PART 8 – ELECTRICAL CONSIDERATIONS FOR UPGRADED LPF PUMPS
The key questions I want to answer in this post are:
1) Is the original wiring adequate with respect to voltage drop when a Stage 2 (19 Amps) or Stage 2.5 (22 Amps) pump is installed? (No, so I upgraded to 10 AWG.)
2) Is the original wiring adequate with respect to resistive heating when a Stage 2 (19 Amps) or Stage 2.5 (22 Amps) pump is installed? (Yes.)
3) Is the original 20 Amp fuse adequate? (Yes, but a larger fuse is appropriate for upgraded wiring.)
4) Can an original EKPM2 adequately control a Stage 2 pump? (Not in my experience.)
I will show that the OE wiring is inadequate to properly support a Stage 2 or Stage 2.5 LPFP. BMW did a decent job of optimizing the wiring for the original pump that draws 11 Amps intermittently but upgraded pumps that draw significantly more than that will not reach their full flow potential due to excessive voltage drops in the wiring. I will show that upgrading to 10 AWG wiring will support approximately 20% more flow from a Stage 2 or Stage 2.5 LPFP vs. the original wiring, and that this corresponds to support for more than 100 additional hp in both cases. To put it another way, the original wire derates the potential performance of Stage2 or Stage 2.5 LPF pumps by about 20%, which is equivalent to more than a 100 hp penalty.
Excessive voltage drops in the system also force the EKPM to operate at a higher duty cycle to output the voltage needed to overcome that voltage drop. In the case of an EKPM2, it will overtax and already marginal component. My initial testing of a Stage 2 pump with my original EKPM2 immediately revealed this weakness and prompted an upgrade to a Stage 2 EKPM3.
SIMPLIFIED CIRCUIT
The following simplified representation of the LPFP electrical circuit is useful to discuss the various voltages, voltage drops and currents. Voltage drops occur along each leg of the circuit and they reduce the voltage available to operate the LPF pump. I measured the voltage drops using a voltmeter at a known current to calculate the resistance of each leg of the circuit.
Fig. 1: Voltage Drops in the LPFP System
The battery voltage (AKA Vt30) measured between the battery + and – terminals is the highest voltage in the system. The current consumed by the EKPM for its internal function (Io) and bypassing the fuel pump is small enough relative to the pump current as to be negligible, so we can assume that all the current flowing in the EKPM flows through the pump.
Fig. 2: PT-CAN Measured Voltage and Current Parameters
Because of the resistance in the wiring, the current flowing in the circuit results in a voltage drop in every wire through which it flows.
The entire LPFP circuit is wired with 2.5 mm2 metric primary wire, which does not correspond exactly with an AWG size. The wiring is often described as being 14 AWG as this is the closest North American equivalent, but the difference is materially significant, as the characterization results in an over-estimate of the benefit of a wire upgrade to 12 AWG.
Potential wire size upgrades include 4.0 mm2, 12 AWG and 10 AWG. The two systems can be compared as follows (typical values):
14 AWG (1.88 mm2) => 3702 CMA => 3.21 Ω / 1000 ft
2.5 mm2 => 4934 CMA => 2.32 Ω / 1000 ft
12 AWG (2.96 mm2) => 5833 CMA => 2.04 Ω / 1000 ft
4.0 mm2 => 7894 CMA => 1.43 Ω / 1000 ft
10 AWG (4.73 mm2) => 9543 CMA => 1.27 Ω / 1000 ft
Measuring wire resistance over installed lengths with a conventional ohmmeter function on a digital multimeter is not accurate because the resistances are so low, but measuring the voltage drop across the wire with a substantial known current flowing through it works well. Connectors, crimps, fuses, etc. contribute to the total circuit resistance. This point is illustrated by the measurement described in Fig. 3. The resistance can be determined by using Ohms Law (R = V / I) in conjunction with measured voltage and known current.
