector). The higher the resistance value of the coil, the higher the voltage required to drive it. Resistance is measured in ohms.

Some non standard ECU systems only support high resistance injectors and those who then for various reasons mount "low ohm injectors" connect a resistor in series on each injector circuit to achieve the resistance for which original control systems are built (high resistance). There are variations and exceptions here too, but this is not something you want to achieve, so we do not go any further as you also manipulate the injector function in this way.

Peak and hold (low resistance)
Peak and hold injectors / signals are used to control "low ohm injectors". There are two sequences where the peak is exactly as it sounds, a first high voltage (peak) is sent out from the control system to quickly open the injector. Then a second sequence (hold) where the voltage is lowered to just keep the injector open.

Saturated (high resistance)
These are "low ohm injectors" that open and close with a low voltage.

Low or high resistance?
2-4ohm usually has an injector with low resistance and high resistance usually means somewhere between 8-16ohm

Before, everybody wanted Peak and hold injectors as they were available in larger sizes and could handle higher fuel pressures with better control. Today, you choose the injector that best suits the application as most aftermarket control systems support both varieties. Regardless of low or high resistance, peak and hold or not, today's large injectors are often "high-ohm" and are able to open and close at least as well as "low-ohm" due to that they can handle a higher voltage, have a faster response time, lighter internal components and generally better control than the old ones.


Fuel injector inductance

Inductance is an electric current that flows through a circuit (eg a coil in a injector) and causes a magnetic field and thus a current through the circuit. The relationship between the magnetic field and the current is called inductance. This is one of many parts that are measured during injector service to check that all the injectors are in good condition and that no one deviates to prevent future problems and downtime.


Workring pressure

The working pressure / flow of a injector is specified by the manufacturer and it is usually at 3bar. If the fuel pressure increases, the flow also increases at the same opening time. Some injectors provide a better injector spray character at high fuel pressure and some work better at a low. This is difficult to know without getting the data from the manufacturer or distributor.

If you have found the right injector for the power to be used, it is now important to ensure that the injector works within the area in which the fuel pressure will be.

The turbocharged engine provide 3 bar overpressure.
The fuel pressure regulator is linear 1:1 against overpressure
3 bar base fuel pressure.

Now the injector will have to work with 6 bar fuel pressure at top and then you want the injector spray character not to be negatively affected because then the engine efficiency will also be negatively affected.

Often large original injectors only have a fine spray character up to 5 bar, although exceptions can always occur. Here you have to make sure to look for another type of injector design that can handle the pressure. These are often referred to as motorsport ones and can often handle up to 8bar without bad injector spray character.


Working temperature

Injector temperature range
A fuel injector works between -40 degrees to +110 degrees without being significantly affected. There are set frameworks in the car industry to cope with different markets where it is very cold and very hot. This also applies to motorsport injectors and although it may differ a little, these are not temperatures that are usually a problem to stay within. In addition, modern injectors are designed so that they are cooled better by the fuel than before.


Compatibility with different media

Most injectors can run E85 and BF95 / 98. E85 requires more frequent injector service as the injector needle can start to jam that affect flow and injector character. Today's control system (ECU) can often compensate for the amount of injector with opening time, but the problem with the injector flow and injector character still remains. Therefore, a fuel additive that lubricates when using ethanol and injector cleaning once a season is always recommended.


Injector character

There are different types of injector character. These are controlled with different amounts of holes in the tip of the injector. They say one hole technique, two hole technique and multi hole technique. Multi hole technology can be between three and eight holes that distribute the fuel in different character.

The different injector character can be divided into three common ones, although here too there are exceptions with even more variants.
Conical spray
Beam spray
Two jet spray

When talking about injector spray, they are available with different scatterings or angles from 10 to 85 degrees. The most common is 10-30 degrees where larger injectors have a wider spread and smaller ones have a narrower spread.


Spray angle

In addition to the fact that there are different injector spray character, even if it is not as common, you can choose the spray angle. The angle can then often be angled in four different directions with the electrical connector as the reference point

The most common is around a 15 degree angle in one of the four directions.


