Basic operation of the Cartech fmu
The Cartech fmu has two adjustments. One is for the fuel pressure at onset of boost (as manifold pressure passes through zero). This is controlled by the center screw. If the screw is all the way out, then the fmu does not affect fuel pressure at zero manifold pressure, rather it is controlled by the fpr. The more the center screw is screwed in, the higher the fuel pressure will be as manifold pressure transitions from vacuum to boost. In fact, the center screw can be used to increase fuel pressure before onset of boost. This is useful if you want to lower the afr at partial throttle settings when you are at low manifold vacuum but are not boosting. Of course, this will only work at a throttle setting high enough to cause the ecu to go into open loop.
The other adjustment is the rise rate of fuel pressure referenced to the boost pressure. This is controlled by the bleeder valve. If the bleeder valve is all the way closed, you get the maximum rise rate. Minimum rise rate occurs with the valve all the way open.
The Cartech fmu does not have an adjustment for base fuel pressure at idle. You need a separate adjustable fpr in order to make that adjustment. The fpr should be installed in-between the fuel rail and the fmu in the fuel circuit.
In theory, the Cartech fmu allows you to adjust both fuel pressure at zero manifold pressure and the rise rate of fuel pressure as a function of manifold pressure. If you think of the equation of a line, these two settings allow you to set the y-intercept and the slope of the line:
y = mx + b
where: y is fuel pressure as a function of manifold pressure, x,
m is the slope, or rise rate, controlled by the bleeder valve,
b is the y-intercept, or fuel pressure at onset of boost, controlled by the center screw.
Tuning with the Cartech fmu
It is best to tune with a wideband O2 sensor, and all examples I give were done with O2 sensor data collected with an Innovate LM-1 system.
The purpose of tuning by adjusting fuel pressure is to enrich or lean out the afr by controlling how much fuel goes through the injectors during wide-open-throttle (WOT) conditions when the ecu is in open loop. In open loop mode, the ecu is programmed to produce an injector duty cycle based on rpm and maf voltage. Injector duty cycle is simply the fraction of time the injector is open and delivering fuel. The ecu controls duty cycle by pulsing the injectors with a voltage over a calculated time interval. Lookup tables have been programmed into the ecu which tell the ecu how long the pulse width needs to be for a given engine rpm and maf voltage. The injector pulse widths were programmed assuming the fuel pressure is constant.
Since the ecu thinks the fuel pressure is constant, and since the ecu does not adjust afr based on O2 sensor information in open loop mode, we can make adjustments to the afr by changing fuel pressure without the ecu trying to readjust back to what it thinks is right, such as would happen at partial throttle in closed loop mode.
Figure 1. below shows data collected for a second gear run at WOT, using a JWT ecu. For this run I have the fmu disconnected so that it is not affecting fuel pressure during the run. Fuel pressure rise is due solely to my adjustable fpr. I have the base fuel pressure set to 37 psi, which gives 46 psi at onset of boost.
Figure 1. Baseline WOT run in 2nd gear with Cartech fmu disconnected. JWT ecu.
From figure 1 we see that the fuel pressure rises gently from about 46 psi at onset of boosting to just over 51 psi at 6800 psi. This is only a rise of 5 psi, even though boost pressure is about 10 psi at 6800 rpm, which makes me think my cheapo fpr isn’t doing it’s job adequately, but we’ll ignore that for now. Notice that the JWT tune starts out in the low 13’s, and transitions to the mid 12’s at 5000 rpm.
Effect of bleeder valve
Figure 2 shows another 2nd gear WOT run, this time with the Cartech fmu connected. The center screw is backed all the way out, and the bleeder valve has been screwed in about 1.5 turns.
Figure 2. WOT run with Cartech fmu connected, center screw all the way out, bleeder valve 1.5 turns in.
With the center screw backed all the way out the fmu is not affecting fuel pressure at the onset of boost, but the fmu is causing the fuel pressure to rise with increasing boost pressure. So instead of 51 psi fuel pressure at 6800 rpm, we have 56.5 psi, a rise of 10 psi from 46 psi at onset of boost, and we see a concomitant lowering of afr to the high 11’s by the end of the run. The afr at the beginning of the run is unchanged and still in the mid 13’s. If the bleeder valve was turned in more than the 1.5 times used for this run, fuel pressure would increase at a faster rate and the afr at the end of the run would be even lower.
Effect of center screw
The center screw adjusts fuel pressure at onset of boost, thus raising the fuel pressure throughout the rpm range at WOT. Figure 3 shows the 3rd WOT run, this time with the bleeder valve in 1.5 turns and with the center screw turned in to give 53 psi of fuel pressure at onset of boost
Figure 3. WOT run with Cartech fmu connected, center screw turned in to give 53 psi fuel pressure at onset of boost, bleeder valve 1.5 turns in.
This time we see fuel pressure starting out at about 53 psi at onset of boost and rising from 53 psi to 68 psi by the end of the run, a rise of 15 psi. The afr at onset of boost is now lowered from the mid 13’s to the mid 12’s, transitions to the mid 11’s at 5000 rpm and keeps lowering to the high 10’s by the end of the run. Thus the afr has been lowered throughout the rpm range.
This is a broad intro to the tuning of the FMU but a good reference.