Fig. 3: Measuring Wire Resistance using 10 Amp Current
The measured resistance of the conductor alone corresponds to a resistance per unit length of 2.6 Ω/1000 ft. This is too low for the wire to be 14 AWG (3.2 Ω/1000 ft) and is close to the rating for 2.5 mm2 wire (7.6 Ω/km = 2.3 Ω/1000 ft. The ISTA wiring diagram in Fig. 4 (2008 N54 335i) confirms this wire size. I am flogging this issue to death because the wiring is widely reported to be 14 AWG which makes the readily available 12 AWG upgrades look much more beneficial than they actually are.
Fig. 4: ITSA Wiring Diagram (N54 335i)
ORIGINAL WIRING
I calculated the various circuit resistance values by means in situ measurements with the engine running. Current was measured using the calibrated PT-CAN current data as described in Part 7. Voltage drops were measured using a digital voltmeter from point to point.
Fig. 5: Measured Resistances for Original Wiring
The resistance of the ground wire is surprisingly high, given the proximity of the ground lug to the EKPM. Clearly this wire does not run directly from the EKPM to the ground lug as the measured resistance corresponds to about 4 ft of wire. There is a splicing of ground wires within the harness in the luggage compartment, so I assume the EKPM ground wire takes that convoluted route to the ground lug just above it. Running a direct wire to this lug takes only 18” of conductor and is very worthwhile.
I confirmed these resistance values using the PT-CAN data parameters accessible on my data logger (described in Part 7). I added a resistor load bank (consisting of eight individually switchable 2Ω resistors in parallel) in parallel with the LPFP to allow me to examine circuit currents from about 7.5 Amps to 18 Amps. Fig. 5 shows the load profile applied in Amps, along with the fuel pressure observed during the test. The load was applied in eight 1.3 Amp steps but unloaded in four 2.6 Amp steps. The fuel pressure was stable with only a small transient response to the unloading.
Fig. 6: EKPM Current Loading Profile at Idle
During this testing, the LPFP voltage and current remained constant and sufficient to maintain stable idle at 72 psi LPF pressure. The additional current flowing in the circuit bypasses the LPF pump and flows through the resistor load bank. The effect on the measured parameters is shown in Fig. 7.
Fig. 7: Voltage Response to Current Loading
The red line represents battery voltage, which declines only slightly as the pump load increases, as the alternator is easily able to supply the current required to maintain the battery voltage. The green line represents the voltage at the EKPM input terminals. It declines with increasing current due to the resistance in its power and ground wires. The blue line represents the EKPM output voltage. It increases slightly as the increased current flowing through the LPFP supply and return wires results in an increased voltage drop in those wires. The EKPM adjusts its duty cycle as required to provide the additional output voltage needed to maintain a constant pump voltage.
It is interesting to plot this same data against circuit current, in addition to the duty cycle parameters. Fig. 8 shows this with the original wiring.
Fig. 8: Original Wiring - Voltage and Duty Cycle vs. EKPM Current
Because R = V / I, the slope of the voltage vs. current regression lines represents resistance values. The slope of the green line represents the sum of dVp and dVg from Fig. 1. The calculated value of 76 mΩ closely matches the measured value of (55 + 20) = 75 mΩ from Fig. 5. The slope of the dark blue line represents the sum of dVs and dVr from Fig. 1. The calculated value of 25 mΩ closely matches the measured value of (12 + 12) = 24 mΩ from Fig. 5.
The light blue line represents the normalized duty cycle. The increasing voltage drop in the pump feeders resulting from increasing current requires the DME to request additional duty cycle to maintain a constant voltage at the LPFP terminals. The flow rate and fuel pressure remain constant.
The purple line represents the adjusted duty cycle. As the increased current pulls the EKPM input voltage below 14.2 Volts, the EKPM must adjust the duty cycle upward to achieve the desired output voltage implied by the normalized duty cycle.
The olive line represents the actual duty cycle. It incorporates the learned flow characteristics of the installed pump through the adaptation process. Luckily, our pump is strong and this results in a reduction from the adjusted duty cycle.
The loss of LPFP capability in response to the voltage drops can be framed in a two equivalent ways:
1) The decline in EKPM input voltage with increasing current reduces the maximum voltage that can be supplied to the pump and so reduces its flow potential.
2) The increase in adjusted duty cycle in response to the voltage drops at increased current reduces the available duty cycle to accommodate actual increased flow requirements.