Fuel injector placement

Selection of injectors is usually made only based on four things

1. How much it flows to cope with a peak power
2. If it is compatible with the control system to be used
3. Physical size
4. Is it compatible with the fuel to be used.

Something that is forgotten is what pressure it should work with, if it should be run in everyday use or only 2 hours per season, injector character and spray angle. Here you should review whether you should adapt the injector to the intake you have with fixed mounting points for the injector or whether you should adapt the intake in order to have specific requirements for the type of injector that is suitable for the area of ​​use for the engine. The choice of electrical connector must also be taken into account depending on the area of ​​use.


Fuel injector voltage supply

In a car, the electrical system is often not completely stable and therefore components that are mounted must be able to withstand fluctuations in voltage. In this case the injectors. The car's generator is built to deliver a stable voltage around 14 volts.

Deviations occur when the starter motor runs or when the battery is bad or generator and charge regulator to name a few examples. In addition, there are a variety of electrical working areas for each of said components, so a injector that can handle a wide working area is preferable to be able to adjust and compensate for this in the control system.

Injectors can often work between 6-18 volts, the specified current only applies to what the manufacturer has measured, which is often 13.8 or 14 volts. It is therefore important to adjust the injector's opening hours in relation to the voltage.

When the coil gets a pulse from the ECU to open, it takes a while before there is any fuel as the injector needle must have time to leave the seat and allow the fuel to get out. This is called injector dead time. When you measure this, you clearly see the difference between old and new technology as well as peak and hold (low resistance) and saturated (high resistance). It is usually around a millisecond opening time and all these details make the big picture.


Fuel injector connector

The injector electrical connector is usually not included with the purchase. However, there is a plethora of this and the choice often falls on what kind of connector there is on the injector that you have chosen according to specification. There may be times when you absolutely must have a weatherproof connector and must then include it in the calculations when choosing injectors. But most of the time, the design of connectors does not play as big a role in choosing a injector, but you adapt the harness with the connectors that fit the injector of coice.

What you never hear anyone talk about is the quality of the connector and pins. This is very important as the injector  installation and disassembly during inspection and service means that the connector piece and pin must have good contact and time again without losing clamping force. A loose connector make the injector not open properly with serious engine damage or engine failure as a result.

Use the correct tool when contacting the cable harness and make sure that you get connectors you can trust and you have eliminated two points that can fail in your installation.

The injector is driven by a coil and means that it is not sensitive to polarity and the pins can be turned, but with a proper installation you set them all uniformly.


Fuel injector components and materials

The body of a fuel injector is often made of plastic.

It has an upper and a lower O-ring that is adapted for the fuel it is to be used with and in the size that fits the mounting hole.

Internally, an injector has looked the same for many many years but has been constantly upgraded with other material choices and designs to make moving parts in better control.

At the top of the injector it is a small pre filter so that dirt does not get in and damage the moving parts.

Then comes the coil that the control system activates with a current. It pulls up a spring loaded piston or injector tip that opens up the fuel. When the power from the ECU disappears, the piston returns to the closed position with the help of the built in spring and the current fuel pressure which also helps to close.

The fuel passes a disk or injector top that forms the spray pattern together with the design of the piston.

This is a very simple basic construction that has been refined over the years with adapted properties.



A new injector often has a very fine spray pattern which deteriorates over the years due to dirt, deposits and fatigue. When using petrol as a fuel, these factors are not a major problem even if they exist. It is when the use of ethanol / E85 that the problems became apparent. During the same time period, modern control systems have received individual cylinder compensation that obscured these problems and compensated for them instead.

Injector service is thus something as obvious as changing the fuel filter, diaphragm in the fuel pressure regulator, changing the oil filter and any other service points. Lack of control will be noticed in the form of uneven engine running, engine wear and minor safety margins when the control system constantly compensates on one or more cylinders.


Concluding remarks

Speeding hopes you have benefited from this fuel injector info. If you have any questions or need help, do not hesitate to contact us.