FICM TUNING
We have been getting alot of questions about FICM tuning, this write-up should be able to answer most questions you have.
To start, the FICM (Fuel Injection Control Module) is the deciding factor to determine injector pulse width (The time the injector is "open"). The FICM will determine the inj PW based off three primary values from the PCM. Those three input values are:
1. Engine RPM2. Injection Control Pressure (ICP)3. Mass Fuel Desired (MFD)
To explain how each of these values correspond to the PW, I will try to break down the logic behind why these three values are important.
Engine RPM is used to determine how many milliseconds the injectors are open. At lower engine RPM, the piston is traveling at a slower speed. Each injection cycle is constrained mechanically by the piston location. As the piston is traveling towards TDC, the injection will begin around 15 degrees TDC. If the start of injection occurs any sooner, you risk over stressing the rods causing engine malfunction. The end of injection will be around 30 degrees. If you exceed this piston location in the cylinder, you will risk “washing the cylinder walls”.
The slower the engine is turning, it will take a greater amount of time for the piston to go from point A (15 degrees pre TDC) to point B ( 30 degrees post TDC). This is often referred to as the injection window.
These are arbitrary numbers for proof of concept.
Injection Control Pressure (ICP) is used to determine fuel quantity in a given window. The way I often explain how ICP affects the injection system is to use an analogy. Think of an injector as your generic spray bottle. If you squeeze the trigger slowly, you will get less fluid to come out of the nozzle than if you squeeze the trigger hard. The HEUI fuel injector operates much like this. If the ICP is low during the injection cycle, you will not get as much fuel from the injector nozzle. If the ICP is high, the injector nozzle will disperse more fuel into the cylinder.
For this reason, the injector PW operates inversely from the ICP. If the injector pressure rises, it will cause more fuel to come out in a given period of time. With low injection pressure, the amount of fuel in that same period of time will be less. Because of this, injector pulse width will expand at lower RPM to attempt to achieve a target amount of fuel. If the injection pressure is high, the injector PW will reduce to attempt to hit that same target amount of fuel.
The PCM measures ICP based off values from MFD and Engine RPM.
Mass Fuel Desired is used to help facilitate what the target amount of fuel is. The PCM measures MFD by calculating between the accelerator pedal position and engine RPM. With the desired outcome of MFD being the amount of fuel, this is measured in mg per stroke.
Spoofing The ICP: With the FICM taking these three values to ultimately calculate the PW, a lot of PCM tuners have taken to “spoofing the ICP”. What spoofing does is to trick the truck even though it is making 4,000 psi ICP, the PCM interprets the psi as actually 2,000 psi ICP.
As mentioned above under ICP, if the truck has less ICP, it will naturally expand the PW to hit the target amount of fuel.
Since we are actually producing 4,000 psi ICP (not the 2,000 psi the PCM thinks) we now have an expanded PW while having full ICP. This is where FICM tuning comes into benefit. Since “spoofing the ICP” is not a precise way to increase PW, FICM tuning steps in to take over.
The ICP spoof only occurs over a set ICP value. On average, tuners begin to spoof the ICP around 2,000 actual psi to 3,000 actual psi. So when you are driving around town at light throttle, there is very little fuel modification via injection PW. That is because the PCM tuning and spoofing the ICP simply cannot work in that situation. When tuners do attempt to spoof the ICP too soon, that is what will cause the ‘light switch effect’ in the way the truck fuels.
Since the FICM is the deciding factor in how the injector fuels the truck, wouldn’t it make sense to tune the actual fuel maps opposed to ‘spoofing’ aka faking values?
This is exactly why FICM tuning is needed in addition to a PCM tune for the truck to run in optimal form.
FICM tuning is able to help your truck achieve better off idle throttle response, increased fuel mileage and more power / torque in the areas that PCM tuning simply cannot achieve on the 6.0 platform due to design.