10 AWG WIRING
To reduce the voltage drops in the circuit, and to increase the flow potential of the installation, I have replaced the wiring with 10 AWG wire, with the power connection going direct to the battery. I left the original EKPM ground wire in place but supplemented it with a 10 AWG wire directly to the adjacent ground lug. For comparison purposes I have measured the circuit resistances as previously, with those results shown in Fig. 9.
Fig. 9: Measured Resistances for 10 AWG Wiring
I tested this in the same manner as previously, with the results shown in Fig. 10.
Fig. 10: 10 AWG Wiring - Voltage and Duty Cycle vs. EKPM Current
The slope of the green line representing the sum of dVp and dVg has a calculated value of 23 mΩ, which matches the measured value of (20 + 3) = 23 mΩ from Fig. 9. The slope of the dark blue line representing the sum of dVs and dVr has a calculated value of 13 mΩ closely matches the measured value of (6 + 6) = 12 mΩ from Fig 9.
The light blue line representing the normalized duty cycle rises with increased current as the DME requests additional duty cycle to maintain a constant voltage at the LPFP terminals. The flow rate and fuel pressure remain constant, but the EKPM must work harder to achieve this as we increase the current, but less so than with the original wiring. As with the original wiring the battery voltage is negligibly affected by the modification.
The purple line representing the adjusted duty cycle is only slightly steeper than the normalized duty cycle because the EKPM input voltage (green line) is never far from the ideal 14.2 Volts. Accordingly, the 10 AWG wiring allows the full potential of a Stage 2 or Stage 2.5 LPFP to be realized without running out of input voltage or duty cycle.
VOLTAGE DROP ACROSS THE EKPM / EFFECTIVE EKPM “ON” RESISTANCE
In Figures 5 and 9 I have indicated that the effective resistance between the EKPM V In and V Out terminals as 10 mΩ. This is a consequence of the on resistance of the half-bridge MOSFET switch, and the resistance of internal conductor traces and solder connections within the EKPM. As a result of this resistance, even at 100% duty cycle, there will be some voltage drop across the device. This is designated dVe in Fig. 11.
Fig. 11: Duty Cycle Calculation Accounting for EKPM Internal Voltage Drop
As a first order approximation it is typically stated that duty cycle equals (V Out / V In) in a PWM circuit. A more accurate representation is obtained by accounting for the voltage drop cross the switch as shown in Fig. 11. Where I have plotted true duty cycle in the previous graphs, it has been calculated in this manner.
THEVENIN’S THEOREM
The LPF circuit can be simplified to a voltage source and resistor in series with the LPF pump according to Thevenin’s theorem. It is useful to consider the case of 100% duty cycle. The voltage source simplifies to the battery voltage, and the series resistance is the sum of the resistances shown in Fig. 5 for original wiring (0.055 + 0.010 + 0.012 + 0.012 + 0.020) = 0.109 Ω, and the sum of resistances shown in Fig. 9 for 10 AWG wiring (0.020 + 0.010 + 0.006 + 0.006 + 0.003) = 0.045 Ω. The total voltage drop between battery and LPFP at 100% duty cycle is then this resistance times the fuel pump current. Based on this we can calculate the potential benefit of 10 AWG in terms of maximum LPFP voltage for a variety of scenarios.
As we showed in Part 3 the LPF system must provide approximately 110 lph to support the pump module and regulator module jet pumps. Thus, any flow penalty due to reduced maximum pump voltage needs to be considered against the usable flow. In Part 6 we established the usable flow rates for the Stage 2 and Stage 2.5 LPFPs at 12.0 Volts and 13.5 Volts. We can use this to estimate the flow penalty associated with the calculated voltage drops. We also calculated the relationship between flow rate and hp. All these concepts are combined in Fig. 12 to calculate the potential benefit of a change to 10 AWG wiring with respect to maximum LPFP voltage via a reduction in total voltage drop, and the associated benefits in terms of fuel flow rate and hp potential.
Fig. 12: Potential Benefits of 10 AWG Wiring with Stage 2 and Stage 2.5 LPFPs
We conclude that the change to 10 AWG wiring will provide the potential for about 19% additional usable fuel flow from either a Stage 2 or Stage 2.5 LPFP over what it could provide with original wiring and that this corresponds with the potential to support at least 100 additional hp. Perhaps a better characterization would be that retaining the original wiring cripples the flow potential by over 100 hp.
