Port injection: wiring and controllers
Past ~500 whp the direct-injection system needs help. Port injection adds a second fuel circuit — here's how the controllers wire in and what protects the engine.
Direct injection gave the N54 its clean combustion and its ceiling. The high-pressure pump can only move so much fuel, and ethanol makes everything worse by demanding more volume for the same energy. Port injection — a second fuel circuit spraying into the intake runners at low pressure — is how the platform gets past it. The mechanical install is a plate, a rail, six injectors, and fuel lines; this guide is the electrical side and the controller logic that keeps it safe.
What PI is, mechanically
A port-injection kit adds a rail fed from the low-pressure side (~5 bar / 72 psi) — upstream of the HPFP entirely — with one spray injector per cylinder in a plate between the manifold and the head. The DI system stays and keeps carrying its share; PI supplements it progressively. Because it rides LP pressure, PI sidesteps the HPFP ceiling completely.
Sizing (on full E85, with adequate LPFP): 550 cc injectors support roughly 650 whp, 750 cc around 750 whp, 950 cc past 800. One gotcha everyone hits: injector flow is quoted at the industry test pressure of 43.5 psi, but the rail here runs ~72 psi — a "750 cc" injector flows about 1000–1050 cc in this service. Compare kits at real pressure or don't compare them.
The controller is the wiring job
The injectors are passive — the controller decides when and how much. Four credible options, matched to how the car is otherwise run:
| Controller | Best for | Control model |
|---|---|---|
| BMS JB4 PI controller | Cars already running a JB4 | Fully integrated — reads LPFP pressure, boost, gear, trims, AFR, injector current |
| Split Second AIC6 | Flash-only cars | Standalone duty table vs. boost pressure and RPM |
| Motiv ReFlex+ / Lite | Flash-only + flex fuel | CAN + analog I/O; AUX outputs can also trigger a second LPFP |
| Fuel-It/BMS CANfuel | B58/G-platform, N54 with JB4 | Sequential 8-injector driver on CAN, Bluetooth config |
None of them tap DME injector pulses — that's a common misconception. Load comes from the JB4's CAN view of the engine (integrated path), or from MAP + RPM (standalone path), or from crank/cam + CAN (sequential path).
JB4 PI controller wiring (N54)
| Wire | Lands on |
|---|---|
| Black ×2 | Ground |
| Red | JB4 power, green connector |
| Yellow | BMS data/Bluetooth dongle spot #9 |
| White | JB4 DB25 harness #15 (the meth-blue position) |
| Purple | JB4 small gray subconnector #21 (read from the female side) |
| Orange (thicker) | +12 V output for a second fuel pump or meth pump — 2 A max, must drive a relay |
Setup side: the board needs the 24K22 slave-chip firmware (G5 boards from 11/2015 on), Fuel Bias = 50 across the table, meth scaling = 99, and Menu 8 = 0 rpm unless a 2-step relay is fitted — with the wrong menu-8 value PI is silently blocked from activating.
AIC6 wiring (standalone path)
| Wire | Lands on |
|---|---|
| Red | +12 V, engine bay |
| Black | Ground |
| Green | T-MAP sensor signal (boost) |
| Yellow/Black | OBD pin 9 (RPM) |
What makes a PI install safe
Fuel added by an aftermarket box is the highest-consequence wiring on the car. In order of importance:
- MHD's PI Safety flash option — cuts ignition to a cylinder whose port injector stops flowing. On any MHD-format file we consider it required, not optional.
- The JB4 PI controller's failsafe set: PI duty dynamically scaled from measured LP pressure (flow stays honest as the pump sags); instant cut on lift-off or traction events (no intake backfires); AFR monitoring with cut on misfire detection; per-injector current monitoring (clogged injector, blown fuse, loose ground); failsafe on bank-to-bank trim mismatch >15%; auto-scaling with flex ethanol content.
- The physical layer: EV6/US-car connectors fully seated, the controller grounded where the engine grounds, and the LP tap done with proper hose and clamps — a PI fuel leak at 72 psi is a fire, not a trim problem.
How PI and the rest of the fueling stack interact
PI moves the ceiling, it doesn't raise the floor: the low-pressure supply feeding the rail has to keep up (the LPFP guide covers pumps and second-pump wiring), the flex sensor tells the blend what's in the tank, and on a single-turbo car the DME may be watching one bank (the single-bank guide). Logs tie it together — LPFP pressure, PI duty, lambda, and ethanol in one pull tell you whether the stack is feeding the engine or flattering it.
Sources
- N54 port injection end-to-end: bmwtuning.co PI guide
- JB4 PI controller wiring + failsafes (vendor thread): JB4tech N54 port injection
- CANfuel controller: burgertuning.com CANfuel
When does an N54 need port injection?
Around 500 wheel horsepower, sooner on ethanol. The stock fueling ceiling is roughly 450–500 whp on pump gas and 425–450 on E30; past that the low-pressure system gives first on ethanol blends, then the high-pressure pump. Port injection is the proven path past 500 whp on heavy E85 and supports 900+ whp on full E85 with the right pumps.
Do port injectors need their own power wiring?
No. PI injectors are passive solenoids on the controller's harness — EV6 connectors, fired by the controller's ground pulses. The only power wiring in a PI install is the controller itself and, if fitted, the relay it triggers for a second fuel pump or meth pump.