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XMS4 - Injector Setup Application | Application Note : AN 1


Perfect Power Application Notes.


Application Note : AN 1


This note describes different injectors, their behavior, and their suitability to be used in your car. The note generalizes some points so that the reader doesn't require an engineering degree to understand the subject.
Injectors are graded as to the FLOW RATE, and spray pattern (construction). The construction can be separated in mechanical properties, and coil resistance. The coil resistance is measured in OHMS and can be checked with a multimeter. 2-4 Ohm is considered low resistance; 12-16 Ohm is considered high resistance.


Resistance : As measured with an OHM-METER (multimeter)


Flow Rate : A very obscure number. The manufacturer gives it. It means the flow when the injector is continuously open. In cubic centimeters per second or cubic inch per minute.

1 Cubic centimeter / sec = 3.66 Cubic Inch / minute

If you have a powerful engine, you need to get some fuel into it. At full power, the injectors should be open 50 to 80% of the time.


Opening Time

It is the time required in milliseconds from applying a voltage to the injector to the injector FULLY open (the armature hitting its mechanical open limit). The injector "sprays" during the process of opening, but the volume is not consistent, nor is it really "spray". It is more a "dribble".
If the open time of the injector is very close to the activation time, then the engine runs very rough, idles badly, and has high hydrocarbons. Not good!
The opening time can be measured with an oscilloscope, as described in the technical manual. See LOW/HIGH RESISTANCE INJECTORS for more details.

Activation Time

This is the time the XMS4 activates the injector. The time is displayed in milliseconds.

Injector Utilization

This is the ratio of the ACTIVATION time to the repeat ACTIVATION. In short, an injector, which is continuously ON, is utilized 100%. The injector utilization can be displayed by the software.

Injector Repeat Time

  • A fully sequential injection repeats every 2 cranks.
  • A batch sequential injection repeats every 1 crank.
  • The XMS4 can do all of the above injection modes.
  • The repeat time decreases as the RPM increases.
  • The three modes relate to fuel delivery as follows :


  4 cyl 6 cyl 8 cyl
Sequential 1 1 1
Batch 2 2 2



Low Resistance Injectors 2-4 Ohm

They normally have a high flow rate and open at 0.4 to 0.6 milliseconds. They require a "stepped" current to reduce the power dissipation when used in parallel, or as a singe injector. The XMS4 cannot provide a "stepped" current, but can activate a 2-OHM injector with the use of an external driver. If this is required, then check the injector temperature after prolonged use.


High Resistance Injectors 12-16 Ohm

These are the most common injectors. They have low to normal flow-rate and open in approximately 1.5 milliseconds. The XMS4 can drive these injectors directly.


Injector Staging

The XMS4 units do not have this ability but it is a method where a second set of injectors is activated once a defined operating point is reached.


Fuel Pressure

A standard fuel pressure is normally between 2-3 bars. Below 2 bars, the injector "SPRAY PATTERN" deteriorates to a "SPLUTTERING", and above 3 bars, the injector may not open under a low voltage condition, as experienced with a cold start. The upper limit can be stretched, if enough fuel is injected BEFORE cranking (because injectors don't open during cranking), to fire the engine, and, once fired, the battery voltage rises and the injectors start to operate.


Progressive Fuel Regulator

This is a device, which has 3 pipe connections, and a double membrane to give it progressive performance. The third connection is the manifold pressure. If set-up correctly, it provides a fuel pressure of, let's say 2 bar at idle, and 0.3 bar AMP, and a 3.4 bar fuel pressure at 1.0 bar AMP. Notice that the AMP changes by 0.7 bars, and the fuel by 1.4 bars.