ACCEPTABLE VOLTAGE DROPS
While there is no universally accepted rule of thumb for acceptable voltage drops, the aviation standard from FAA AC 43.13-1B states that for a 14 Volt circuit, the maximum allowable voltage drop between the bus and the equipment ground is 0.5 Volts for continuous loads and 1.0 Volts for intermittent loads. Fig. 12 shows that the intermittent standard is not met with original wiring but is with 10 AWG, for both Stage 2 and Stage 2.5 pumps. The original wiring woefully fails to meet this standard.
WIRE CURRENT LIMITS (AMPACITY)
All wires have some resistance, and when current flows through them they heat up proportional to the current and their resistance per unit length. The only way to reduce the heating is to provide better cooling, which is generally not practical, or to use heavier gauge wire (numerically smaller AWG or bigger mm2, which has lower resistance). Alternately we can use wire with a higher insulation temperature limit and allow the wire to run hotter, which is generally not desirable. Standard automotive primary wire is typically rated at 105°C. Wiring used for motorsport upgrades is typically either TXL (125°C rated) or M22759/16 (150°C rated). While higher temperature ratings provide an additional margin of safety against wires overheating and insulation failure, running hot risks damage to electrical connectors.
A wire’s resistive heating current limit is referred to as its ampacity, and it is not a fixed value. How hot a wire will get at a given current depends on the ambient temperature and whether the wire is alone in free air or part of a bundle, and how much current the other conductors in the bundle are carrying. There are empirical methods that can establish current limits based on these and other factors. FAA AC 43.13-1B has a method that can be used, but the science is inexact and highly dependent on assumptions, so a healthy margin is prudent. Wire data sheets often provide a current rating, but we need to read the fine print to understand the assumptions involved. The data sheet for 2.5 mm2 metric FLRY B automotive primary wire shows a current limit (ampacity) of 38 Amps, assuming a single wire in free air at an ambient temperature of 40°C and the wire at its temperature limit of 105°C (221°F). While the cooling assumptions are not conservative the Stage 2.5 maximum current of 22 Amps is an intermittent load condition. For these reasons I have concluded that the ampacity of the original wire is sufficient for a Stage 2.5 pump.
Fig. 13: Metric Primary Wire Data Sheet
Fuses don’t overheat/blow instantly, so the 20 Amp fuse protecting the circuit (which is to say the wires) is unlikely to blow at 22 Amps intermittent. Increasing the fuse rating to 25 Amps would be acceptable based on the wire’s ampacity but provides no tangible benefit.
UPGRADED WIRE OPTIONS
In terms of voltage drop, both 12 AWG and 10 AWG are improvements, preferably in TXL or M22759/16 for their higher temperature ratings. Metric primary wire is also an option, but much harder to find. 4.0 mm2 would be a good choice because it is the largest size that the required connector pins will nominally accept. 10 AWG requires that any pins be crimped out of spec, which you may not be comfortable with (of course solder is an option the experts abhor).
Arc Terminator’s aftermarket pump upgrade harness (there are others too) is only 12 AWG because of the crimping issue.
Considering just the wire, and ignoring the benefits of shorter direct wiring to the battery or shorter grounding, the larger wire sizes provide the following voltage drop benefits:
2.5 mm2 = 2.32 Ω / 1000 ft -> status quo
12 AWG = 2.04 Ω / 1000 ft -> about 12% less voltage drop than status quo
4.0 mm2 = 1.43 Ω / 1000 ft -> about 38% less voltage drop than status quo
10 AWG = 1.27 Ω / 1000 ft -> about 45% less voltage drop than status quo
PUMP WIRING OPTIONS
The combined length of the pump supply and return wires from EPKM to pump is 6 ft. A 12 AWG TXL upgrade harness is available from Arc Terminator and others for those disinclined to make their own and would provide some improvement, particularly if paired with an upgraded EKPM ground wire.
The fuel pump connector end of this harness uses 2.5 mm Series crimp sockets. The largest compatible sockets are compatible with 2.5-4 mm2 wire (p/n 61131376206). 10 AWG is strictly slightly larger than 4 mm2, so testing for acceptable crimps is essential. As I don’t know the tensile strength specification for this style of crimp pin, I referred to the specification for AMP Mate-N-Lok 0.140” diameter pins (document 108-1032), which specifies a tensile strength of 65 lbs. I found a crimp tool/cavity combination that achieved a pull-out load of about 60 lbs (with just the conductor crimped, not the insulation), which for 10 AWG wire is a bit less than ideal, but acceptable (in my application, in my engineering judgement) given the benefits. I lack the technology to do an official test at the 1 inch/minute rate but used a tensile load cell and a steady pull by hand. For a commercial solution, using 4.0 mm2 metric primary wire would be a better solution in terms of achieving fully qualified crimps, and it would be objectively (26%) better than 12 AWG in terms of voltage drops.
Fig. 14: Mate-N-Lok Crimp Tensile Strength Requirements
Fig 15: LPFP Pin Test Crimp with 10 AWG M22759/16 Wire
I bought extra pins so that I could do a couple of trials to figure out the best crimp tool for the job. I used 10 AWG M22758/16 wire, which has a CMA of 9354 and a metric area of 4.7 mm2. I crimped the 2.5-4.0 mm2 rated sockets using a Delphi 12085271 ratcheting crimper. 10 AWG TXL wire has a slightly higher CMA of 9704 and a metric area of 4.9 mm2. I doubt that difference will materially affect the ability to crimp these pins on TXL, but I haven’t tried it.
BMW sells a universal connector housing (p/n 61138352390), which can be used for either the pump or gauge connection if your housing is cracked. The housings can be de-pinned relatively easily, with the correct tool. I could not find such a tool in my collection of Amazon cheapies, or my Schwaben kit. A good tool can be made from stainless steel hypodermic tubing (0.180” OD, 0.160” ID, McMaster Carr p/n 8987K9).
Fig. 16: 10 AWG LPFP Harness Installed at Pump End
The EKPM end of the harness uses ELO-Power 2.8x0.63 contacts. These crimp contacts are also available for wires 2.5-4.0 mm2 (6113913612), which would work with either 12 AWG or 4.0 mm2 wire. I suspect 10 AWG would be a problem with these connectors. As my ET Design Stage 2 EKPM3 has screw terminal blocks, I did not need to use this connector, which allowed me to easily go to 10 AWG at the EKPM. Depinning the EKPM connector housing does not take any special tools.
Fig. 17: ET3 Design Stage 2 EKPM3 Installed with 10 AWG
The EKPM draws no standby current unless woken up by the CAS. For that reason, it can be wired direct to the battery without introducing a parasitic load.
Fig. 18: EKPM Direct-to-Battery Wiring Through 30 Amp ATO Fuse
My EKPM main power wire follows the main harness back to the battery compartment and then connecting to battery + through a 30 Amp ATO fuse. Littelfuse makes a nice stackable fuse holder housing (p/n 178.6150.001) that will accept a variety of crimp pins depending on the wire size being used (p/n 178.6116.6001 works for 10 AWG). Again, the Delphi crimper worked well, but I did need to do a bit of fiddling to get decent insulation crimps. Spare pins are your friend when finding a workable crimp setup for a previously untested configuration, as pull testing is always prudent. I took the opportunity to replace a previous pigtail fuse holder used by my trailer module by creating a two-fuse stack. By following the existing wiring routing for protection, rather than taking the most direct route for minimal length, the total conductor length from battery to EKPM is about 7 ft. I installed a 30 Amp fuse as the upgraded wiring will easily support this.
Fig. 19: ATO Fuse Holder
The EKPM connectors are a known weak point, and the ground pin in particular has been known to overheat. For this reason, ET3 Design added a terminal strip to their Stage 2 EKPM3. While it looks a bit clunky, it is very practical, and it supports wires up to 10 AWG. That feature prompted me to adopt the ET3 Design unit when upgrading. As I left my original power wire in place in the main EKPM connector, I removed the glovebox fuse 70 to prevent the new fuse from back feeding the fuse box through that wire.
Fig. 20: Original LPF Fuse 70 Removed